[Beowulf] Westmere EX
Ellis H. Wilson III
ellis at runnersroll.com
Thu Apr 7 08:35:24 PDT 2011
On 04/07/11 09:49, Prentice Bisbal wrote:
> On 04/06/2011 10:15 PM, Jon Forrest wrote:
>> On 4/6/2011 5:39 PM, Mark Hahn wrote:
>>> shrug. does anyone have serious experience with real apps on manycore
>>> machines? (I'm familiar with SGI boxes, where 80 is fairly ho-hum,
>>> but they're substantially more exotic/rare/expensive.)
>> I have a couple 48-core 1U boxes. They can build
>> gcc and other large packages very quickly.
> But are the makes definitely running in parallel to take advantage of
> the multiple cores? I haven't built gcc, so don't know if it uses make's
> -j option to do parallel builds.
In general I see quite nice speedups on my four-core machine at home
running Gentoo, but I find running -j > cores up to 2xcores tends to
produce better results as many packages (especially with recursive
makes) tend to mix configuration (low cpu usage) with makes (high cpu
usage). The gentoo handbook itself suggests cores+1 for the -j
parameter. Higher than core -j counts is purely a heuristic, and a few
packages will degrade a bit because in fact 8 (2x4cores) processes are
spawned, each contending heavily for the 4 cores and context switching
starts to slow things down and hurt locality. Once again, I suppose
this is a YMMV situation. It would be cool to hack make to dynamically
throttle parallelization based on cpu usage within some given bounds...
I have access to a 48 core box, so if I get a chance I'll generate a
graph for the list on gcc build times by -j count. Note however that I
don't have root access so I can't clear caches, which should be taken
into account when examining results.
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