[OE-core] Fetch/Unpack performance dependent on git version

[Richard Purdie]

I thought it might be worth highlighting to people that the performance
of the git fetcher *and* the unpack process is highly dependant on the
version of git.

Firstly, all versions of git 1.6+ appear to default to injecting an
fsync() after fetching objects. fsync() usage significantly hurts our
builds. It is also a bit pointless in our usecase, we can however
disable it with a simple tweak to the fetcher:

-        ud.basecmd = data.getVar("FETCHCMD_git", d, True) or "git"
+        ud.basecmd = data.getVar("FETCHCMD_git", d, True) or "git -c core.fsyncobjectfiles=0"

Secondly, for older versions of git, git clone is running repository
coverage checks even for local cloning. E.g.:

git version 1.8.3.2 on my system:

$ bitbake linux-yocto -c fetch
$ time bitbake linux-yocto -c unpack
51.33user 1.93system 1:09.27elapsed 76%CPU (0avgtext+0avgdata 779380maxresident)k

after updating to git 1.9.2:

$ bitbake linux-yocto -c fetch
$ time bitbake linux-yocto -c unpack
4.28user 0.63system 0:04.36elapsed 112%CPU (0avgtext+0avgdata 260692maxresident)k

so 16 times faster!

The commit in question for anyone interested:

https://github.com/git/git/commit/125a05fd0b45416558923b753f6418c24208d443

Michael/Beth: Can we please upgrade the autobuilder to a daisy release
buildtools tarball which has git 1.9.x in it? This should massively
speed up certain parts of the autobuilder.

Stefan: Could you see which version of git is on the performance
benchmark machine? A run with the daisy build tools tarball would be
extremely interesting if its before 1.9.

Cheers,

Richard