Re: x86 SHA1: Faster than OpenSSL

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, 3 Aug 2009, Linus Torvalds wrote:
> 
> The thing that I'd prefer is simply
> 
> 	git fsck --full
> 
> on the Linux kernel archive. For me (with a fast machine), it takes about 
> 4m30s with the OpenSSL SHA1, and takes 6m40s with the Mozilla SHA1 (ie 
> using a NO_OPENSSL=1 build).
> 
> So that's an example of a load that is actually very sensitive to SHA1 
> performance (more so than _most_ git loads, I suspect), and at the same 
> time is a real git load rather than some SHA1-only microbenchmark. It also 
> shows very clearly why we default to the OpenSSL version over the Mozilla 
> one.

"perf report --sort comm,dso,symbol" profiling shows the following for 
'git fsck --full' on the kernel repo, using the Mozilla SHA1:

    47.69%               git  /home/torvalds/git/git     [.] moz_SHA1_Update
    22.98%               git  /lib64/libz.so.1.2.3       [.] inflate_fast
     7.32%               git  /lib64/libc-2.10.1.so      [.] __GI_memcpy
     4.66%               git  /lib64/libz.so.1.2.3       [.] inflate
     3.76%               git  /lib64/libz.so.1.2.3       [.] adler32
     2.86%               git  /lib64/libz.so.1.2.3       [.] inflate_table
     2.41%               git  /home/torvalds/git/git     [.] lookup_object
     1.31%               git  /lib64/libc-2.10.1.so      [.] _int_malloc
     0.84%               git  /home/torvalds/git/git     [.] patch_delta
     0.78%               git  [kernel]                   [k] hpet_next_event

so yeah, SHA1 performance matters. Judging by the OpenSSL numbers, the 
OpenSSL SHA1 implementation must be about twice as fast as the C version 
we use.

That said, under "normal" git usage models, the SHA1 costs are almost 
invisible. So git-fsck is definitely a fairly unusual case that stresses 
the SHA1 performance more than most git lods.

		Linus