[Libreoffice] ccache support

[Norbert Thiebaud]

On Fri, Sep 9, 2011 at 1:22 PM, Lubos Lunak <l.lunak at suse.cz> wrote:
>  So if ccache has 0% hit ratio, there is 17% overhead in this specific case.
> With 100% hit ratio, 73% is saved. That, if my math is right, means at least
> 19% ccache hit ratio is required for breaking even.

I'm am gathering numbers, in the mean time
http://tinderbox.libreoffice.org/MASTER/status.html give me some good
data about the ccache hit.
by looking at the time it take for each build iteration... most of
them show that the hit ration is almost always very very high (90%+),
unless some commit touch a commonly included header.

Now granted, once we can reliably do incremental build, the question
of the effectiveness of ccache will be much more acute.
presumable incremental build only try to compile stuff that _need_ to
be compiled and therefore should be a ccache miss... so with perfect
incremental build the ccache hit should be 0%

when that is true, we could indeed change the default for ccache to disable....

FYI: my ccachs -s for my MacOSX buildbot is

n_th at tpamac ~/.ssh $ccache -s
cache directory                     /Volumes/ccache_ramdisk
cache hit (direct)                481347
cache hit (preprocessed)           19049
cache miss                         28588
called for link                    25314
multiple source files                102
compile failed                      8524
preprocessor error                  4318
bad compiler arguments              1768
unsupported source language          408
autoconf compile/link              35802
unsupported compiler option        60295
no input file                       7446
files in cache                     43163
cache size                           1.8 Gbytes
max cache size                       2.3 Gbytes


so a hit ration of  94.6%  (481347 + 19049) / (481347 + 19049 + 25588)

(note that cache had been wiped out about 2 days ago due to an IPL of
the box -- the cache is in ramdisk, so it does not survive a reboot)

Norbert