| Junio C Hamano | 4078a55 | 2021-04-30 06:08:10 | [diff] [blame] | 1 | <?xml version="1.0" encoding="UTF-8"?> |
| 2 | <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" |
| 3 | "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> |
| 4 | <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"> |
| 5 | <head> |
| 6 | <meta http-equiv="Content-Type" content="application/xhtml+xml; charset=UTF-8" /> |
| Junio C Hamano | a85030a | 2022-07-27 16:48:21 | [diff] [blame] | 7 | <meta name="generator" content="AsciiDoc 10.2.0" /> |
| Junio C Hamano | 4078a55 | 2021-04-30 06:08:10 | [diff] [blame] | 8 | <title>Parallel Checkout Design Notes</title> |
| 9 | <style type="text/css"> |
| 10 | /* Shared CSS for AsciiDoc xhtml11 and html5 backends */ |
| 11 | |
| 12 | /* Default font. */ |
| 13 | body { |
| 14 | font-family: Georgia,serif; |
| 15 | } |
| 16 | |
| 17 | /* Title font. */ |
| 18 | h1, h2, h3, h4, h5, h6, |
| 19 | div.title, caption.title, |
| 20 | thead, p.table.header, |
| 21 | #toctitle, |
| 22 | #author, #revnumber, #revdate, #revremark, |
| 23 | #footer { |
| 24 | font-family: Arial,Helvetica,sans-serif; |
| 25 | } |
| 26 | |
| 27 | body { |
| 28 | margin: 1em 5% 1em 5%; |
| 29 | } |
| 30 | |
| 31 | a { |
| 32 | color: blue; |
| 33 | text-decoration: underline; |
| 34 | } |
| 35 | a:visited { |
| 36 | color: fuchsia; |
| 37 | } |
| 38 | |
| 39 | em { |
| 40 | font-style: italic; |
| 41 | color: navy; |
| 42 | } |
| 43 | |
| 44 | strong { |
| 45 | font-weight: bold; |
| 46 | color: #083194; |
| 47 | } |
| 48 | |
| 49 | h1, h2, h3, h4, h5, h6 { |
| 50 | color: #527bbd; |
| 51 | margin-top: 1.2em; |
| 52 | margin-bottom: 0.5em; |
| 53 | line-height: 1.3; |
| 54 | } |
| 55 | |
| 56 | h1, h2, h3 { |
| 57 | border-bottom: 2px solid silver; |
| 58 | } |
| 59 | h2 { |
| 60 | padding-top: 0.5em; |
| 61 | } |
| 62 | h3 { |
| 63 | float: left; |
| 64 | } |
| 65 | h3 + * { |
| 66 | clear: left; |
| 67 | } |
| 68 | h5 { |
| 69 | font-size: 1.0em; |
| 70 | } |
| 71 | |
| 72 | div.sectionbody { |
| 73 | margin-left: 0; |
| 74 | } |
| 75 | |
| 76 | hr { |
| 77 | border: 1px solid silver; |
| 78 | } |
| 79 | |
| 80 | p { |
| 81 | margin-top: 0.5em; |
| 82 | margin-bottom: 0.5em; |
| 83 | } |
| 84 | |
| 85 | ul, ol, li > p { |
| 86 | margin-top: 0; |
| 87 | } |
| 88 | ul > li { color: #aaa; } |
| 89 | ul > li > * { color: black; } |
| 90 | |
| 91 | .monospaced, code, pre { |
| 92 | font-family: "Courier New", Courier, monospace; |
| 93 | font-size: inherit; |
| 94 | color: navy; |
| 95 | padding: 0; |
| 96 | margin: 0; |
| 97 | } |
| 98 | pre { |
| 99 | white-space: pre-wrap; |
| 100 | } |
| 101 | |
| 102 | #author { |
| 103 | color: #527bbd; |
| 104 | font-weight: bold; |
| 105 | font-size: 1.1em; |
| 106 | } |
| 107 | #email { |
| 108 | } |
| 109 | #revnumber, #revdate, #revremark { |
| 110 | } |
| 111 | |
| 112 | #footer { |
| 113 | font-size: small; |
| 114 | border-top: 2px solid silver; |
| 115 | padding-top: 0.5em; |
| 116 | margin-top: 4.0em; |
| 117 | } |
| 118 | #footer-text { |
| 119 | float: left; |
| 120 | padding-bottom: 0.5em; |
| 121 | } |
| 122 | #footer-badges { |
| 123 | float: right; |
| 124 | padding-bottom: 0.5em; |
| 125 | } |
| 126 | |
| 127 | #preamble { |
| 128 | margin-top: 1.5em; |
| 129 | margin-bottom: 1.5em; |
| 130 | } |
| 131 | div.imageblock, div.exampleblock, div.verseblock, |
| 132 | div.quoteblock, div.literalblock, div.listingblock, div.sidebarblock, |
| 133 | div.admonitionblock { |
| 134 | margin-top: 1.0em; |
| 135 | margin-bottom: 1.5em; |
| 136 | } |
| 137 | div.admonitionblock { |
| 138 | margin-top: 2.0em; |
| 139 | margin-bottom: 2.0em; |
| 140 | margin-right: 10%; |
| 141 | color: #606060; |
| 142 | } |
| 143 | |
| 144 | div.content { /* Block element content. */ |
| 145 | padding: 0; |
| 146 | } |
| 147 | |
| 148 | /* Block element titles. */ |
| 149 | div.title, caption.title { |
| 150 | color: #527bbd; |
| 151 | font-weight: bold; |
| 152 | text-align: left; |
| 153 | margin-top: 1.0em; |
| 154 | margin-bottom: 0.5em; |
| 155 | } |
| 156 | div.title + * { |
| 157 | margin-top: 0; |
| 158 | } |
| 159 | |
| 160 | td div.title:first-child { |
| 161 | margin-top: 0.0em; |
| 162 | } |
| 163 | div.content div.title:first-child { |
| 164 | margin-top: 0.0em; |
| 165 | } |
| 166 | div.content + div.title { |
| 167 | margin-top: 0.0em; |
| 168 | } |
| 169 | |
| 170 | div.sidebarblock > div.content { |
| 171 | background: #ffffee; |
| 172 | border: 1px solid #dddddd; |
| 173 | border-left: 4px solid #f0f0f0; |
| 174 | padding: 0.5em; |
| 175 | } |
| 176 | |
| 177 | div.listingblock > div.content { |
| 178 | border: 1px solid #dddddd; |
| 179 | border-left: 5px solid #f0f0f0; |
| 180 | background: #f8f8f8; |
| 181 | padding: 0.5em; |
| 182 | } |
| 183 | |
| 184 | div.quoteblock, div.verseblock { |
| 185 | padding-left: 1.0em; |
| 186 | margin-left: 1.0em; |
| 187 | margin-right: 10%; |
| 188 | border-left: 5px solid #f0f0f0; |
| 189 | color: #888; |
| 190 | } |
| 191 | |
| 192 | div.quoteblock > div.attribution { |
| 193 | padding-top: 0.5em; |
| 194 | text-align: right; |
| 195 | } |
| 196 | |
| 197 | div.verseblock > pre.content { |
| 198 | font-family: inherit; |
| 199 | font-size: inherit; |
| 200 | } |
| 201 | div.verseblock > div.attribution { |
| 202 | padding-top: 0.75em; |
| 203 | text-align: left; |
| 204 | } |
| 205 | /* DEPRECATED: Pre version 8.2.7 verse style literal block. */ |
| 206 | div.verseblock + div.attribution { |
| 207 | text-align: left; |
| 208 | } |
| 209 | |
| 210 | div.admonitionblock .icon { |
| 211 | vertical-align: top; |
| 212 | font-size: 1.1em; |
| 213 | font-weight: bold; |
| 214 | text-decoration: underline; |
| 215 | color: #527bbd; |
| 216 | padding-right: 0.5em; |
| 217 | } |
| 218 | div.admonitionblock td.content { |
| 219 | padding-left: 0.5em; |
| 220 | border-left: 3px solid #dddddd; |
| 221 | } |
| 222 | |
| 223 | div.exampleblock > div.content { |
| 224 | border-left: 3px solid #dddddd; |
| 225 | padding-left: 0.5em; |
| 226 | } |
| 227 | |
| 228 | div.imageblock div.content { padding-left: 0; } |
| 229 | span.image img { border-style: none; vertical-align: text-bottom; } |
| 230 | a.image:visited { color: white; } |
| 231 | |
| 232 | dl { |
| 233 | margin-top: 0.8em; |
| 234 | margin-bottom: 0.8em; |
| 235 | } |
| 236 | dt { |
| 237 | margin-top: 0.5em; |
| 238 | margin-bottom: 0; |
| 239 | font-style: normal; |
| 240 | color: navy; |
| 241 | } |
| 242 | dd > *:first-child { |
| 243 | margin-top: 0.1em; |
| 244 | } |
| 245 | |
| 246 | ul, ol { |
| 247 | list-style-position: outside; |
| 248 | } |
| 249 | ol.arabic { |
| 250 | list-style-type: decimal; |
| 251 | } |
| 252 | ol.loweralpha { |
| 253 | list-style-type: lower-alpha; |
| 254 | } |
| 255 | ol.upperalpha { |
| 256 | list-style-type: upper-alpha; |
| 257 | } |
| 258 | ol.lowerroman { |
| 259 | list-style-type: lower-roman; |
| 260 | } |
| 261 | ol.upperroman { |
| 262 | list-style-type: upper-roman; |
| 263 | } |
| 264 | |
| 265 | div.compact ul, div.compact ol, |
| 266 | div.compact p, div.compact p, |
| 267 | div.compact div, div.compact div { |
| 268 | margin-top: 0.1em; |
| 269 | margin-bottom: 0.1em; |
| 270 | } |
| 271 | |
| 272 | tfoot { |
| 273 | font-weight: bold; |
| 274 | } |
| 275 | td > div.verse { |
| 276 | white-space: pre; |
| 277 | } |
| 278 | |
| 279 | div.hdlist { |
| 280 | margin-top: 0.8em; |
| 281 | margin-bottom: 0.8em; |
| 282 | } |
| 283 | div.hdlist tr { |
| 284 | padding-bottom: 15px; |
| 285 | } |
| 286 | dt.hdlist1.strong, td.hdlist1.strong { |
| 287 | font-weight: bold; |
| 288 | } |
| 289 | td.hdlist1 { |
| 290 | vertical-align: top; |
| 291 | font-style: normal; |
| 292 | padding-right: 0.8em; |
| 293 | color: navy; |
| 294 | } |
| 295 | td.hdlist2 { |
| 296 | vertical-align: top; |
| 297 | } |
| 298 | div.hdlist.compact tr { |
| 299 | margin: 0; |
| 300 | padding-bottom: 0; |
| 301 | } |
| 302 | |
| 303 | .comment { |
| 304 | background: yellow; |
| 305 | } |
| 306 | |
| 307 | .footnote, .footnoteref { |
| 308 | font-size: 0.8em; |
| 309 | } |
| 310 | |
| 311 | span.footnote, span.footnoteref { |
| 312 | vertical-align: super; |
| 313 | } |
| 314 | |
| 315 | #footnotes { |
| 316 | margin: 20px 0 20px 0; |
| 317 | padding: 7px 0 0 0; |
| 318 | } |
| 319 | |
| 320 | #footnotes div.footnote { |
| 321 | margin: 0 0 5px 0; |
| 322 | } |
| 323 | |
| 324 | #footnotes hr { |
| 325 | border: none; |
| 326 | border-top: 1px solid silver; |
| 327 | height: 1px; |
| 328 | text-align: left; |
| 329 | margin-left: 0; |
| 330 | width: 20%; |
| 331 | min-width: 100px; |
| 332 | } |
| 333 | |
| 334 | div.colist td { |
| 335 | padding-right: 0.5em; |
| 336 | padding-bottom: 0.3em; |
| 337 | vertical-align: top; |
| 338 | } |
| 339 | div.colist td img { |
| 340 | margin-top: 0.3em; |
| 341 | } |
| 342 | |
| 343 | @media print { |
| 344 | #footer-badges { display: none; } |
| 345 | } |
| 346 | |
| 347 | #toc { |
| 348 | margin-bottom: 2.5em; |
| 349 | } |
| 350 | |
| 351 | #toctitle { |
| 352 | color: #527bbd; |
| 353 | font-size: 1.1em; |
| 354 | font-weight: bold; |
| 355 | margin-top: 1.0em; |
| 356 | margin-bottom: 0.1em; |
| 357 | } |
| 358 | |
| 359 | div.toclevel0, div.toclevel1, div.toclevel2, div.toclevel3, div.toclevel4 { |
| 360 | margin-top: 0; |
| 361 | margin-bottom: 0; |
| 362 | } |
| 363 | div.toclevel2 { |
| 364 | margin-left: 2em; |
| 365 | font-size: 0.9em; |
| 366 | } |
| 367 | div.toclevel3 { |
| 368 | margin-left: 4em; |
| 369 | font-size: 0.9em; |
| 370 | } |
| 371 | div.toclevel4 { |
| 372 | margin-left: 6em; |
| 373 | font-size: 0.9em; |
| 374 | } |
| 375 | |
| 376 | span.aqua { color: aqua; } |
| 377 | span.black { color: black; } |
| 378 | span.blue { color: blue; } |
| 379 | span.fuchsia { color: fuchsia; } |
| 380 | span.gray { color: gray; } |
| 381 | span.green { color: green; } |
| 382 | span.lime { color: lime; } |
| 383 | span.maroon { color: maroon; } |
| 384 | span.navy { color: navy; } |
| 385 | span.olive { color: olive; } |
| 386 | span.purple { color: purple; } |
| 387 | span.red { color: red; } |
| 388 | span.silver { color: silver; } |
| 389 | span.teal { color: teal; } |
| 390 | span.white { color: white; } |
| 391 | span.yellow { color: yellow; } |
| 392 | |
| 393 | span.aqua-background { background: aqua; } |
| 394 | span.black-background { background: black; } |
| 395 | span.blue-background { background: blue; } |
| 396 | span.fuchsia-background { background: fuchsia; } |
| 397 | span.gray-background { background: gray; } |
| 398 | span.green-background { background: green; } |
| 399 | span.lime-background { background: lime; } |
| 400 | span.maroon-background { background: maroon; } |
| 401 | span.navy-background { background: navy; } |
| 402 | span.olive-background { background: olive; } |
| 403 | span.purple-background { background: purple; } |
| 404 | span.red-background { background: red; } |
| 405 | span.silver-background { background: silver; } |
| 406 | span.teal-background { background: teal; } |
| 407 | span.white-background { background: white; } |
| 408 | span.yellow-background { background: yellow; } |
| 409 | |
| 410 | span.big { font-size: 2em; } |
| 411 | span.small { font-size: 0.6em; } |
| 412 | |
| 413 | span.underline { text-decoration: underline; } |
| 414 | span.overline { text-decoration: overline; } |
| 415 | span.line-through { text-decoration: line-through; } |
| 416 | |
| 417 | div.unbreakable { page-break-inside: avoid; } |
| 418 | |
| 419 | |
| 420 | /* |
| 421 | * xhtml11 specific |
| 422 | * |
| 423 | * */ |
| 424 | |
| 425 | div.tableblock { |
| 426 | margin-top: 1.0em; |
| 427 | margin-bottom: 1.5em; |
| 428 | } |
| 429 | div.tableblock > table { |
| 430 | border: 3px solid #527bbd; |
| 431 | } |
| 432 | thead, p.table.header { |
| 433 | font-weight: bold; |
| 434 | color: #527bbd; |
| 435 | } |
| 436 | p.table { |
| 437 | margin-top: 0; |
| 438 | } |
| 439 | /* Because the table frame attribute is overridden by CSS in most browsers. */ |
| 440 | div.tableblock > table[frame="void"] { |
| 441 | border-style: none; |
| 442 | } |
| 443 | div.tableblock > table[frame="hsides"] { |
| 444 | border-left-style: none; |
| 445 | border-right-style: none; |
| 446 | } |
| 447 | div.tableblock > table[frame="vsides"] { |
| 448 | border-top-style: none; |
| 449 | border-bottom-style: none; |
| 450 | } |
| 451 | |
| 452 | |
| 453 | /* |
| 454 | * html5 specific |
| 455 | * |
| 456 | * */ |
| 457 | |
| 458 | table.tableblock { |
| 459 | margin-top: 1.0em; |
| 460 | margin-bottom: 1.5em; |
| 461 | } |
| 462 | thead, p.tableblock.header { |
| 463 | font-weight: bold; |
| 464 | color: #527bbd; |
| 465 | } |
| 466 | p.tableblock { |
| 467 | margin-top: 0; |
| 468 | } |
| 469 | table.tableblock { |
| 470 | border-width: 3px; |
| 471 | border-spacing: 0px; |
| 472 | border-style: solid; |
| 473 | border-color: #527bbd; |
| 474 | border-collapse: collapse; |
| 475 | } |
| 476 | th.tableblock, td.tableblock { |
| 477 | border-width: 1px; |
| 478 | padding: 4px; |
| 479 | border-style: solid; |
| 480 | border-color: #527bbd; |
| 481 | } |
| 482 | |
| 483 | table.tableblock.frame-topbot { |
| 484 | border-left-style: hidden; |
| 485 | border-right-style: hidden; |
| 486 | } |
| 487 | table.tableblock.frame-sides { |
| 488 | border-top-style: hidden; |
| 489 | border-bottom-style: hidden; |
| 490 | } |
| 491 | table.tableblock.frame-none { |
| 492 | border-style: hidden; |
| 493 | } |
| 494 | |
| 495 | th.tableblock.halign-left, td.tableblock.halign-left { |
| 496 | text-align: left; |
| 497 | } |
| 498 | th.tableblock.halign-center, td.tableblock.halign-center { |
| 499 | text-align: center; |
| 500 | } |
| 501 | th.tableblock.halign-right, td.tableblock.halign-right { |
| 502 | text-align: right; |
| 503 | } |
| 504 | |
| 505 | th.tableblock.valign-top, td.tableblock.valign-top { |
| 506 | vertical-align: top; |
| 507 | } |
| 508 | th.tableblock.valign-middle, td.tableblock.valign-middle { |
| 509 | vertical-align: middle; |
| 510 | } |
| 511 | th.tableblock.valign-bottom, td.tableblock.valign-bottom { |
| 512 | vertical-align: bottom; |
| 513 | } |
| 514 | |
| 515 | |
| 516 | /* |
| 517 | * manpage specific |
| 518 | * |
| 519 | * */ |
| 520 | |
| 521 | body.manpage h1 { |
| 522 | padding-top: 0.5em; |
| 523 | padding-bottom: 0.5em; |
| 524 | border-top: 2px solid silver; |
| 525 | border-bottom: 2px solid silver; |
| 526 | } |
| 527 | body.manpage h2 { |
| 528 | border-style: none; |
| 529 | } |
| 530 | body.manpage div.sectionbody { |
| 531 | margin-left: 3em; |
| 532 | } |
| 533 | |
| 534 | @media print { |
| 535 | body.manpage div#toc { display: none; } |
| 536 | } |
| 537 | |
| 538 | |
| 539 | </style> |
| 540 | <script type="text/javascript"> |
| 541 | /*<![CDATA[*/ |
| Junio C Hamano | 2b15318 | 2021-12-15 21:00:31 | [diff] [blame] | 542 | var asciidoc = { // Namespace. |
| 543 | |
| 544 | ///////////////////////////////////////////////////////////////////// |
| 545 | // Table Of Contents generator |
| 546 | ///////////////////////////////////////////////////////////////////// |
| 547 | |
| 548 | /* Author: Mihai Bazon, September 2002 |
| 549 | * http://students.infoiasi.ro/~mishoo |
| 550 | * |
| 551 | * Table Of Content generator |
| 552 | * Version: 0.4 |
| 553 | * |
| 554 | * Feel free to use this script under the terms of the GNU General Public |
| 555 | * License, as long as you do not remove or alter this notice. |
| 556 | */ |
| 557 | |
| 558 | /* modified by Troy D. Hanson, September 2006. License: GPL */ |
| 559 | /* modified by Stuart Rackham, 2006, 2009. License: GPL */ |
| 560 | |
| 561 | // toclevels = 1..4. |
| 562 | toc: function (toclevels) { |
| 563 | |
| 564 | function getText(el) { |
| 565 | var text = ""; |
| 566 | for (var i = el.firstChild; i != null; i = i.nextSibling) { |
| 567 | if (i.nodeType == 3 /* Node.TEXT_NODE */) // IE doesn't speak constants. |
| 568 | text += i.data; |
| 569 | else if (i.firstChild != null) |
| 570 | text += getText(i); |
| 571 | } |
| 572 | return text; |
| 573 | } |
| 574 | |
| 575 | function TocEntry(el, text, toclevel) { |
| 576 | this.element = el; |
| 577 | this.text = text; |
| 578 | this.toclevel = toclevel; |
| 579 | } |
| 580 | |
| 581 | function tocEntries(el, toclevels) { |
| 582 | var result = new Array; |
| 583 | var re = new RegExp('[hH]([1-'+(toclevels+1)+'])'); |
| 584 | // Function that scans the DOM tree for header elements (the DOM2 |
| 585 | // nodeIterator API would be a better technique but not supported by all |
| 586 | // browsers). |
| 587 | var iterate = function (el) { |
| 588 | for (var i = el.firstChild; i != null; i = i.nextSibling) { |
| 589 | if (i.nodeType == 1 /* Node.ELEMENT_NODE */) { |
| 590 | var mo = re.exec(i.tagName); |
| 591 | if (mo && (i.getAttribute("class") || i.getAttribute("className")) != "float") { |
| 592 | result[result.length] = new TocEntry(i, getText(i), mo[1]-1); |
| 593 | } |
| 594 | iterate(i); |
| 595 | } |
| 596 | } |
| 597 | } |
| 598 | iterate(el); |
| 599 | return result; |
| 600 | } |
| 601 | |
| 602 | var toc = document.getElementById("toc"); |
| 603 | if (!toc) { |
| 604 | return; |
| 605 | } |
| 606 | |
| 607 | // Delete existing TOC entries in case we're reloading the TOC. |
| 608 | var tocEntriesToRemove = []; |
| 609 | var i; |
| 610 | for (i = 0; i < toc.childNodes.length; i++) { |
| 611 | var entry = toc.childNodes[i]; |
| 612 | if (entry.nodeName.toLowerCase() == 'div' |
| 613 | && entry.getAttribute("class") |
| 614 | && entry.getAttribute("class").match(/^toclevel/)) |
| 615 | tocEntriesToRemove.push(entry); |
| 616 | } |
| 617 | for (i = 0; i < tocEntriesToRemove.length; i++) { |
| 618 | toc.removeChild(tocEntriesToRemove[i]); |
| 619 | } |
| 620 | |
| 621 | // Rebuild TOC entries. |
| 622 | var entries = tocEntries(document.getElementById("content"), toclevels); |
| 623 | for (var i = 0; i < entries.length; ++i) { |
| 624 | var entry = entries[i]; |
| 625 | if (entry.element.id == "") |
| 626 | entry.element.id = "_toc_" + i; |
| 627 | var a = document.createElement("a"); |
| 628 | a.href = "#" + entry.element.id; |
| 629 | a.appendChild(document.createTextNode(entry.text)); |
| 630 | var div = document.createElement("div"); |
| 631 | div.appendChild(a); |
| 632 | div.className = "toclevel" + entry.toclevel; |
| 633 | toc.appendChild(div); |
| 634 | } |
| 635 | if (entries.length == 0) |
| 636 | toc.parentNode.removeChild(toc); |
| 637 | }, |
| 638 | |
| 639 | |
| 640 | ///////////////////////////////////////////////////////////////////// |
| 641 | // Footnotes generator |
| 642 | ///////////////////////////////////////////////////////////////////// |
| 643 | |
| 644 | /* Based on footnote generation code from: |
| 645 | * http://www.brandspankingnew.net/archive/2005/07/format_footnote.html |
| 646 | */ |
| 647 | |
| 648 | footnotes: function () { |
| 649 | // Delete existing footnote entries in case we're reloading the footnodes. |
| 650 | var i; |
| 651 | var noteholder = document.getElementById("footnotes"); |
| 652 | if (!noteholder) { |
| 653 | return; |
| 654 | } |
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| 732 | /*]]>*/ |
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| 734 | </head> |
| 735 | <body class="article"> |
| 736 | <div id="header"> |
| 737 | <h1>Parallel Checkout Design Notes</h1> |
| Junio C Hamano | f9b2d64 | 2023-10-31 06:31:53 | [diff] [blame^] | 738 | <span id="revdate">2023-10-31</span> |
| Junio C Hamano | 4078a55 | 2021-04-30 06:08:10 | [diff] [blame] | 739 | </div> |
| 740 | <div id="content"> |
| 741 | <div id="preamble"> |
| 742 | <div class="sectionbody"> |
| 743 | <div class="paragraph"><p>The "Parallel Checkout" feature attempts to use multiple processes to |
| 744 | parallelize the work of uncompressing the blobs, applying in-core |
| 745 | filters, and writing the resulting contents to the working tree during a |
| 746 | checkout operation. It can be used by all checkout-related commands, |
| 747 | such as <code>clone</code>, <code>checkout</code>, <code>reset</code>, <code>sparse-checkout</code>, and others.</p></div> |
| 748 | <div class="paragraph"><p>These commands share the following basic structure:</p></div> |
| 749 | <div class="ulist"><ul> |
| 750 | <li> |
| 751 | <p> |
| 752 | Step 1: Read the current index file into memory. |
| 753 | </p> |
| 754 | </li> |
| 755 | <li> |
| 756 | <p> |
| 757 | Step 2: Modify the in-memory index based upon the command, and |
| 758 | temporarily mark all cache entries that need to be updated. |
| 759 | </p> |
| 760 | </li> |
| 761 | <li> |
| 762 | <p> |
| 763 | Step 3: Populate the working tree to match the new candidate index. |
| 764 | This includes iterating over all of the to-be-updated cache entries |
| 765 | and delete, create, or overwrite the associated files in the working |
| 766 | tree. |
| 767 | </p> |
| 768 | </li> |
| 769 | <li> |
| 770 | <p> |
| 771 | Step 4: Write the new index to disk. |
| 772 | </p> |
| 773 | </li> |
| 774 | </ul></div> |
| 775 | <div class="paragraph"><p>Step 3 is the focus of the "parallel checkout" effort described here.</p></div> |
| 776 | </div> |
| 777 | </div> |
| 778 | <div class="sect1"> |
| 779 | <h2 id="_sequential_implementation">Sequential Implementation</h2> |
| 780 | <div class="sectionbody"> |
| 781 | <div class="paragraph"><p>For the purposes of discussion here, the current sequential |
| 782 | implementation of Step 3 is divided in 3 parts, each one implemented in |
| 783 | its own function:</p></div> |
| 784 | <div class="ulist"><ul> |
| 785 | <li> |
| 786 | <p> |
| 787 | Step 3a: <code>unpack-trees.c:check_updates()</code> contains a series of |
| 788 | sequential loops iterating over the <code>cache_entry</code>'s array. The main |
| 789 | loop in this function calls the Step 3b function for each of the |
| 790 | to-be-updated entries. |
| 791 | </p> |
| 792 | </li> |
| 793 | <li> |
| 794 | <p> |
| 795 | Step 3b: <code>entry.c:checkout_entry()</code> examines the existing working tree |
| 796 | for file conflicts, collisions, and unsaved changes. It removes files |
| 797 | and creates leading directories as necessary. It calls the Step 3c |
| 798 | function for each entry to be written. |
| 799 | </p> |
| 800 | </li> |
| 801 | <li> |
| 802 | <p> |
| 803 | Step 3c: <code>entry.c:write_entry()</code> loads the blob into memory, smudges |
| 804 | it if necessary, creates the file in the working tree, writes the |
| 805 | smudged contents, calls <code>fstat()</code> or <code>lstat()</code>, and updates the |
| 806 | associated <code>cache_entry</code> struct with the stat information gathered. |
| 807 | </p> |
| 808 | </li> |
| 809 | </ul></div> |
| 810 | <div class="paragraph"><p>It wouldn’t be safe to perform Step 3b in parallel, as there could be |
| 811 | race conditions between file creations and removals. Instead, the |
| 812 | parallel checkout framework lets the sequential code handle Step 3b, |
| 813 | and uses parallel workers to replace the sequential |
| 814 | <code>entry.c:write_entry()</code> calls from Step 3c.</p></div> |
| 815 | </div> |
| 816 | </div> |
| 817 | <div class="sect1"> |
| 818 | <h2 id="_rejected_multi_threaded_solution">Rejected Multi-Threaded Solution</h2> |
| 819 | <div class="sectionbody"> |
| 820 | <div class="paragraph"><p>The most "straightforward" implementation would be to spread the set of |
| 821 | to-be-updated cache entries across multiple threads. But due to the |
| Junio C Hamano | e8c74d8 | 2022-11-12 07:59:36 | [diff] [blame] | 822 | thread-unsafe functions in the object database code, we would have to use locks to |
| Junio C Hamano | 4078a55 | 2021-04-30 06:08:10 | [diff] [blame] | 823 | coordinate the parallel operation. An early prototype of this solution |
| 824 | showed that the multi-threaded checkout would bring performance |
| 825 | improvements over the sequential code, but there was still too much lock |
| 826 | contention. A <code>perf</code> profiling indicated that around 20% of the runtime |
| 827 | during a local Linux clone (on an SSD) was spent in locking functions. |
| 828 | For this reason this approach was rejected in favor of using multiple |
| Junio C Hamano | 33be821 | 2023-10-23 21:45:54 | [diff] [blame] | 829 | child processes, which led to better performance.</p></div> |
| Junio C Hamano | 4078a55 | 2021-04-30 06:08:10 | [diff] [blame] | 830 | </div> |
| 831 | </div> |
| 832 | <div class="sect1"> |
| 833 | <h2 id="_multi_process_solution">Multi-Process Solution</h2> |
| 834 | <div class="sectionbody"> |
| 835 | <div class="paragraph"><p>Parallel checkout alters the aforementioned Step 3 to use multiple |
| 836 | <code>checkout--worker</code> background processes to distribute the work. The |
| 837 | long-running worker processes are controlled by the foreground Git |
| 838 | command using the existing run-command API.</p></div> |
| 839 | <div class="sect2"> |
| 840 | <h3 id="_overview">Overview</h3> |
| 841 | <div class="paragraph"><p>Step 3b is only slightly altered; for each entry to be checked out, the |
| 842 | main process performs the following steps:</p></div> |
| 843 | <div class="ulist"><ul> |
| 844 | <li> |
| 845 | <p> |
| 846 | M1: Check whether there is any untracked or unclean file in the |
| 847 | working tree which would be overwritten by this entry, and decide |
| 848 | whether to proceed (removing the file(s)) or not. |
| 849 | </p> |
| 850 | </li> |
| 851 | <li> |
| 852 | <p> |
| 853 | M2: Create the leading directories. |
| 854 | </p> |
| 855 | </li> |
| 856 | <li> |
| 857 | <p> |
| 858 | M3: Load the conversion attributes for the entry’s path. |
| 859 | </p> |
| 860 | </li> |
| 861 | <li> |
| 862 | <p> |
| 863 | M4: Check, based on the entry’s type and conversion attributes, |
| 864 | whether the entry is eligible for parallel checkout (more on this |
| 865 | later). If it is eligible, enqueue the entry and the loaded |
| 866 | attributes to later write the entry in parallel. If not, write the |
| 867 | entry right away, using the default sequential code. |
| 868 | </p> |
| 869 | </li> |
| 870 | </ul></div> |
| 871 | <div class="paragraph"><p>Note: we save the conversion attributes associated with each entry |
| 872 | because the workers don’t have access to the main process' index state, |
| 873 | so they can’t load the attributes by themselves (and the attributes are |
| 874 | needed to properly smudge the entry). Additionally, this has a positive |
| 875 | impact on performance as (1) we don’t need to load the attributes twice |
| 876 | and (2) the attributes machinery is optimized to handle paths in |
| 877 | sequential order.</p></div> |
| 878 | <div class="paragraph"><p>After all entries have passed through the above steps, the main process |
| 879 | checks if the number of enqueued entries is sufficient to spread among |
| 880 | the workers. If not, it just writes them sequentially. Otherwise, it |
| 881 | spawns the workers and distributes the queued entries uniformly in |
| 882 | continuous chunks. This aims to minimize the chances of two workers |
| 883 | writing to the same directory simultaneously, which could increase lock |
| 884 | contention in the kernel.</p></div> |
| 885 | <div class="paragraph"><p>Then, for each assigned item, each worker:</p></div> |
| 886 | <div class="ulist"><ul> |
| 887 | <li> |
| 888 | <p> |
| 889 | W1: Checks if there is any non-directory file in the leading part of |
| 890 | the entry’s path or if there already exists a file at the entry' path. |
| 891 | If so, mark the entry with <code>PC_ITEM_COLLIDED</code> and skip it (more on |
| 892 | this later). |
| 893 | </p> |
| 894 | </li> |
| 895 | <li> |
| 896 | <p> |
| 897 | W2: Creates the file (with O_CREAT and O_EXCL). |
| 898 | </p> |
| 899 | </li> |
| 900 | <li> |
| 901 | <p> |
| 902 | W3: Loads the blob into memory (inflating and delta reconstructing |
| 903 | it). |
| 904 | </p> |
| 905 | </li> |
| 906 | <li> |
| 907 | <p> |
| 908 | W4: Applies any required in-process filter, like end-of-line |
| 909 | conversion and re-encoding. |
| 910 | </p> |
| 911 | </li> |
| 912 | <li> |
| 913 | <p> |
| 914 | W5: Writes the result to the file descriptor opened at W2. |
| 915 | </p> |
| 916 | </li> |
| 917 | <li> |
| 918 | <p> |
| Junio C Hamano | 33be821 | 2023-10-23 21:45:54 | [diff] [blame] | 919 | W6: Calls <code>fstat()</code> or <code>lstat()</code> on the just-written path, and sends |
| Junio C Hamano | 4078a55 | 2021-04-30 06:08:10 | [diff] [blame] | 920 | the result back to the main process, together with the end status of |
| 921 | the operation and the item’s identification number. |
| 922 | </p> |
| 923 | </li> |
| 924 | </ul></div> |
| 925 | <div class="paragraph"><p>Note that, when possible, steps W3 to W5 are delegated to the streaming |
| 926 | machinery, removing the need to keep the entire blob in memory.</p></div> |
| 927 | <div class="paragraph"><p>If the worker fails to read the blob or to write it to the working tree, |
| 928 | it removes the created file to avoid leaving empty files behind. This is |
| 929 | the <strong>only</strong> time a worker is allowed to remove a file.</p></div> |
| 930 | <div class="paragraph"><p>As mentioned earlier, it is the responsibility of the main process to |
| 931 | remove any file that blocks the checkout operation (or abort if the |
| 932 | removal(s) would cause data loss and the user didn’t ask to <code>--force</code>). |
| 933 | This is crucial to avoid race conditions and also to properly detect |
| 934 | path collisions at Step W1.</p></div> |
| 935 | <div class="paragraph"><p>After the workers finish writing the items and sending back the required |
| 936 | information, the main process handles the results in two steps:</p></div> |
| 937 | <div class="ulist"><ul> |
| 938 | <li> |
| 939 | <p> |
| 940 | First, it updates the in-memory index with the <code>lstat()</code> information |
| 941 | sent by the workers. (This must be done first as this information |
| Junio C Hamano | 33be821 | 2023-10-23 21:45:54 | [diff] [blame] | 942 | might be required in the following step.) |
| Junio C Hamano | 4078a55 | 2021-04-30 06:08:10 | [diff] [blame] | 943 | </p> |
| 944 | </li> |
| 945 | <li> |
| 946 | <p> |
| 947 | Then it writes the items which collided on disk (i.e. items marked |
| 948 | with <code>PC_ITEM_COLLIDED</code>). More on this below. |
| 949 | </p> |
| 950 | </li> |
| 951 | </ul></div> |
| 952 | </div> |
| 953 | </div> |
| 954 | </div> |
| 955 | <div class="sect1"> |
| 956 | <h2 id="_path_collisions">Path Collisions</h2> |
| 957 | <div class="sectionbody"> |
| 958 | <div class="paragraph"><p>Path collisions happen when two different paths correspond to the same |
| 959 | entry in the file system. E.g. the paths <em>a</em> and <em>A</em> would collide in a |
| 960 | case-insensitive file system.</p></div> |
| 961 | <div class="paragraph"><p>The sequential checkout deals with collisions in the same way that it |
| 962 | deals with files that were already present in the working tree before |
| 963 | checkout. Basically, it checks if the path that it wants to write |
| 964 | already exists on disk, makes sure the existing file doesn’t have |
| 965 | unsaved data, and then overwrites it. (To be more pedantic: it deletes |
| 966 | the existing file and creates the new one.) So, if there are multiple |
| 967 | colliding files to be checked out, the sequential code will write each |
| 968 | one of them but only the last will actually survive on disk.</p></div> |
| 969 | <div class="paragraph"><p>Parallel checkout aims to reproduce the same behavior. However, we |
| 970 | cannot let the workers racily write to the same file on disk. Instead, |
| 971 | the workers detect when the entry that they want to check out would |
| 972 | collide with an existing file, and mark it with <code>PC_ITEM_COLLIDED</code>. |
| 973 | Later, the main process can sequentially feed these entries back to |
| 974 | <code>checkout_entry()</code> without the risk of race conditions. On clone, this |
| 975 | also has the effect of marking the colliding entries to later emit a |
| 976 | warning for the user, like the classic sequential checkout does.</p></div> |
| 977 | <div class="paragraph"><p>The workers are able to detect both collisions among the entries being |
| 978 | concurrently written and collisions between a parallel-eligible entry |
| 979 | and an ineligible entry. The general idea for collision detection is |
| 980 | quite straightforward: for each parallel-eligible entry, the main |
| 981 | process must remove all files that prevent this entry from being written |
| 982 | (before enqueueing it). This includes any non-directory file in the |
| 983 | leading path of the entry. Later, when a worker gets assigned the entry, |
| Junio C Hamano | 33be821 | 2023-10-23 21:45:54 | [diff] [blame] | 984 | it looks again for the non-directory files and for an already existing |
| Junio C Hamano | 4078a55 | 2021-04-30 06:08:10 | [diff] [blame] | 985 | file at the entry’s path. If any of these checks finds something, the |
| 986 | worker knows that there was a path collision.</p></div> |
| 987 | <div class="paragraph"><p>Because parallel checkout can distinguish path collisions from the case |
| 988 | where the file was already present in the working tree before checkout, |
| 989 | we could alternatively choose to skip the checkout of colliding entries. |
| 990 | However, each entry that doesn’t get written would have NULL <code>lstat()</code> |
| 991 | fields on the index. This could cause performance penalties for |
| 992 | subsequent commands that need to refresh the index, as they would have |
| 993 | to go to the file system to see if the entry is dirty. Thus, if we have |
| 994 | N entries in a colliding group and we decide to write and <code>lstat()</code> only |
| 995 | one of them, every subsequent <code>git-status</code> will have to read, convert, |
| 996 | and hash the written file N - 1 times. By checking out all colliding |
| 997 | entries (like the sequential code does), we only pay the overhead once, |
| 998 | during checkout.</p></div> |
| 999 | </div> |
| 1000 | </div> |
| 1001 | <div class="sect1"> |
| 1002 | <h2 id="_eligible_entries_for_parallel_checkout">Eligible Entries for Parallel Checkout</h2> |
| 1003 | <div class="sectionbody"> |
| 1004 | <div class="paragraph"><p>As previously mentioned, not all entries passed to <code>checkout_entry()</code> |
| 1005 | will be considered eligible for parallel checkout. More specifically, we |
| 1006 | exclude:</p></div> |
| 1007 | <div class="ulist"><ul> |
| 1008 | <li> |
| 1009 | <p> |
| 1010 | Symbolic links; to avoid race conditions that, in combination with |
| 1011 | path collisions, could cause workers to write files at the wrong |
| 1012 | place. For example, if we were to concurrently check out a symlink |
| 1013 | <em>a</em> → <em>b</em> and a regular file <em>A/f</em> in a case-insensitive file system, |
| 1014 | we could potentially end up writing the file <em>A/f</em> at <em>a/f</em>, due to a |
| 1015 | race condition. |
| 1016 | </p> |
| 1017 | </li> |
| 1018 | <li> |
| 1019 | <p> |
| 1020 | Regular files that require external filters (either "one shot" filters |
| 1021 | or long-running process filters). These filters are black-boxes to Git |
| 1022 | and may have their own internal locking or non-concurrent assumptions. |
| 1023 | So it might not be safe to run multiple instances in parallel. |
| 1024 | </p> |
| 1025 | <div class="paragraph"><p>Besides, long-running filters may use the delayed checkout feature to |
| 1026 | postpone the return of some filtered blobs. The delayed checkout queue |
| 1027 | and the parallel checkout queue are not compatible and should remain |
| 1028 | separate.</p></div> |
| 1029 | <div class="paragraph"><p>Note: regular files that only require internal filters, like end-of-line |
| 1030 | conversion and re-encoding, are eligible for parallel checkout.</p></div> |
| 1031 | </li> |
| 1032 | </ul></div> |
| 1033 | <div class="paragraph"><p>Ineligible entries are checked out by the classic sequential codepath |
| 1034 | <strong>before</strong> spawning workers.</p></div> |
| Junio C Hamano | 33be821 | 2023-10-23 21:45:54 | [diff] [blame] | 1035 | <div class="paragraph"><p>Note: submodules' files are also eligible for parallel checkout (as |
| Junio C Hamano | 4078a55 | 2021-04-30 06:08:10 | [diff] [blame] | 1036 | long as they don’t fall into any of the excluding categories mentioned |
| 1037 | above). But since each submodule is checked out in its own child |
| 1038 | process, we don’t mix the superproject’s and the submodules' files in |
| 1039 | the same parallel checkout process or queue.</p></div> |
| 1040 | </div> |
| 1041 | </div> |
| 1042 | <div class="sect1"> |
| 1043 | <h2 id="_the_api">The API</h2> |
| 1044 | <div class="sectionbody"> |
| 1045 | <div class="paragraph"><p>The parallel checkout API was designed with the goal of minimizing |
| 1046 | changes to the current users of the checkout machinery. This means that |
| 1047 | they don’t have to call a different function for sequential or parallel |
| 1048 | checkout. As already mentioned, <code>checkout_entry()</code> will automatically |
| 1049 | insert the given entry in the parallel checkout queue when this feature |
| 1050 | is enabled and the entry is eligible; otherwise, it will just write the |
| 1051 | entry right away, using the sequential code. In general, callers of the |
| 1052 | parallel checkout API should look similar to this:</p></div> |
| 1053 | <div class="listingblock"> |
| 1054 | <div class="content"> |
| 1055 | <pre><code>int pc_workers, pc_threshold, err = 0; |
| 1056 | struct checkout state; |
| 1057 | |
| 1058 | get_parallel_checkout_configs(&pc_workers, &pc_threshold); |
| 1059 | |
| 1060 | /* |
| 1061 | * This check is not strictly required, but it |
| 1062 | * should save some time in sequential mode. |
| 1063 | */ |
| 1064 | if (pc_workers > 1) |
| 1065 | init_parallel_checkout(); |
| 1066 | |
| 1067 | for (each cache_entry ce to-be-updated) |
| 1068 | err |= checkout_entry(ce, &state, NULL, NULL); |
| 1069 | |
| 1070 | err |= run_parallel_checkout(&state, pc_workers, pc_threshold, NULL, NULL);</code></pre> |
| 1071 | </div></div> |
| 1072 | </div> |
| 1073 | </div> |
| 1074 | </div> |
| 1075 | <div id="footnotes"><hr /></div> |
| 1076 | <div id="footer"> |
| 1077 | <div id="footer-text"> |
| 1078 | Last updated |
| Junio C Hamano | 918a697 | 2023-10-29 23:44:11 | [diff] [blame] | 1079 | 2023-10-24 06:43:46 JST |
| Junio C Hamano | 4078a55 | 2021-04-30 06:08:10 | [diff] [blame] | 1080 | </div> |
| 1081 | </div> |
| 1082 | </body> |
| 1083 | </html> |
