'git describe' is very slow on development trees with lots of commits
From: Ingo Molnar <mingo@kernel.org>
Date: 2012-10-27 13:33:59
(Cc:-ed the Git development list.) * David Ahern [off-list ref] wrote:
PERF-VERSION-GEN and specifically the git commands are the cause of more delay than the config checks, especially when doing the build in a VM with the kernel source on an NFS mount.
Yes, I have noticed that too.
So, the problem is that we use 'git describe' on the kernel tree
to generate the version string, which is very, very slow if we
are far away from any tagged release - which is the case for the
-tip tree:
comet:~/tip> perf stat --null --repeat 3 git describe
v3.7-rc2-2007-g83e8223
v3.7-rc2-2007-g83e8223
v3.7-rc2-2007-g83e8223
'git describe' is much faster if we are on or near to a tag:
$ git checkout v3.6
$ perf stat --null --repeat 3 git describe
v3.6
v3.6
v3.6
Performance counter stats for 'git describe' (3 runs):
0.020171640 seconds time elapsed ( +- 3.64% )
$ git checkout b34e5f55a1e6
$ perf stat --null --repeat 3 git describe
v3.6-41-gb34e5f5
v3.6-41-gb34e5f5
v3.6-41-gb34e5f5
Performance counter stats for 'git describe' (3 runs):
0.155603676 seconds time elapsed ( +- 0.23% )
The cost on this pretty fast machine is about 1 msecs per commit
- which adds up to about 2.5 seconds during much of the
development cycle.
So maybe we should be using a different version string, for
example, instead of:
v3.7-rc2-2007-g83e8223
this would be perfectly fine:
v3.7-rc2-g83e8223
the 'commit count' is informative but not essential - and in
counting the number of off-tag commits is where much of the
overhead is:
#
# Overhead Command Shared Object Symbol
# ........ ....... .................. ..........................................
#
39.79% git libz.so.1.2.5 [.] 0x000000000000c1fe
26.39% git libz.so.1.2.5 [.] inflate
22.42% git git [.] 0x000000000009bd1e
2.99% git libz.so.1.2.5 [.] adler32
1.23% git libc-2.15.so [.] _int_malloc
0.72% git libc-2.15.so [.] __GI_____strtoull_l_internal
0.67% git libc-2.15.so [.] _int_free
0.62% git libc-2.15.so [.] malloc_consolidate
0.54% git [kernel.kallsyms] [k] clear_page_c
0.32% git [kernel.kallsyms] [k] page_fault
So by switching to the shorter version string that still embedds
the tag and the exact sha1 we'd be able to run this script a
*lot* faster.
Thanks,
Ingo