Not as much ccache win as I expected

[Tim Bird <tim.bird@am.sony.com>]

I'm running an automated test which does numerous compiles
of the Linux kernel.  One of the things I do is create a localversion
file at the root of the kernel source tree with a unique identifier
that I use later on in testing.

I started using ccache to improve the performance of my builds,
but found that the hit rate on the cache was not very good.
 $ ccache -s
cache directory                     /home/tbird/.ccache
cache hit                          74416
cache miss                         59400
called for link                    87252
compile failed                        21
not a C/C++ file                  143449
no input file                      49336
files in cache                     42844
cache size                           1.8 Gbytes
max cache size                       2.0 Gbytes

Thinking that the problem might be having a unique version for
every build (and that this change flowed to every file via the
version.h file), I tried building without this change.  I saw
an improvement, but not much.

Is there anything else obvious which is prevents ccache from
working well with a kernel build (that is, anything else that
would, for otherwise identical C files with a similar build,
cause a difference?)

Any tips would be appreciated.
 -- Tim

=============================
Tim Bird
Architecture Group Chair, CE Linux Forum
Senior Staff Engineer, Sony Corporation of America
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