On Thu, 14 Aug 2008, Andi Kleen wrote:

quoted text
>

> I would have thought that zlib has a sequential access pattern that the

> CPU prefetchers have a easy time with hiding latency.

No, the lookup tables for the patterns are quite non-sequential. It does

do a lot of indirect accesses, ie it loads data from the input stream and

then looks things up through that. 
But it's quite possible that we should use different compression factors

for different object types. Right now we have different (configurable)

compression levels for loose objects and packs, but it might be

interesting to see what happens for just "packed tree objects".
The trees really end up having rather different access patterns in

pack-files. They also tend to be rather less compressible than other

blobs, since the SHA1's in there are just random binary data. They also

delta very well - obviously regular blobs do that _too_, but regular blobs

are seldom as performance-critical in git (ie once you actually unpack a

blob, there are other things going on like actually generating a diff -

but trees get unpacked over and over for "internal git reasons")
		Linus

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