Re: cleaner/better zlib sources?

From: Jeff Garzik <jeff@garzik.org>
Date: Mar 16, 2007, 16:35 UTC
Message-ID: <45FAC75B.3030902@garzik.org>
In-Reply-To: <Pine.LNX.4.64.0703160913361.3816@woody.linux-foundation.org>

Linus Torvalds wrote:

Show 7 quoted lines

> The normal size for the performance-critical git objects are in the couple 
> of *hundred* bytes. Not kilobytes, and not megabytes.
> 
> The most performance-critical objects for uncompression are commits and 
> trees. At least for the kernel, the average size of a tree object is 678
> bytes. And that's ignoring the fact that most of them are then deltified, 
> so about 80% of them are likely just a ~60-byte delta.

Ahhh. At least for me, that explains a lot. Rather than spending all its time in inflate_fast(), git is dealing with lots of zlib startup/shutdown overhead.

Although it sounds like zlib could indeed be optimized to reduce its startup and shutdown overhead, I wonder if switching compression algorithms to a pure Huffman or even RLE compression (with associated lower startup/shutdown costs) would perform better in the face of all those small objects.

And another random thought, though it may be useless in this thread: I bet using a pre-built (compiled into git) static zlib dictionary for git commit and tree objects might improve things a bit.

	Jeff