On Thu, Sep 27, 2012 at 07:09:28AM +0200, Mike Galbraith wrote:
> > The way I understand it is, you either want to share L2 with a process,
> > because, for example, both working sets fit in the L2 and/or there's
> > some sharing which saves you moving everything over the L3. This is
> > where selecting a core on the same L2 is actually a good thing.
>
> Yeah, and if the wakee can't get to the L2 hot data instantly, it may be
> better to let wakee drag the data to an instantly accessible spot.
Yep, then moving it to another L2 is the same.
[ … ]
> > A crazy thought: one could go and sample tasks while running their
> > timeslices with the perf counters to know exactly what type of workload
> > we're looking at. I.e., do I have a large number of L2 evictions? Yes,
> > then spread them out. No, then select the other core on the L2. And so
> > on.
>
> Hm. That sampling better be really cheap. Might help...
Yeah, that's why I said sampling and not run the perfcounters during
every timeslice.
But if you count the proper events, you should be able to know exactly
what the workload is doing (compute-bound, io-bound, contention, etc...)
> but how does that affect pgbench and ilk that must spread regardless
> of footprints.
Well, how do you measure latency of the 1 process in the 1:N case? Maybe
pipeline stalls of the 1 along with some way to recognize it is the 1 in
the 1:N case.
Hmm.
--
Regards/Gruss,
Boris.
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