technical/api-trace2.html - external/gitster/git-htmldocs - Git at Google

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 <body class="article">
 <div id="header">
 <h1>Trace2 API</h1>
 <span id="revdate">2024-02-06</span>
 </div>
 <div id="content">
 <div id="preamble">
 <div class="sectionbody">
 <div class="paragraph"><p>The Trace2 API can be used to print debug, performance, and telemetry
 information to stderr or a file. The Trace2 feature is inactive unless
 explicitly enabled by enabling one or more Trace2 Targets.</p></div>
 <div class="paragraph"><p>The Trace2 API is intended to replace the existing (Trace1)
 <code>printf()</code>-style tracing provided by the existing <code>GIT_TRACE</code> and
 <code>GIT_TRACE_PERFORMANCE</code> facilities. During initial implementation,
 Trace2 and Trace1 may operate in parallel.</p></div>
 <div class="paragraph"><p>The Trace2 API defines a set of high-level messages with known fields,
 such as (<code>start</code>: <code>argv</code>) and (<code>exit</code>: {<code>exit-code</code>, <code>elapsed-time</code>}).</p></div>
 <div class="paragraph"><p>Trace2 instrumentation throughout the Git code base sends Trace2
 messages to the enabled Trace2 Targets. Targets transform these
 messages content into purpose-specific formats and write events to
 their data streams. In this manner, the Trace2 API can drive
 many different types of analysis.</p></div>
 <div class="paragraph"><p>Targets are defined using a VTable allowing easy extension to other
 formats in the future. This might be used to define a binary format,
 for example.</p></div>
 <div class="paragraph"><p>Trace2 is controlled using <code>trace2.*</code> config values in the system and
 global config files and <code>GIT_TRACE2*</code> environment variables. Trace2 does
 not read from repo local or worktree config files, nor does it respect
 <code>-c</code> command line config settings.</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_trace2_targets">Trace2 Targets</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>Trace2 defines the following set of Trace2 Targets.
 Format details are given in a later section.</p></div>
 <div class="sect2">
 <h3 id="_the_normal_format_target">The Normal Format Target</h3>
 <div class="paragraph"><p>The normal format target is a traditional <code>printf()</code> format and similar
 to the <code>GIT_TRACE</code> format. This format is enabled with the <code>GIT_TRACE2</code>
 environment variable or the <code>trace2.normalTarget</code> system or global
 config setting.</p></div>
 <div class="paragraph"><p>For example</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ export GIT_TRACE2=~/log.normal
 $ git version
 git version 2.20.1.155.g426c96fcdb</code></pre>
 </div></div>
 <div class="paragraph"><p>or</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git config --global trace2.normalTarget ~/log.normal
 $ git version
 git version 2.20.1.155.g426c96fcdb</code></pre>
 </div></div>
 <div class="paragraph"><p>yields</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ cat ~/log.normal
 12:28:42.620009 common-main.c:38 version 2.20.1.155.g426c96fcdb
 12:28:42.620989 common-main.c:39 start git version
 12:28:42.621101 git.c:432 cmd_name version (version)
 12:28:42.621215 git.c:662 exit elapsed:0.001227 code:0
 12:28:42.621250 trace2/tr2_tgt_normal.c:124 atexit elapsed:0.001265 code:0</code></pre>
 </div></div>
 </div>
 <div class="sect2">
 <h3 id="_the_performance_format_target">The Performance Format Target</h3>
 <div class="paragraph"><p>The performance format target (PERF) is a column-based format to
 replace <code>GIT_TRACE_PERFORMANCE</code> and is suitable for development and
 testing, possibly to complement tools like <code>gprof</code>. This format is
 enabled with the <code>GIT_TRACE2_PERF</code> environment variable or the
 <code>trace2.perfTarget</code> system or global config setting.</p></div>
 <div class="paragraph"><p>For example</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ export GIT_TRACE2_PERF=~/log.perf
 $ git version
 git version 2.20.1.155.g426c96fcdb</code></pre>
 </div></div>
 <div class="paragraph"><p>or</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git config --global trace2.perfTarget ~/log.perf
 $ git version
 git version 2.20.1.155.g426c96fcdb</code></pre>
 </div></div>
 <div class="paragraph"><p>yields</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ cat ~/log.perf
 12:28:42.620675 common-main.c:38 | d0 | main | version | | | | | 2.20.1.155.g426c96fcdb
 12:28:42.621001 common-main.c:39 | d0 | main | start | | 0.001173 | | | git version
 12:28:42.621111 git.c:432 | d0 | main | cmd_name | | | | | version (version)
 12:28:42.621225 git.c:662 | d0 | main | exit | | 0.001227 | | | code:0
 12:28:42.621259 trace2/tr2_tgt_perf.c:211 | d0 | main | atexit | | 0.001265 | | | code:0</code></pre>
 </div></div>
 </div>
 <div class="sect2">
 <h3 id="_the_event_format_target">The Event Format Target</h3>
 <div class="paragraph"><p>The event format target is a JSON-based format of event data suitable
 for telemetry analysis. This format is enabled with the <code>GIT_TRACE2_EVENT</code>
 environment variable or the <code>trace2.eventTarget</code> system or global config
 setting.</p></div>
 <div class="paragraph"><p>For example</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ export GIT_TRACE2_EVENT=~/log.event
 $ git version
 git version 2.20.1.155.g426c96fcdb</code></pre>
 </div></div>
 <div class="paragraph"><p>or</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git config --global trace2.eventTarget ~/log.event
 $ git version
 git version 2.20.1.155.g426c96fcdb</code></pre>
 </div></div>
 <div class="paragraph"><p>yields</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ cat ~/log.event
 {"event":"version","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.620713Z","file":"common-main.c","line":38,"evt":"3","exe":"2.20.1.155.g426c96fcdb"}
 {"event":"start","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621027Z","file":"common-main.c","line":39,"t_abs":0.001173,"argv":["git","version"]}
 {"event":"cmd_name","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621122Z","file":"git.c","line":432,"name":"version","hierarchy":"version"}
 {"event":"exit","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621236Z","file":"git.c","line":662,"t_abs":0.001227,"code":0}
 {"event":"atexit","sid":"20190408T191610.507018Z-H9b68c35f-P000059a8","thread":"main","time":"2019-01-16T17:28:42.621268Z","file":"trace2/tr2_tgt_event.c","line":163,"t_abs":0.001265,"code":0}</code></pre>
 </div></div>
 </div>
 <div class="sect2">
 <h3 id="_enabling_a_target">Enabling a Target</h3>
 <div class="paragraph"><p>To enable a target, set the corresponding environment variable or
 system or global config value to one of the following:</p></div>
 <div class="openblock">
 <div class="content">
 <div class="ulist"><ul>
 <li>
 <p>
 <code>0</code> or <code>false</code> - Disables the target.
 </p>
 </li>
 <li>
 <p>
 <code>1</code> or <code>true</code> - Writes to <code>STDERR</code>.
 </p>
 </li>
 <li>
 <p>
 <code>[2-9]</code> - Writes to the already opened file descriptor.
 </p>
 </li>
 <li>
 <p>
 <code>&lt;absolute-pathname&gt;</code> - Writes to the file in append mode. If the target
 already exists and is a directory, the traces will be written to files (one
 per process) underneath the given directory.
 </p>
 </li>
 <li>
 <p>
 <code>af_unix:[&lt;socket_type&gt;:]&lt;absolute-pathname&gt;</code> - Write to a
 Unix DomainSocket (on platforms that support them). Socket
 type can be either <code>stream</code> or <code>dgram</code>; if omitted Git will
 try both.
 </p>
 </li>
 </ul></div>
 </div></div>
 <div class="paragraph"><p>When trace files are written to a target directory, they will be named according
 to the last component of the SID (optionally followed by a counter to avoid
 filename collisions).</p></div>
 </div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_trace2_api">Trace2 API</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>The Trace2 public API is defined and documented in <code>trace2.h</code>; refer to it for
 more information. All public functions and macros are prefixed
 with <code>trace2_</code> and are implemented in <code>trace2.c</code>.</p></div>
 <div class="paragraph"><p>There are no public Trace2 data structures.</p></div>
 <div class="paragraph"><p>The Trace2 code also defines a set of private functions and data types
 in the <code>trace2/</code> directory. These symbols are prefixed with <code>tr2_</code>
 and should only be used by functions in <code>trace2.c</code> (or other private
 source files in <code>trace2/</code>).</p></div>
 <div class="sect2">
 <h3 id="_conventions_for_public_functions_and_macros">Conventions for Public Functions and Macros</h3>
 <div class="paragraph"><p>Some functions have a <code>_fl()</code> suffix to indicate that they take <code>file</code>
 and <code>line-number</code> arguments.</p></div>
 <div class="paragraph"><p>Some functions have a <code>_va_fl()</code> suffix to indicate that they also
 take a <code>va_list</code> argument.</p></div>
 <div class="paragraph"><p>Some functions have a <code>_printf_fl()</code> suffix to indicate that they also
 take a <code>printf()</code> style format with a variable number of arguments.</p></div>
 <div class="paragraph"><p>CPP wrapper macros are defined to hide most of these details.</p></div>
 </div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_trace2_target_formats">Trace2 Target Formats</h2>
 <div class="sectionbody">
 <div class="sect2">
 <h3 id="_normal_format">NORMAL Format</h3>
 <div class="paragraph"><p>Events are written as lines of the form:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>[&lt;time&gt; SP &lt;filename&gt;:&lt;line&gt; SP+] &lt;event-name&gt; [[SP] &lt;event-message&gt;] LF</code></pre>
 </div></div>
 <div class="dlist"><dl>
 <dt class="hdlist1">
 <code>&lt;event-name&gt;</code>
 </dt>
 <dd>
 <p>
  is the event name.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>&lt;event-message&gt;</code>
 </dt>
 <dd>
 <p>
  is a free-form <code>printf()</code> message intended for human consumption.
 </p>
 <div class="paragraph"><p>Note that this may contain embedded LF or CRLF characters that are
 not escaped, so the event may spill across multiple lines.</p></div>
 </dd>
 </dl></div>
 <div class="paragraph"><p>If <code>GIT_TRACE2_BRIEF</code> or <code>trace2.normalBrief</code> is true, the <code>time</code>, <code>filename</code>,
 and <code>line</code> fields are omitted.</p></div>
 <div class="paragraph"><p>This target is intended to be more of a summary (like GIT_TRACE) and
 less detailed than the other targets. It ignores thread, region, and
 data messages, for example.</p></div>
 </div>
 <div class="sect2">
 <h3 id="_perf_format">PERF Format</h3>
 <div class="paragraph"><p>Events are written as lines of the form:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>[&lt;time&gt; SP &lt;filename&gt;:&lt;line&gt; SP+
  BAR SP] d&lt;depth&gt; SP
  BAR SP &lt;thread-name&gt; SP+
  BAR SP &lt;event-name&gt; SP+
  BAR SP [r&lt;repo-id&gt;] SP+
  BAR SP [&lt;t_abs&gt;] SP+
  BAR SP [&lt;t_rel&gt;] SP+
  BAR SP [&lt;category&gt;] SP+
  BAR SP DOTS* &lt;perf-event-message&gt;
  LF</code></pre>
 </div></div>
 <div class="dlist"><dl>
 <dt class="hdlist1">
 <code>&lt;depth&gt;</code>
 </dt>
 <dd>
 <p>
  is the git process depth. This is the number of parent
  git processes. A top-level git command has depth value "d0".
  A child of it has depth value "d1". A second level child
  has depth value "d2" and so on.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>&lt;thread-name&gt;</code>
 </dt>
 <dd>
 <p>
  is a unique name for the thread. The primary thread
  is called "main". Other thread names are of the form "th%d:%s"
  and include a unique number and the name of the thread-proc.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>&lt;event-name&gt;</code>
 </dt>
 <dd>
 <p>
  is the event name.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>&lt;repo-id&gt;</code>
 </dt>
 <dd>
 <p>
  when present, is a number indicating the repository
  in use. A <code>def_repo</code> event is emitted when a repository is
  opened. This defines the repo-id and associated worktree.
  Subsequent repo-specific events will reference this repo-id.
 </p>
 <div class="paragraph"><p>Currently, this is always "r1" for the main repository.
 This field is in anticipation of in-proc submodules in the future.</p></div>
 </dd>
 <dt class="hdlist1">
 <code>&lt;t_abs&gt;</code>
 </dt>
 <dd>
 <p>
  when present, is the absolute time in seconds since the
  program started.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>&lt;t_rel&gt;</code>
 </dt>
 <dd>
 <p>
  when present, is time in seconds relative to the start of
  the current region. For a thread-exit event, it is the elapsed
  time of the thread.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>&lt;category&gt;</code>
 </dt>
 <dd>
 <p>
  is present on region and data events and is used to
  indicate a broad category, such as "index" or "status".
 </p>
 </dd>
 <dt class="hdlist1">
 <code>&lt;perf-event-message&gt;</code>
 </dt>
 <dd>
 <p>
  is a free-form <code>printf()</code> message intended for human consumption.
 </p>
 </dd>
 </dl></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>15:33:33.532712 wt-status.c:2310 | d0 | main | region_enter | r1 | 0.126064 | | status | label:print
 15:33:33.532712 wt-status.c:2331 | d0 | main | region_leave | r1 | 0.127568 | 0.001504 | status | label:print</code></pre>
 </div></div>
 <div class="paragraph"><p>If <code>GIT_TRACE2_PERF_BRIEF</code> or <code>trace2.perfBrief</code> is true, the <code>time</code>, <code>file</code>,
 and <code>line</code> fields are omitted.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>d0 | main | region_leave | r1 | 0.011717 | 0.009122 | index | label:preload</code></pre>
 </div></div>
 <div class="paragraph"><p>The PERF target is intended for interactive performance analysis
 during development and is quite noisy.</p></div>
 </div>
 <div class="sect2">
 <h3 id="_event_format">EVENT Format</h3>
 <div class="paragraph"><p>Each event is a JSON-object containing multiple key/value pairs
 written as a single line and followed by a LF.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>'{' &lt;key&gt; ':' &lt;value&gt; [',' &lt;key&gt; ':' &lt;value&gt;]* '}' LF</code></pre>
 </div></div>
 <div class="paragraph"><p>Some key/value pairs are common to all events and some are
 event-specific.</p></div>
 <div class="sect3">
 <h4 id="_common_key_value_pairs">Common Key/Value Pairs</h4>
 <div class="paragraph"><p>The following key/value pairs are common to all events:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"version",
  "sid":"20190408T191827.272759Z-H9b68c35f-P00003510",
  "thread":"main",
  "time":"2019-04-08T19:18:27.282761Z",
  "file":"common-main.c",
  "line":42,
  ...
 }</code></pre>
 </div></div>
 <div class="dlist"><dl>
 <dt class="hdlist1">
 <code>"event":&lt;event&gt;</code>
 </dt>
 <dd>
 <p>
  is the event name.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>"sid":&lt;sid&gt;</code>
 </dt>
 <dd>
 <p>
  is the session-id. This is a unique string to identify the
  process instance to allow all events emitted by a process to
  be identified. A session-id is used instead of a PID because
  PIDs are recycled by the OS. For child git processes, the
  session-id is prepended with the session-id of the parent git
  process to allow parent-child relationships to be identified
  during post-processing.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>"thread":&lt;thread&gt;</code>
 </dt>
 <dd>
 <p>
  is the thread name.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>"time":&lt;time&gt;</code>
 </dt>
 <dd>
 <p>
  is the UTC time of the event.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>"file":&lt;filename&gt;</code>
 </dt>
 <dd>
 <p>
  is source file generating the event.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>"line":&lt;line-number&gt;</code>
 </dt>
 <dd>
 <p>
  is the integer source line number generating the event.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>"repo":&lt;repo-id&gt;</code>
 </dt>
 <dd>
 <p>
  when present, is the integer repo-id as described previously.
 </p>
 </dd>
 </dl></div>
 <div class="paragraph"><p>If <code>GIT_TRACE2_EVENT_BRIEF</code> or <code>trace2.eventBrief</code> is true, the <code>file</code>
 and <code>line</code> fields are omitted from all events and the <code>time</code> field is
 only present on the "start" and "atexit" events.</p></div>
 </div>
 <div class="sect3">
 <h4 id="_event_specific_key_value_pairs">Event-Specific Key/Value Pairs</h4>
 <div class="dlist"><dl>
 <dt class="hdlist1">
 <code>"version"</code>
 </dt>
 <dd>
 <p>
  This event gives the version of the executable and the EVENT format. It
  should always be the first event in a trace session. The EVENT format
  version will be incremented if new event types are added, if existing
  fields are removed, or if there are significant changes in
  interpretation of existing events or fields. Smaller changes, such as
  adding a new field to an existing event, will not require an increment
  to the EVENT format version.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"version",
  ...
  "evt":"3", # EVENT format version
  "exe":"2.20.1.155.g426c96fcdb" # git version
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"too_many_files"</code>
 </dt>
 <dd>
 <p>
  This event is written to the git-trace2-discard sentinel file if there
  are too many files in the target trace directory (see the
  trace2.maxFiles config option).
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"too_many_files",
  ...
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"start"</code>
 </dt>
 <dd>
 <p>
  This event contains the complete argv received by main().
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"start",
  ...
  "t_abs":0.001227, # elapsed time in seconds
  "argv":["git","version"]
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"exit"</code>
 </dt>
 <dd>
 <p>
  This event is emitted when git calls <code>exit()</code>.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"exit",
  ...
  "t_abs":0.001227, # elapsed time in seconds
  "code":0 # exit code
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"atexit"</code>
 </dt>
 <dd>
 <p>
  This event is emitted by the Trace2 <code>atexit</code> routine during
  final shutdown. It should be the last event emitted by the
  process.
 </p>
 <div class="paragraph"><p>(The elapsed time reported here is greater than the time reported in
 the "exit" event because it runs after all other atexit tasks have
 completed.)</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"atexit",
  ...
  "t_abs":0.001227, # elapsed time in seconds
  "code":0 # exit code
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"signal"</code>
 </dt>
 <dd>
 <p>
  This event is emitted when the program is terminated by a user
  signal. Depending on the platform, the signal event may
  prevent the "atexit" event from being generated.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"signal",
  ...
  "t_abs":0.001227, # elapsed time in seconds
  "signo":13 # SIGTERM, SIGINT, etc.
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"error"</code>
 </dt>
 <dd>
 <p>
  This event is emitted when one of the <code>BUG()</code>, <code>bug()</code>, <code>error()</code>,
  <code>die()</code>, <code>warning()</code>, or <code>usage()</code> functions are called.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"error",
  ...
  "msg":"invalid option: --cahced", # formatted error message
  "fmt":"invalid option: %s" # error format string
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>The error event may be emitted more than once. The format string
 allows post-processors to group errors by type without worrying
 about specific error arguments.</p></div>
 </dd>
 <dt class="hdlist1">
 <code>"cmd_path"</code>
 </dt>
 <dd>
 <p>
  This event contains the discovered full path of the git
  executable (on platforms that are configured to resolve it).
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"cmd_path",
  ...
  "path":"C:/work/gfw/git.exe"
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"cmd_ancestry"</code>
 </dt>
 <dd>
 <p>
  This event contains the text command name for the parent (and earlier
  generations of parents) of the current process, in an array ordered from
  nearest parent to furthest great-grandparent. It may not be implemented
  on all platforms.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"cmd_ancestry",
  ...
  "ancestry":["bash","tmux: server","systemd"]
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"cmd_name"</code>
 </dt>
 <dd>
 <p>
  This event contains the command name for this git process
  and the hierarchy of commands from parent git processes.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"cmd_name",
  ...
  "name":"pack-objects",
  "hierarchy":"push/pack-objects"
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>Normally, the "name" field contains the canonical name of the
 command. When a canonical name is not available, one of
 these special values are used:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>"_query_" # "git --html-path"
 "_run_dashed_" # when "git foo" tries to run "git-foo"
 "_run_shell_alias_" # alias expansion to a shell command
 "_run_git_alias_" # alias expansion to a git command
 "_usage_" # usage error</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"cmd_mode"</code>
 </dt>
 <dd>
 <p>
  This event, when present, describes the command variant. This
  event may be emitted more than once.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"cmd_mode",
  ...
  "name":"branch"
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>The "name" field is an arbitrary string to describe the command mode.
 For example, checkout can checkout a branch or an individual file.
 And these variations typically have different performance
 characteristics that are not comparable.</p></div>
 </dd>
 <dt class="hdlist1">
 <code>"alias"</code>
 </dt>
 <dd>
 <p>
  This event is present when an alias is expanded.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"alias",
  ...
  "alias":"l", # registered alias
  "argv":["log","--graph"] # alias expansion
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"child_start"</code>
 </dt>
 <dd>
 <p>
  This event describes a child process that is about to be
  spawned.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"child_start",
  ...
  "child_id":2,
  "child_class":"?",
  "use_shell":false,
  "argv":["git","rev-list","--objects","--stdin","--not","--all","--quiet"]

  "hook_name":"&lt;hook_name&gt;" # present when child_class is "hook"
  "cd":"&lt;path&gt;" # present when cd is required
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>The "child_id" field can be used to match this child_start with the
 corresponding child_exit event.</p></div>
 <div class="paragraph"><p>The "child_class" field is a rough classification, such as "editor",
 "pager", "transport/*", and "hook". Unclassified children are classified
 with "?".</p></div>
 </dd>
 <dt class="hdlist1">
 <code>"child_exit"</code>
 </dt>
 <dd>
 <p>
  This event is generated after the current process has returned
  from the <code>waitpid()</code> and collected the exit information from the
  child.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"child_exit",
  ...
  "child_id":2,
  "pid":14708, # child PID
  "code":0, # child exit-code
  "t_rel":0.110605 # observed run-time of child process
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>Note that the session-id of the child process is not available to
 the current/spawning process, so the child&#8217;s PID is reported here as
 a hint for post-processing. (But it is only a hint because the child
 process may be a shell script which doesn&#8217;t have a session-id.)</p></div>
 <div class="paragraph"><p>Note that the <code>t_rel</code> field contains the observed run time in seconds
 for the child process (starting before the fork/exec/spawn and
 stopping after the <code>waitpid()</code> and includes OS process creation overhead).
 So this time will be slightly larger than the atexit time reported by
 the child process itself.</p></div>
 </dd>
 <dt class="hdlist1">
 <code>"child_ready"</code>
 </dt>
 <dd>
 <p>
  This event is generated after the current process has started
  a background process and released all handles to it.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"child_ready",
  ...
  "child_id":2,
  "pid":14708, # child PID
  "ready":"ready", # child ready state
  "t_rel":0.110605 # observed run-time of child process
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>Note that the session-id of the child process is not available to
 the current/spawning process, so the child&#8217;s PID is reported here as
 a hint for post-processing. (But it is only a hint because the child
 process may be a shell script which doesn&#8217;t have a session-id.)</p></div>
 <div class="paragraph"><p>This event is generated after the child is started in the background
 and given a little time to boot up and start working. If the child
 starts up normally while the parent is still waiting, the "ready"
 field will have the value "ready".
 If the child is too slow to start and the parent times out, the field
 will have the value "timeout".
 If the child starts but the parent is unable to probe it, the field
 will have the value "error".</p></div>
 <div class="paragraph"><p>After the parent process emits this event, it will release all of its
 handles to the child process and treat the child as a background
 daemon. So even if the child does eventually finish booting up,
 the parent will not emit an updated event.</p></div>
 <div class="paragraph"><p>Note that the <code>t_rel</code> field contains the observed run time in seconds
 when the parent released the child process into the background.
 The child is assumed to be a long-running daemon process and may
 outlive the parent process. So the parent&#8217;s child event times should
 not be compared to the child&#8217;s atexit times.</p></div>
 </dd>
 <dt class="hdlist1">
 <code>"exec"</code>
 </dt>
 <dd>
 <p>
  This event is generated before git attempts to <code>exec()</code>
  another command rather than starting a child process.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"exec",
  ...
  "exec_id":0,
  "exe":"git",
  "argv":["foo", "bar"]
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>The "exec_id" field is a command-unique id and is only useful if the
 <code>exec()</code> fails and a corresponding exec_result event is generated.</p></div>
 </dd>
 <dt class="hdlist1">
 <code>"exec_result"</code>
 </dt>
 <dd>
 <p>
  This event is generated if the <code>exec()</code> fails and control
  returns to the current git command.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"exec_result",
  ...
  "exec_id":0,
  "code":1 # error code (errno) from exec()
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"thread_start"</code>
 </dt>
 <dd>
 <p>
  This event is generated when a thread is started. It is
  generated from <strong>within</strong> the new thread&#8217;s thread-proc (because
  it needs to access data in the thread&#8217;s thread-local storage).
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"thread_start",
  ...
  "thread":"th02:preload_thread" # thread name
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"thread_exit"</code>
 </dt>
 <dd>
 <p>
  This event is generated when a thread exits. It is generated
  from <strong>within</strong> the thread&#8217;s thread-proc.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"thread_exit",
  ...
  "thread":"th02:preload_thread", # thread name
  "t_rel":0.007328 # thread elapsed time
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"def_param"</code>
 </dt>
 <dd>
 <p>
  This event is generated to log a global parameter, such as a config
  setting, command-line flag, or environment variable.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"def_param",
  ...
  "scope":"global",
  "param":"core.abbrev",
  "value":"7"
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"def_repo"</code>
 </dt>
 <dd>
 <p>
  This event defines a repo-id and associates it with the root
  of the worktree.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"def_repo",
  ...
  "repo":1,
  "worktree":"/Users/jeffhost/work/gfw"
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>As stated earlier, the repo-id is currently always 1, so there will
 only be one def_repo event. Later, if in-proc submodules are
 supported, a def_repo event should be emitted for each submodule
 visited.</p></div>
 </dd>
 <dt class="hdlist1">
 <code>"region_enter"</code>
 </dt>
 <dd>
 <p>
  This event is generated when entering a region.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"region_enter",
  ...
  "repo":1, # optional
  "nesting":1, # current region stack depth
  "category":"index", # optional
  "label":"do_read_index", # optional
  "msg":".git/index" # optional
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>The <code>category</code> field may be used in a future enhancement to
 do category-based filtering.</p></div>
 <div class="paragraph"><p><code>GIT_TRACE2_EVENT_NESTING</code> or <code>trace2.eventNesting</code> can be used to
 filter deeply nested regions and data events. It defaults to "2".</p></div>
 </dd>
 <dt class="hdlist1">
 <code>"region_leave"</code>
 </dt>
 <dd>
 <p>
  This event is generated when leaving a region.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"region_leave",
  ...
  "repo":1, # optional
  "t_rel":0.002876, # time spent in region in seconds
  "nesting":1, # region stack depth
  "category":"index", # optional
  "label":"do_read_index", # optional
  "msg":".git/index" # optional
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"data"</code>
 </dt>
 <dd>
 <p>
  This event is generated to log a thread- and region-local
  key/value pair.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"data",
  ...
  "repo":1, # optional
  "t_abs":0.024107, # absolute elapsed time
  "t_rel":0.001031, # elapsed time in region/thread
  "nesting":2, # region stack depth
  "category":"index",
  "key":"read/cache_nr",
  "value":"3552"
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>The "value" field may be an integer or a string.</p></div>
 </dd>
 <dt class="hdlist1">
 <code>"data-json"</code>
 </dt>
 <dd>
 <p>
  This event is generated to log a pre-formatted JSON string
  containing structured data.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"data_json",
  ...
  "repo":1, # optional
  "t_abs":0.015905,
  "t_rel":0.015905,
  "nesting":1,
  "category":"process",
  "key":"windows/ancestry",
  "value":["bash.exe","bash.exe"]
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"th_timer"</code>
 </dt>
 <dd>
 <p>
  This event logs the amount of time that a stopwatch timer was
  running in the thread. This event is generated when a thread
  exits for timers that requested per-thread events.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"th_timer",
  ...
  "category":"my_category",
  "name":"my_timer",
  "intervals":5, # number of time it was started/stopped
  "t_total":0.052741, # total time in seconds it was running
  "t_min":0.010061, # shortest interval
  "t_max":0.011648 # longest interval
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"timer"</code>
 </dt>
 <dd>
 <p>
  This event logs the amount of time that a stopwatch timer was
  running aggregated across all threads. This event is generated
  when the process exits.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"timer",
  ...
  "category":"my_category",
  "name":"my_timer",
  "intervals":5, # number of time it was started/stopped
  "t_total":0.052741, # total time in seconds it was running
  "t_min":0.010061, # shortest interval
  "t_max":0.011648 # longest interval
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"th_counter"</code>
 </dt>
 <dd>
 <p>
  This event logs the value of a counter variable in a thread.
  This event is generated when a thread exits for counters that
  requested per-thread events.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"th_counter",
  ...
  "category":"my_category",
  "name":"my_counter",
  "count":23
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 <code>"counter"</code>
 </dt>
 <dd>
 <p>
  This event logs the value of a counter variable across all threads.
  This event is generated when the process exits. The total value
  reported here is the sum across all threads.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>{
  "event":"counter",
  ...
  "category":"my_category",
  "name":"my_counter",
  "count":23
 }</code></pre>
 </div></div>
 </dd>
 </dl></div>
 </div>
 </div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_example_trace2_api_usage">Example Trace2 API Usage</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>Here is a hypothetical usage of the Trace2 API showing the intended
 usage (without worrying about the actual Git details).</p></div>
 <div class="dlist"><dl>
 <dt class="hdlist1">
 Initialization
 </dt>
 <dd>
 <p>
  Initialization happens in <code>main()</code>. Behind the scenes, an
  <code>atexit</code> and <code>signal</code> handler are registered.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>int main(int argc, const char **argv)
 {
  int exit_code;

  trace2_initialize();
  trace2_cmd_start(argv);

  exit_code = cmd_main(argc, argv);

  trace2_cmd_exit(exit_code);

  return exit_code;
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 Command Details
 </dt>
 <dd>
 <p>
  After the basics are established, additional command
  information can be sent to Trace2 as it is discovered.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>int cmd_checkout(int argc, const char **argv)
 {
  trace2_cmd_name("checkout");
  trace2_cmd_mode("branch");
  trace2_def_repo(the_repository);

  // emit "def_param" messages for "interesting" config settings.
  trace2_cmd_list_config();

  if (do_something())
  trace2_cmd_error("Path '%s': cannot do something", path);

  return 0;
 }</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 Child Processes
 </dt>
 <dd>
 <p>
  Wrap code spawning child processes.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>void run_child(...)
 {
  int child_exit_code;
  struct child_process cmd = CHILD_PROCESS_INIT;
  ...
  cmd.trace2_child_class = "editor";

  trace2_child_start(&amp;cmd);
  child_exit_code = spawn_child_and_wait_for_it();
  trace2_child_exit(&amp;cmd, child_exit_code);
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>For example, the following fetch command spawned ssh, index-pack,
 rev-list, and gc. This example also shows that fetch took
 5.199 seconds and of that 4.932 was in ssh.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ export GIT_TRACE2_BRIEF=1
 $ export GIT_TRACE2=~/log.normal
 $ git fetch origin
 ...</code></pre>
 </div></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ cat ~/log.normal
 version 2.20.1.vfs.1.1.47.g534dbe1ad1
 start git fetch origin
 worktree /Users/jeffhost/work/gfw
 cmd_name fetch (fetch)
 child_start[0] ssh git@github.com ...
 child_start[1] git index-pack ...
 ... (Trace2 events from child processes omitted)
 child_exit[1] pid:14707 code:0 elapsed:0.076353
 child_exit[0] pid:14706 code:0 elapsed:4.931869
 child_start[2] git rev-list ...
 ... (Trace2 events from child process omitted)
 child_exit[2] pid:14708 code:0 elapsed:0.110605
 child_start[3] git gc --auto
 ... (Trace2 events from child process omitted)
 child_exit[3] pid:14709 code:0 elapsed:0.006240
 exit elapsed:5.198503 code:0
 atexit elapsed:5.198541 code:0</code></pre>
 </div></div>
 <div class="paragraph"><p>When a git process is a (direct or indirect) child of another
 git process, it inherits Trace2 context information. This
 allows the child to print the command hierarchy. This example
 shows gc as child[3] of fetch. When the gc process reports
 its name as "gc", it also reports the hierarchy as "fetch/gc".
 (In this example, trace2 messages from the child process is
 indented for clarity.)</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ export GIT_TRACE2_BRIEF=1
 $ export GIT_TRACE2=~/log.normal
 $ git fetch origin
 ...</code></pre>
 </div></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ cat ~/log.normal
 version 2.20.1.160.g5676107ecd.dirty
 start git fetch official
 worktree /Users/jeffhost/work/gfw
 cmd_name fetch (fetch)
 ...
 child_start[3] git gc --auto
  version 2.20.1.160.g5676107ecd.dirty
  start /Users/jeffhost/work/gfw/git gc --auto
  worktree /Users/jeffhost/work/gfw
  cmd_name gc (fetch/gc)
  exit elapsed:0.001959 code:0
  atexit elapsed:0.001997 code:0
 child_exit[3] pid:20303 code:0 elapsed:0.007564
 exit elapsed:3.868938 code:0
 atexit elapsed:3.868970 code:0</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 Regions
 </dt>
 <dd>
 <p>
  Regions can be used to time an interesting section of code.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>void wt_status_collect(struct wt_status *s)
 {
  trace2_region_enter("status", "worktrees", s-&gt;repo);
  wt_status_collect_changes_worktree(s);
  trace2_region_leave("status", "worktrees", s-&gt;repo);

  trace2_region_enter("status", "index", s-&gt;repo);
  wt_status_collect_changes_index(s);
  trace2_region_leave("status", "index", s-&gt;repo);

  trace2_region_enter("status", "untracked", s-&gt;repo);
  wt_status_collect_untracked(s);
  trace2_region_leave("status", "untracked", s-&gt;repo);
 }

 void wt_status_print(struct wt_status *s)
 {
  trace2_region_enter("status", "print", s-&gt;repo);
  switch (s-&gt;status_format) {
  ...
  }
  trace2_region_leave("status", "print", s-&gt;repo);
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>In this example, scanning for untracked files ran from +0.012568 to
 +0.027149 (since the process started) and took 0.014581 seconds.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ export GIT_TRACE2_PERF_BRIEF=1
 $ export GIT_TRACE2_PERF=~/log.perf
 $ git status
 ...

 $ cat ~/log.perf
 d0 | main | version | | | | | 2.20.1.160.g5676107ecd.dirty
 d0 | main | start | | 0.001173 | | | git status
 d0 | main | def_repo | r1 | | | | worktree:/Users/jeffhost/work/gfw
 d0 | main | cmd_name | | | | | status (status)
 ...
 d0 | main | region_enter | r1 | 0.010988 | | status | label:worktrees
 d0 | main | region_leave | r1 | 0.011236 | 0.000248 | status | label:worktrees
 d0 | main | region_enter | r1 | 0.011260 | | status | label:index
 d0 | main | region_leave | r1 | 0.012542 | 0.001282 | status | label:index
 d0 | main | region_enter | r1 | 0.012568 | | status | label:untracked
 d0 | main | region_leave | r1 | 0.027149 | 0.014581 | status | label:untracked
 d0 | main | region_enter | r1 | 0.027411 | | status | label:print
 d0 | main | region_leave | r1 | 0.028741 | 0.001330 | status | label:print
 d0 | main | exit | | 0.028778 | | | code:0
 d0 | main | atexit | | 0.028809 | | | code:0</code></pre>
 </div></div>
 <div class="paragraph"><p>Regions may be nested. This causes messages to be indented in the
 PERF target, for example.
 Elapsed times are relative to the start of the corresponding nesting
 level as expected. For example, if we add region message to:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>static enum path_treatment read_directory_recursive(struct dir_struct *dir,
  struct index_state *istate, const char *base, int baselen,
  struct untracked_cache_dir *untracked, int check_only,
  int stop_at_first_file, const struct pathspec *pathspec)
 {
  enum path_treatment state, subdir_state, dir_state = path_none;

  trace2_region_enter_printf("dir", "read_recursive", NULL, "%.*s", baselen, base);
  ...
  trace2_region_leave_printf("dir", "read_recursive", NULL, "%.*s", baselen, base);
  return dir_state;
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>We can further investigate the time spent scanning for untracked files.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ export GIT_TRACE2_PERF_BRIEF=1
 $ export GIT_TRACE2_PERF=~/log.perf
 $ git status
 ...
 $ cat ~/log.perf
 d0 | main | version | | | | | 2.20.1.162.gb4ccea44db.dirty
 d0 | main | start | | 0.001173 | | | git status
 d0 | main | def_repo | r1 | | | | worktree:/Users/jeffhost/work/gfw
 d0 | main | cmd_name | | | | | status (status)
 ...
 d0 | main | region_enter | r1 | 0.015047 | | status | label:untracked
 d0 | main | region_enter | | 0.015132 | | dir | ..label:read_recursive
 d0 | main | region_enter | | 0.016341 | | dir | ....label:read_recursive vcs-svn/
 d0 | main | region_leave | | 0.016422 | 0.000081 | dir | ....label:read_recursive vcs-svn/
 d0 | main | region_enter | | 0.016446 | | dir | ....label:read_recursive xdiff/
 d0 | main | region_leave | | 0.016522 | 0.000076 | dir | ....label:read_recursive xdiff/
 d0 | main | region_enter | | 0.016612 | | dir | ....label:read_recursive git-gui/
 d0 | main | region_enter | | 0.016698 | | dir | ......label:read_recursive git-gui/po/
 d0 | main | region_enter | | 0.016810 | | dir | ........label:read_recursive git-gui/po/glossary/
 d0 | main | region_leave | | 0.016863 | 0.000053 | dir | ........label:read_recursive git-gui/po/glossary/
 ...
 d0 | main | region_enter | | 0.031876 | | dir | ....label:read_recursive builtin/
 d0 | main | region_leave | | 0.032270 | 0.000394 | dir | ....label:read_recursive builtin/
 d0 | main | region_leave | | 0.032414 | 0.017282 | dir | ..label:read_recursive
 d0 | main | region_leave | r1 | 0.032454 | 0.017407 | status | label:untracked
 ...
 d0 | main | exit | | 0.034279 | | | code:0
 d0 | main | atexit | | 0.034322 | | | code:0</code></pre>
 </div></div>
 <div class="paragraph"><p>Trace2 regions are similar to the existing trace_performance_enter()
 and trace_performance_leave() routines, but are thread safe and
 maintain per-thread stacks of timers.</p></div>
 </dd>
 <dt class="hdlist1">
 Data Messages
 </dt>
 <dd>
 <p>
  Data messages added to a region.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>int read_index_from(struct index_state *istate, const char *path,
  const char *gitdir)
 {
  trace2_region_enter_printf("index", "do_read_index", the_repository, "%s", path);

  ...

  trace2_data_intmax("index", the_repository, "read/version", istate-&gt;version);
  trace2_data_intmax("index", the_repository, "read/cache_nr", istate-&gt;cache_nr);

  trace2_region_leave_printf("index", "do_read_index", the_repository, "%s", path);
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>This example shows that the index contained 3552 entries.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ export GIT_TRACE2_PERF_BRIEF=1
 $ export GIT_TRACE2_PERF=~/log.perf
 $ git status
 ...
 $ cat ~/log.perf
 d0 | main | version | | | | | 2.20.1.156.gf9916ae094.dirty
 d0 | main | start | | 0.001173 | | | git status
 d0 | main | def_repo | r1 | | | | worktree:/Users/jeffhost/work/gfw
 d0 | main | cmd_name | | | | | status (status)
 d0 | main | region_enter | r1 | 0.001791 | | index | label:do_read_index .git/index
 d0 | main | data | r1 | 0.002494 | 0.000703 | index | ..read/version:2
 d0 | main | data | r1 | 0.002520 | 0.000729 | index | ..read/cache_nr:3552
 d0 | main | region_leave | r1 | 0.002539 | 0.000748 | index | label:do_read_index .git/index
 ...</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 Thread Events
 </dt>
 <dd>
 <p>
  Thread messages added to a thread-proc.
 </p>
 <div class="paragraph"><p>For example, the multi-threaded preload-index code can be
 instrumented with a region around the thread pool and then
 per-thread start and exit events within the thread-proc.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>static void *preload_thread(void *_data)
 {
  // start the per-thread clock and emit a message.
  trace2_thread_start("preload_thread");

  // report which chunk of the array this thread was assigned.
  trace2_data_intmax("index", the_repository, "offset", p-&gt;offset);
  trace2_data_intmax("index", the_repository, "count", nr);

  do {
  ...
  } while (--nr &gt; 0);
  ...

  // report elapsed time taken by this thread.
  trace2_thread_exit();
  return NULL;
 }

 void preload_index(struct index_state *index,
  const struct pathspec *pathspec,
  unsigned int refresh_flags)
 {
  trace2_region_enter("index", "preload", the_repository);

  for (i = 0; i &lt; threads; i++) {
  ... /* create thread */
  }

  for (i = 0; i &lt; threads; i++) {
  ... /* join thread */
  }

  trace2_region_leave("index", "preload", the_repository);
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>In this example preload_index() was executed by the <code>main</code> thread
 and started the <code>preload</code> region. Seven threads, named
 <code>th01:preload_thread</code> through <code>th07:preload_thread</code>, were started.
 Events from each thread are atomically appended to the shared target
 stream as they occur so they may appear in random order with respect
 other threads. Finally, the main thread waits for the threads to
 finish and leaves the region.</p></div>
 <div class="paragraph"><p>Data events are tagged with the active thread name. They are used
 to report the per-thread parameters.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ export GIT_TRACE2_PERF_BRIEF=1
 $ export GIT_TRACE2_PERF=~/log.perf
 $ git status
 ...
 $ cat ~/log.perf
 ...
 d0 | main | region_enter | r1 | 0.002595 | | index | label:preload
 d0 | th01:preload_thread | thread_start | | 0.002699 | | |
 d0 | th02:preload_thread | thread_start | | 0.002721 | | |
 d0 | th01:preload_thread | data | r1 | 0.002736 | 0.000037 | index | offset:0
 d0 | th02:preload_thread | data | r1 | 0.002751 | 0.000030 | index | offset:2032
 d0 | th03:preload_thread | thread_start | | 0.002711 | | |
 d0 | th06:preload_thread | thread_start | | 0.002739 | | |
 d0 | th01:preload_thread | data | r1 | 0.002766 | 0.000067 | index | count:508
 d0 | th06:preload_thread | data | r1 | 0.002856 | 0.000117 | index | offset:2540
 d0 | th03:preload_thread | data | r1 | 0.002824 | 0.000113 | index | offset:1016
 d0 | th04:preload_thread | thread_start | | 0.002710 | | |
 d0 | th02:preload_thread | data | r1 | 0.002779 | 0.000058 | index | count:508
 d0 | th06:preload_thread | data | r1 | 0.002966 | 0.000227 | index | count:508
 d0 | th07:preload_thread | thread_start | | 0.002741 | | |
 d0 | th07:preload_thread | data | r1 | 0.003017 | 0.000276 | index | offset:3048
 d0 | th05:preload_thread | thread_start | | 0.002712 | | |
 d0 | th05:preload_thread | data | r1 | 0.003067 | 0.000355 | index | offset:1524
 d0 | th05:preload_thread | data | r1 | 0.003090 | 0.000378 | index | count:508
 d0 | th07:preload_thread | data | r1 | 0.003037 | 0.000296 | index | count:504
 d0 | th03:preload_thread | data | r1 | 0.002971 | 0.000260 | index | count:508
 d0 | th04:preload_thread | data | r1 | 0.002983 | 0.000273 | index | offset:508
 d0 | th04:preload_thread | data | r1 | 0.007311 | 0.004601 | index | count:508
 d0 | th05:preload_thread | thread_exit | | 0.008781 | 0.006069 | |
 d0 | th01:preload_thread | thread_exit | | 0.009561 | 0.006862 | |
 d0 | th03:preload_thread | thread_exit | | 0.009742 | 0.007031 | |
 d0 | th06:preload_thread | thread_exit | | 0.009820 | 0.007081 | |
 d0 | th02:preload_thread | thread_exit | | 0.010274 | 0.007553 | |
 d0 | th07:preload_thread | thread_exit | | 0.010477 | 0.007736 | |
 d0 | th04:preload_thread | thread_exit | | 0.011657 | 0.008947 | |
 d0 | main | region_leave | r1 | 0.011717 | 0.009122 | index | label:preload
 ...
 d0 | main | exit | | 0.029996 | | | code:0
 d0 | main | atexit | | 0.030027 | | | code:0</code></pre>
 </div></div>
 <div class="paragraph"><p>In this example, the preload region took 0.009122 seconds. The 7 threads
 took between 0.006069 and 0.008947 seconds to work on their portion of
 the index. Thread "th01" worked on 508 items at offset 0. Thread "th02"
 worked on 508 items at offset 2032. Thread "th04" worked on 508 items
 at offset 508.</p></div>
 <div class="paragraph"><p>This example also shows that thread names are assigned in a racy manner
 as each thread starts.</p></div>
 </dd>
 <dt class="hdlist1">
 Config (def param) Events
 </dt>
 <dd>
 <p>
  Dump "interesting" config values to trace2 log.
 </p>
 <div class="paragraph"><p>We can optionally emit configuration events, see
 <code>trace2.configparams</code> in <a href="../git-config.html">git-config(1)</a> for how to enable
 it.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git config --system color.ui never
 $ git config --global color.ui always
 $ git config --local color.ui auto
 $ git config --list --show-scope | grep 'color.ui'
 system color.ui=never
 global color.ui=always
 local color.ui=auto</code></pre>
 </div></div>
 <div class="paragraph"><p>Then, mark the config <code>color.ui</code> as "interesting" config with
 <code>GIT_TRACE2_CONFIG_PARAMS</code>:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ export GIT_TRACE2_PERF_BRIEF=1
 $ export GIT_TRACE2_PERF=~/log.perf
 $ export GIT_TRACE2_CONFIG_PARAMS=color.ui
 $ git version
 ...
 $ cat ~/log.perf
 d0 | main | version | | | | | ...
 d0 | main | start | | 0.001642 | | | /usr/local/bin/git version
 d0 | main | cmd_name | | | | | version (version)
 d0 | main | def_param | | | | scope:system | color.ui:never
 d0 | main | def_param | | | | scope:global | color.ui:always
 d0 | main | def_param | | | | scope:local | color.ui:auto
 d0 | main | data | r0 | 0.002100 | 0.002100 | fsync | fsync/writeout-only:0
 d0 | main | data | r0 | 0.002126 | 0.002126 | fsync | fsync/hardware-flush:0
 d0 | main | exit | | 0.000470 | | | code:0
 d0 | main | atexit | | 0.000477 | | | code:0</code></pre>
 </div></div>
 </dd>
 <dt class="hdlist1">
 Stopwatch Timer Events
 </dt>
 <dd>
 <p>
  Measure the time spent in a function call or span of code
  that might be called from many places within the code
  throughout the life of the process.
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>static void expensive_function(void)
 {
  trace2_timer_start(TRACE2_TIMER_ID_TEST1);
  ...
  sleep_millisec(1000); // Do something expensive
  ...
  trace2_timer_stop(TRACE2_TIMER_ID_TEST1);
 }

 static int ut_100timer(int argc, const char **argv)
 {
  ...

  expensive_function();

  // Do something else 1...

  expensive_function();

  // Do something else 2...

  expensive_function();

  return 0;
 }</code></pre>
 </div></div>
 <div class="paragraph"><p>In this example, we measure the total time spent in
 <code>expensive_function()</code> regardless of when it is called
 in the overall flow of the program.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ export GIT_TRACE2_PERF_BRIEF=1
 $ export GIT_TRACE2_PERF=~/log.perf
 $ t/helper/test-tool trace2 100timer 3 1000
 ...
 $ cat ~/log.perf
 d0 | main | version | | | | | ...
 d0 | main | start | | 0.001453 | | | t/helper/test-tool trace2 100timer 3 1000
 d0 | main | cmd_name | | | | | trace2 (trace2)
 d0 | main | exit | | 3.003667 | | | code:0
 d0 | main | timer | | | | test | name:test1 intervals:3 total:3.001686 min:1.000254 max:1.000929
 d0 | main | atexit | | 3.003796 | | | code:0</code></pre>
 </div></div>
 </dd>
 </dl></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_future_work">Future Work</h2>
 <div class="sectionbody">
 <div class="sect2">
 <h3 id="_relationship_to_the_existing_trace_api_api_trace_txt">Relationship to the Existing Trace Api (api-trace.txt)</h3>
 <div class="paragraph"><p>There are a few issues to resolve before we can completely
 switch to Trace2.</p></div>
 <div class="ulist"><ul>
 <li>
 <p>
 Updating existing tests that assume <code>GIT_TRACE</code> format messages.
 </p>
 </li>
 <li>
 <p>
 How to best handle custom <code>GIT_TRACE_&lt;key&gt;</code> messages?
 </p>
 <div class="ulist"><ul>
 <li>
 <p>
 The <code>GIT_TRACE_&lt;key&gt;</code> mechanism allows each &lt;key&gt; to write to a
 different file (in addition to just stderr).
 </p>
 </li>
 <li>
 <p>
 Do we want to maintain that ability or simply write to the existing
 Trace2 targets (and convert &lt;key&gt; to a "category").
 </p>
 </li>
 </ul></div>
 </li>
 </ul></div>
 </div>
 </div>
 </div>
 </div>
 <div id="footnotes"><hr /></div>
 <div id="footer">
 <div id="footer-text">
 Last updated
  2022-11-04 21:49:36 PDT
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