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diff --git a/Documentation/technical/pack-heuristics.txt b/Documentation/technical/pack-heuristics.txt new file mode 100644 index 0000000000..95a07db6e8 --- /dev/null +++ b/Documentation/technical/pack-heuristics.txt @@ -0,0 +1,460 @@ +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 SHA-1. + + 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 + + <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. |