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Date: Tue, 25 Aug 2015 18:53:43 -0400 (EDT)
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC 05/38] thread-posix: inline qemu_spin
	functions
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To: "Emilio G. Cota" <cota@braap.org>
Cc: mttcg@greensocs.com, mark burton <mark.burton@greensocs.com>, a rigo <a.rigo@virtualopensystems.com>, qemu-devel@nongnu.org, guillaume delbergue <guillaume.delbergue@greensocs.com>, alex bennee <alex.bennee@linaro.org>, Frederic Konrad <fred.konrad@greensocs.com>


> > I just tested a fetch-and-swap+exp.backoff spinlock with usermode on a
> > program that spawns N threads and each thread performs an 2**M atomic
> > increments
> > on the same variable. That is, a degenerate worst-case kind of contention.
> > N varies from 1 to 64, and M=15 on all runs, 5 runs per experiment:
> > 
> >   http://imgur.com/XpYctyT
> >   With backoff, the per-access latency grows roughly linearly with the
> >   number of
> >   cores, i.e. this is scalable. The other two are clearly superlinear.
> 
> Just tried MCS, CLH and ticket spinlocks (with and without backoff).
> They take essentially forever for this (admittedly worst-case) test;

[snip interesting stuff]

Yeah, fair spinlocks in userspace wasn't a smart suggestion. :)

Paolo