[ccache] PATCH: Look at include files' mtimes

Joel Rosdahl joel at rosdahl.net
Wed May 23 15:37:25 MDT 2012

On 20 May 2012 22:49, Justin Lebar <justin.lebar at gmail.com> wrote:
> This patch lets ccache examine an include file's mtime and size in
> lieu of hashing it, during direct mode. [...]

Cool. I've had this thought myself but never tried it out, partly
because (as mentioned in the thread) it's quite brittle.

> The net result is roughly a factor-of-two speedup in ccache hits (*),
> on my machine.

Same for me when running ccache's perf.py.

> I'm not sure if we'd want mtime-snooping enabled by default.

I think it's a feature worth having in ccache, but it needs to be
disabled by default. Checking ctime as well sounds good, but I don't
think we can rely on it - there are just too many strange file systems
to consider.

Somewhat related to this: I'm seriously considering making the whole
direct mode opt-in as well in ccache 3.2 because of the design bug
described in https://bugzilla.samba.org/show_bug.cgi?id=8728#c1. (Any
ideas on how to fix the problem, or make it less likely, are welcome.)

-- Joel

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