[ccache] [patch] Massive performance degradation with direct mode due to bypassing filesystem cache with mmap (Mac/PPC)

[Janne Johansson]

2010/8/23 Christian Lohmaier
<lohmaier+ccache-ml at googlemail.com<lohmaier%2Bccache-ml at googlemail.com>
>

> Hi *,
>
> nice to see ccache back active :-) - but I was shocked when I tried
> direct mode on Mac/PPC. For comparison:
>
> real time for compiling OOo's binfilter module in preprocessor mode:
> 8:30 (8 Minutes, 30 Seconds), with filled cache
> real time for compiling OOo's binfilter module in direct mode: 35:50
> (no typo thirty-five minutes compared to eight)
>
> While investigating this problem, I found the root cause: Using mmap
> to load all the include files bypasses the filesystem cache, and I/O
> performance on Mac isn't great to begin with… (On Mac Intel with
> RAID0, direct mode performed slightly better than preprocessor mode,
> thus you might only experience it on systems with pathetic I/O)
>
>
Haven't done a similar benchmark, but I know OpenBSD will not share caches
between mmap()
and read()s, so I could expect similar results there.


-- 
 To our sweethearts and wives.  May they never meet. -- 19th century toast