Re: [PATCH 0/5] Suggested for PU: revision caching system to significantly speed up packing/walking

On Sat, 8 Aug 2009, Johannes Schindelin wrote:

Hi,

On Fri, 7 Aug 2009, Nicolas Pitre wrote:

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On Fri, 7 Aug 2009, Johannes Schindelin wrote:

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Hi,

On Fri, 7 Aug 2009, Nicolas Pitre wrote:

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On Fri, 7 Aug 2009, Sam Vilain wrote:

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Johannes Schindelin wrote:

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the short answer is that cache slices are totally independant of 
pack files.
    

My idea with that was that you already have a SHA-1 map in the pack 
index, and if all you want to be able to accelerate the revision 
walker, you'd probably need something that adds yet another mapping, 
from commit to parents and tree, and from tree to sub-tree and blob 
(so you can avoid unpacking commit and tree objects).
  

Tying indexes together like that is not a good idea in the database 
world. Especially as in this case as Nick mentions, the domain is 
subtly different (ie pack vs dag). Unfortunately you just can't try to 
pretend that they will always be the same; you can't force a full 
repack on every ref change!

Right.  And the rev cache must work even if the repository is not 
packed.

Umm, why?  AFAICT the principal purpose of the rev cache is to help work 
loads on, say, www.kernel.org.

So what?

Speeding up rev-list with a rev cache is completely orthogonal to 
whether the repository is packed or not.

No, it is not.

For both technical and practical reasons, caching revision walker data is
very closely related to packing.

No it is not.

You are _very_ unlikely helped by speeding up revision walking in the 
general case, _especially_ when you do stuff like blame or -S that needs 
to unpack tons of objects _anyway_.

I completely agree.  And this is why I wish for the rev cache to be 
enabled with a config variable.  Why?  Because "client" repositories are 
unlikely to benefit as much as "server" repositories from this cache.

The one big kicker argument for speeding up revision walking _is_ to 
relieve the loads on big ass servers, and they _should_ be as packed as 
possible (as I will patiently explain over and over again).

Please do a shortlog on builtin-pack-objects.c and realize who spent a 
lot of energy making git repository packing what it is now.

Then re-read what I said on this list for more than a year now about the 
remaining latency on large git clone operations.  After explaining it a 
couple times already I will need some of your patience to make people 
understand that fully packing your repo is _not_ going to help it as 
we're always talking about fully packed repos to start with.

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It is like having a "git diff" result cache: no one would think of 
stuffing that in the pack index.

Do you want to try to kid me?  You'll have to try harder.  Caching "git 
diff" results... no, really!

Why not?  You worked on git diff yourself, so you certainly are well 
positionned to appreciate my suggestion, no?

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If we want to improve on the repository packing format, that must be 
doable without bothering with an independent concept such as a rev 
cache.

I would love to tell you that you're right, but the single fact that 
pack v4 is startig to compete with Duke Nukem Forever just prevents me 
from doing that.

Because of the current economy, I was waiting to be laid off so to have 
the time to make pack v4 a reality.  Unfortunately for git they didn't 
fire me yet.  Oh well...

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I am unlikely to notice the improvements in my regular "git log" calls 
that only show a couple of pages before I quit the pager.

Indeed.  But what is your point again?

Oh?  My point?  Being that the rev cache has a certain target audience, 
and that the regular user is not part of that audience, and that it just 
so happens that the _technical_ similarities with the pack index can be 
exploited in those scenarios?

I don't see a similarity with the pack index at all, certainly not a 
technical one.

IOW we can be pretty certain that a heavy-load server has a fully (or 
next-to-fully) packed object database.  The pack indices already contain a 
SHA-1 table that we can simply reuse.  And it should not be hard (or 
fragile) at all to put the "cached" information about parents, 
referenced tree and blob objects into that file, into a different section.

And then someone does a few pushes.  So most of the time your repository 
contains a few packs and not only a single one.  So in which pack index 
files should you put the rev cache?  What do yo do with that cache if it 
happens to be split across multiple pack index files when a repack is 
performed?  Can't you see all the disadvantages to tie rev cache data 
which happens to share no issue with repacking into the same file?

So what do you do to keep the code simple and maintainable?  Yes, you 
abstract things and use separate files, thank you very much.  After all, 
the packed refs are not stored in the pack index file either even if the 
packed-refs file contains a list of SHA1's.


Nicolas