Re: [PATCH 0/6] x86: reduce paravirtualized spinlock overhead

[Juergen Gross]

On 05/05/2015 07:21 PM, Jeremy Fitzhardinge wrote:
> On 05/03/2015 10:55 PM, Juergen Gross wrote:
> > I did a small measurement of the pure locking functions on bare metal
> > without and with my patches.
> >
> > spin_lock() for the first time (lock and code not in cache) dropped from
> > about 600 to 500 cycles.
> >
> > spin_unlock() for first time dropped from 145 to 87 cycles.
> >
> > spin_lock() in a loop dropped from 48 to 45 cycles.
> >
> > spin_unlock() in the same loop dropped from 24 to 22 cycles.
>
> Did you isolate icache hot/cold from dcache hot/cold? It seems to me the
> main difference will be whether the branch predictor is warmed up rather
> than if the lock itself is in dcache, but its much more likely that the
> lock code is icache if the code is lock intensive, making the cold case
> moot. But that's pure speculation.
>
> Could you see any differences in workloads beyond microbenchmarks?
>
> Not that its my call at all, but I think we'd need to see some concrete
> improvements in real workloads before adding the complexity of more pvops.

I did another test on a larger machine:

25 kernel builds (time make -j 32) on a 32 core machine. Before each
build "make clean" was called, the first result after boot was omitted
to avoid disk cache warmup effects.

System time without my patches: 861.5664 +/- 3.3665 s
               with my patches: 852.2269 +/- 3.6629 s


Juergen
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