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Date: Tue, 24 May 2016 20:10:04 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20160525001004.GA3650@flamenco>
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Subject: Re: [Qemu-devel] [PATCH v5 13/18] qht: support parallel writes
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To: Sergey Fedorov <serge.fdrv@gmail.com>
Cc: QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@listserver.greensocs.com>, Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Richard Henderson <rth@twiddle.net>

On Wed, May 25, 2016 at 01:17:21 +0300, Sergey Fedorov wrote:
> >> With this implementation we could:
> >>  (1) get rid of qht_map::stale
> >>  (2) don't waste cycles waiting for resize to complete
> > I'll include this in v6.
> 
> How is it by perf?

Not much of a difference, since resize is a slow path. Calling
qht-bench with lots of update and resize threads performs very
poorly either way =D

I like the change though because using the mutex here simplifies
the resize code; there's no guilt anymore attached to holding the lock
for some time (e.g. when allocating a new, possible quite large,
map), whereas with the spinlock we would allocate it before
acquiring the lock, without knowing whether the allocation would
be needed in the end.

		Emilio