(Cc:-ed the Git development list.)

* David Ahern <dsah...@gmail.com> 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

'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

 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

 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:


this would be perfectly fine:


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                                      
# ........  .......  ..................  
    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.


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