Added Packing Heursitics IRC writeup.

Signed-off-by: Jon Loeliger <jdl@jdl.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 466 insertions, 0 deletions

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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.