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authorJon Loeliger <jdl@jdl.com>2006-03-03 01:19:29 (GMT)
committerJunio C Hamano <junkio@cox.net>2006-04-07 09:06:18 (GMT)
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Added Packing Heursitics IRC writeup.
Signed-off-by: Jon Loeliger <jdl@jdl.com> Signed-off-by: Junio C Hamano <junkio@cox.net>
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+ 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 Bable-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 paramater) 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
+ (ie 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 homogenous 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 obious 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.