technical/pack-heuristics.txt

 Concerning Git's Packing Heuristics
 ===================================

 Oh, here's a really stupid question:

 Where do I go
 to learn the details
 of git's packing heuristics?

Be careful what you ask!

Followers of the git, please open the git IRC Log and turn to
February 10, 2006.

It's a rare occasion, and we are joined by the King Git Himself,
Linus Torvalds (linus). Nathaniel Smith, (njs`), has the floor
and seeks enlightenment. Others are present, but silent.

Let's listen in!

 <njs`> Oh, here's a really stupid question -- where do I go to
 learn the details of git's packing heuristics? google avails
 me not, reading the source didn't help a lot, and wading
 through the whole mailing list seems less efficient than any
 of that.

It is a bold start! A plea for help combined with a simultaneous
tri-part attack on some of the tried and true mainstays in the quest
for enlightenment. Brash accusations of google being useless. Hubris!
Maligning the source. Heresy! Disdain for the mailing list archives.
Woe.

 <pasky> yes, the packing-related delta stuff is somewhat
 mysterious even for me ;)

Ah! Modesty after all.

 <linus> njs, I don't think the docs exist. That's something where
 I don't think anybody else than me even really got involved.
 Most of the rest of git others have been busy with (especially
 Junio), but packing nobody touched after I did it.

It's cryptic, yet vague. Linus in style for sure. Wise men
interpret this as an apology. A few argue it is merely a
statement of fact.

 <njs`> I guess the next step is "read the source again", but I
 have to build up a certain level of gumption first :-)

Indeed! On both points.

 <linus> The packing heuristic is actually really really simple.

Bait...

 <linus> But strange.

And switch. That ought to do it!

 <linus> Remember: git really doesn't follow files. So what it does is
 - generate a list of all objects
 - sort the list according to magic heuristics
 - walk the list, using a sliding window, seeing if an object
 can be diffed against another object in the window
 - write out the list in recency order

The traditional understatement:

 <njs`> I suspect that what I'm missing is the precise definition of
 the word "magic"

The traditional insight:

 <pasky> yes

And Babel-like confusion flowed.

 <njs`> oh, hmm, and I'm not sure what this sliding window means either

 <pasky> iirc, it appeared to me to be just the sha1 of the object
 when reading the code casually ...

 ... which simply doesn't sound as a very good heuristics, though ;)

 <njs`> .....and recency order. okay, I think it's clear I didn't
 even realize how much I wasn't realizing :-)

Ah, grasshopper! And thus the enlightenment begins anew.

 <linus> The "magic" is actually in theory totally arbitrary.
 ANY order will give you a working pack, but no, it's not
 ordered by SHA1.

 Before talking about the ordering for the sliding delta
 window, let's talk about the recency order. That's more
 important in one way.

 <njs`> Right, but if all you want is a working way to pack things
 together, you could just use cat and save yourself some
 trouble...

Waaait for it....

 <linus> The recency ordering (which is basically: put objects
 _physically_ into the pack in the order that they are
 "reachable" from the head) is important.

 <njs`> okay

 <linus> It's important because that's the thing that gives packs
 good locality. It keeps the objects close to the head (whether
 they are old or new, but they are _reachable_ from the head)
 at the head of the pack. So packs actually have absolutely
 _wonderful_ IO patterns.

Read that again, because it is important.

 <linus> But recency ordering is totally useless for deciding how
 to actually generate the deltas, so the delta ordering is
 something else.

 The delta ordering is (wait for it):
 - first sort by the "basename" of the object, as defined by
 the name the object was _first_ reached through when
 generating the object list
 - within the same basename, sort by size of the object
 - but always sort different types separately (commits first).

 That's not exactly it, but it's very close.

 <njs`> The "_first_ reached" thing is not too important, just you
 need some way to break ties since the same objects may be
 reachable many ways, yes?

And as if to clarify:

 <linus> The point is that it's all really just any random
 heuristic, and the ordering is totally unimportant for
 correctness, but it helps a lot if the heuristic gives
 "clumping" for things that are likely to delta well against
 each other.

It is an important point, so secretly, I did my own research and have
included my results below. To be fair, it has changed some over time.
And through the magic of Revisionistic History, I draw upon this entry
from The Git IRC Logs on my father's birthday, March 1:

 <gitster> The quote from the above linus should be rewritten a
 bit (wait for it):
 - first sort by type. Different objects never delta with
 each other.
 - then sort by filename/dirname. hash of the basename
 occupies the top BITS_PER_INT-DIR_BITS bits, and bottom
 DIR_BITS are for the hash of leading path elements.
 - then if we are doing "thin" pack, the objects we are _not_
 going to pack but we know about are sorted earlier than
 other objects.
 - and finally sort by size, larger to smaller.

In one swell-foop, clarification and obscurification! Nonetheless,
authoritative. Cryptic, yet concise. It even solicits notions of
quotes from The Source Code. Clearly, more study is needed.

 <gitster> That's the sort order. What this means is:
 - we do not delta different object types.
- we prefer to delta the objects with the same full path, but
 allow files with the same name from different directories.
- we always prefer to delta against objects we are not going
 to send, if there are some.
- we prefer to delta against larger objects, so that we have
 lots of removals.

 The penultimate rule is for "thin" packs. It is used when
 the other side is known to have such objects.

There it is again. "Thin" packs. I'm thinking to myself, "What
is a 'thin' pack?" So I ask:

 <jdl> What is a "thin" pack?

 <gitster> Use of --objects-edge to rev-list as the upstream of
 pack-objects. The pack transfer protocol negotiates that.

Woo hoo! Cleared that _right_ up!

 <gitster> There are two directions - push and fetch.

There! Did you see it? It is not '"push" and "pull"'! How often the
confusion has started here. So casually mentioned, too!

 <gitster> For push, git-send-pack invokes git-receive-pack on the
 other end. The receive-pack says "I have up to these commits".
 send-pack looks at them, and computes what are missing from
 the other end. So "thin" could be the default there.

 In the other direction, fetch, git-fetch-pack and
 git-clone-pack invokes git-upload-pack on the other end
(via ssh or by talking to the daemon).

There are two cases: fetch-pack with -k and clone-pack is one,
 fetch-pack without -k is the other. clone-pack and fetch-pack
 with -k will keep the downloaded packfile without expanded, so
 we do not use thin pack transfer. Otherwise, the generated
 pack will have delta without base object in the same pack.

 But fetch-pack without -k will explode the received pack into
 individual objects, so we automatically ask upload-pack to
 give us a thin pack if upload-pack supports it.

OK then.

Uh.

Let's return to the previous conversation still in progress.

 <njs`> and "basename" means something like "the tail of end of
 path of file objects and dir objects, as per basename(3), and
 we just declare all commit and tag objects to have the same
 basename" or something?

Luckily, that too is a point that gitster clarified for us!

If I might add, the trick is to make files that _might_ be similar be
located close to each other in the hash buckets based on their file
names. It used to be that "foo/Makefile", "bar/baz/quux/Makefile" and
"Makefile" all landed in the same bucket due to their common basename,
"Makefile". However, now they land in "close" buckets.

The algorithm allows not just for the _same_ bucket, but for _close_
buckets to be considered delta candidates. The rationale is
essentially that files, like Makefiles, often have very similar
content no matter what directory they live in.

 <linus> I played around with different delta algorithms, and with
 making the "delta window" bigger, but having too big of a
 sliding window makes it very expensive to generate the pack:
 you need to compare every object with a _ton_ of other objects.

 There are a number of other trivial heuristics too, which
 basically boil down to "don't bother even trying to delta this
 pair" if we can tell before-hand that the delta isn't worth it
 (due to size differences, where we can take a previous delta
 result into account to decide that "ok, no point in trying
 that one, it will be worse").

 End result: packing is actually very size efficient. It's
 somewhat CPU-wasteful, but on the other hand, since you're
 really only supposed to do it maybe once a month (and you can
 do it during the night), nobody really seems to care.

Nice Engineering Touch, there. Find when it doesn't matter, and
proclaim it a non-issue. Good style too!

 <njs`> So, just to repeat to see if I'm following, we start by
 getting a list of the objects we want to pack, we sort it by
 this heuristic (basically lexicographically on the tuple
 (type, basename, size)).

 Then we walk through this list, and calculate a delta of
 each object against the last n (tunable parameter) objects,
 and pick the smallest of these deltas.

Vastly simplified, but the essence is there!

 <linus> Correct.

 <njs`> And then once we have picked a delta or fulltext to
 represent each object, we re-sort by recency, and write them
 out in that order.

 <linus> Yup. Some other small details:

And of course there is the "Other Shoe" Factor too.

 <linus> - We limit the delta depth to another magic value (right
 now both the window and delta depth magic values are just "10")

 <njs`> Hrm, my intuition is that you'd end up with really _bad_ IO
 patterns, because the things you want are near by, but to
 actually reconstruct them you may have to jump all over in
 random ways.

 <linus> - When we write out a delta, and we haven't yet written
 out the object it is a delta against, we write out the base
 object first. And no, when we reconstruct them, we actually
 get nice IO patterns, because:
 - larger objects tend to be "more recent" (Linus' law: files grow)
 - we actively try to generate deltas from a larger object to a
 smaller one
 - this means that the top-of-tree very seldom has deltas
 (i.e. deltas in _practice_ are "backwards deltas")

Again, we should reread that whole paragraph. Not just because
Linus has slipped Linus's Law in there on us, but because it is
important. Let's make sure we clarify some of the points here:

 <njs`> So the point is just that in practice, delta order and
 recency order match each other quite well.

 <linus> Yes. There's another nice side to this (and yes, it was
designed that way ;):
 - the reason we generate deltas against the larger object is
 actually a big space saver too!

 <njs`> Hmm, but your last comment (if "we haven't yet written out
 the object it is a delta against, we write out the base object
 first"), seems like it would make these facts mostly
 irrelevant because even if in practice you would not have to
 wander around much, in fact you just brute-force say that in
 the cases where you might have to wander, don't do that :-)

 <linus> Yes and no. Notice the rule: we only write out the base
 object first if the delta against it was more recent. That
 means that you can actually have deltas that refer to a base
 object that is _not_ close to the delta object, but that only
 happens when the delta is needed to generate an _old_ object.

 <linus> See?

Yeah, no. I missed that on the first two or three readings myself.

 <linus> This keeps the front of the pack dense. The front of the
 pack never contains data that isn't relevant to a "recent"
 object. The size optimization comes from our use of xdelta
 (but is true for many other delta algorithms): removing data
 is cheaper (in size) than adding data.

 When you remove data, you only need to say "copy bytes n--m".
In contrast, in a delta that _adds_ data, you have to say "add
 these bytes: 'actual data goes here'"

 *** njs` has quit: Read error: 104 (Connection reset by peer)

 <linus> Uhhuh. I hope I didn't blow njs` mind.

 *** njs` has joined channel #git

 <pasky> :)

The silent observers are amused. Of course.

And as if njs` was expected to be omniscient:

 <linus> njs - did you miss anything?

OK, I'll spell it out. That's Geek Humor. If njs` was not actually
connected for a little bit there, how would he know if missed anything
while he was disconnected? He's a benevolent dictator with a sense of
humor! Well noted!

 <njs`> Stupid router. Or gremlins, or whatever.

It's a cheap shot at Cisco. Take 'em when you can.

 <njs`> Yes and no. Notice the rule: we only write out the base
 object first if the delta against it was more recent.

 I'm getting lost in all these orders, let me re-read :-)
So the write-out order is from most recent to least recent?
 (Conceivably it could be the opposite way too, I'm not sure if
 we've said) though my connection back at home is logging, so I
 can just read what you said there :-)

And for those of you paying attention, the Omniscient Trick has just
been detailed!

 <linus> Yes, we always write out most recent first

For the other record:

 <pasky> njs`: http://pastebin.com/547965

The 'net never forgets, so that should be good until the end of time.

 <njs`> And, yeah, I got the part about deeper-in-history stuff
 having worse IO characteristics, one sort of doesn't care.

 <linus> With the caveat that if the "most recent" needs an older
 object to delta against (hey, shrinking sometimes does
 happen), we write out the old object with the delta.

 <njs`> (if only it happened more...)

 <linus> Anyway, the pack-file could easily be denser still, but
 because it's used both for streaming (the git protocol) and
 for on-disk, it has a few pessimizations.

Actually, it is a made-up word. But it is a made-up word being
used as setup for a later optimization, which is a real word:

 <linus> In particular, while the pack-file is then compressed,
 it's compressed just one object at a time, so the actual
 compression factor is less than it could be in theory. But it
 means that it's all nice random-access with a simple index to
 do "object name->location in packfile" translation.

 <njs`> I'm assuming the real win for delta-ing large->small is
 more homogeneous statistics for gzip to run over?

 (You have to put the bytes in one place or another, but
 putting them in a larger blob wins on compression)

 Actually, what is the compression strategy -- each delta
 individually gzipped, the whole file gzipped, somewhere in
 between, no compression at all, ....?

 Right.

Reality IRC sets in. For example:

 <pasky> I'll read the rest in the morning, I really have to go
 sleep or there's no hope whatsoever for me at the today's
 exam... g'nite all.

Heh.

 <linus> pasky: g'nite

 <njs`> pasky: 'luck

 <linus> Right: large->small matters exactly because of compression
 behaviour. If it was non-compressed, it probably wouldn't make
 any difference.

 <njs`> yeah

 <linus> Anyway: I'm not even trying to claim that the pack-files
 are perfect, but they do tend to have a nice balance of
 density vs ease-of use.

Gasp! OK, saved. That's a fair Engineering trade off. Close call!
In fact, Linus reflects on some Basic Engineering Fundamentals,
design options, etc.

 <linus> More importantly, they allow git to still _conceptually_
 never deal with deltas at all, and be a "whole object" store.

 Which has some problems (we discussed bad huge-file
 behaviour on the git lists the other day), but it does mean
 that the basic git concepts are really really simple and
 straightforward.

 It's all been quite stable.

 Which I think is very much a result of having very simple
 basic ideas, so that there's never any confusion about what's
 going on.

 Bugs happen, but they are "simple" bugs. And bugs that
 actually get some object store detail wrong are almost always
 so obvious that they never go anywhere.

 <njs`> Yeah.

Nuff said.

 <linus> Anyway. I'm off for bed. It's not 6AM here, but I've got
 three kids, and have to get up early in the morning to send
 them off. I need my beauty sleep.

 <njs`> :-)

 <njs`> appreciate the infodump, I really was failing to find the
 details on git packs :-)

And now you know the rest of the story.