linux-kernel - Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

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Date:	Thu, 06 Oct 2011 10:40:01 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Stephan Diestelhorst <stephan.diestelhorst@....com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Jan Beulich <JBeulich@...e.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Piggin <npiggin@...nel.dk>,
	the arch/x86 maintainers <x86@...nel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

On 10/06/2011 07:04 AM, Stephan Diestelhorst wrote:
> On Wednesday 28 September 2011, 14:49:56 Linus Torvalds wrote:
>> Which certainly should *work*, but from a conceptual standpoint, isn't
>> it just *much* nicer to say "we actually know *exactly* what the upper
>> bits were".
> Well, we really do NOT want atomicity here. What we really rather want
> is sequentiality: free the lock, make the update visible, and THEN
> check if someone has gone sleeping on it.
>
> Atomicity only conveniently enforces that the three do not happen in a
> different order (with the store becoming visible after the checking
> load).
>
> This does not have to be atomic, since spurious wakeups are not a
> problem, in particular not with the FIFO-ness of ticket locks.
>
> For that the fence, additional atomic etc. would be IMHO much cleaner
> than the crazy overflow logic.

All things being equal I'd prefer lock-xadd just because its easier to
analyze the concurrency for, crazy overflow tests or no.  But if
add+mfence turned out to be a performance win, then that would obviously
tip the scales.

However, it looks like locked xadd is also has better performance:  on
my Sandybridge laptop (2 cores, 4 threads), the add+mfence is 20% slower
than locked xadd, so that pretty much settles it unless you think
there'd be a dramatic difference on an AMD system.

(On Nehalem it was much less dramatic 2% difference, but still in favour
of locked xadd.)

This is with dumb-as-rocks run it in a loop with "time" benchmark, but
the results are not very subtle.

    J

View attachment "add-barrier.c" of type "text/x-csrc" (285 bytes)

View attachment "locked-xadd.c" of type "text/x-csrc" (423 bytes)