PCH build times (Re: new gerrit-builds & unit tests ...)

On Friday 05 of April 2013, Bjoern Michaelsen wrote:
> On Fri, Apr 05, 2013 at 11:16:06AM -0500, Norbert Thiebaud wrote:
> > I enabled pch because it was claimed to speed up things... I have not
> > benchmark that claim.
>
> I would benchmark that, at least in the old days the results of PCH where
> really mixed:
>
> http://wiki.openoffice.org/wiki/Build_Environment_Effort/Performance
>
> and only reliably gave an advantage on incremental builds. OTOH, generating
> the PCH for a library was a singlethreaded bottleneck in the old build
> system and isnt anymore in gbuild, when doing a toplevel make, so this
> might be mitigated.
>
> Anyway, worth a benchmark IMHO.

 make Library_sw

PCH: 7:01
non-PCH: 20:31

 That's on Win86-6, MSVC2010, hot caches. The difference is of course smaller 
for modules lower in the stack, sw must use tons of includes. Full rebuild of 
master on Win86-6 is now something like 3 hours, without PCH it'd be at least 
4 (I don't remember anymore and it'd take a while to measure).

 I have not tried with GCC, and with Clang it's actually not worth it at all.

-- 
 Lubos Lunak
 l.lunak@suse.cz