Re: [PATCH] mingw: use OpenSSL's SHA-1 routines

Hi Peff,

On Fri, 10 Feb 2017, Jeff King wrote:

On Thu, Feb 09, 2017 at 11:27:49PM +0100, Johannes Schindelin wrote:

quoted

From: Jeff Hostetler <redacted>

Use OpenSSL's SHA-1 routines rather than builtin block-sha1 routines.
This improves performance on SHA1 operations on Intel processors.

OpenSSL 1.0.2 has made considerable performance improvements and
support the Intel hardware acceleration features.  See:
https://software.intel.com/en-us/articles/improving-openssl-performance
https://software.intel.com/en-us/articles/intel-sha-extensions

To test this I added/staged a single file in a gigantic repository
having a 450MB index file.  The code in read-cache.c verifies the
header SHA as it reads the index and computes a new header SHA as it
writes out the new index.  Therefore, in this test the SHA code must
process 900MB of data.  Testing was done on an Intel I7-4770 CPU @
3.40GHz (Intel64, Family 6, Model 60) CPU.

I think this is only half the story. A heavy-sha1 workload is faster,
which is good. But one of the original reasons to prefer blk-sha1 (at
least on Linux) is that resolving libcrypto.so symbols takes a
non-trivial amount of time. I just timed it again, and it seems to be
consistently 1ms slower to run "git rev-parse --git-dir" on my machine
(from the top-level of a repo).

1ms is mostly irrelevant, but it adds up on scripted workloads that
start a lot of git processes.

You know my answer to that. If scripting slows things down, we should
avoid it in production code. As it is, scripting slows us down. Therefore
I work slowly but steadily to get rid of scripting where it hurts most.

Ciao,
Dscho