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Message-ID: <20130930065703.GB13584@gmail.com>
Date: Mon, 30 Sep 2013 08:57:03 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Davidlohr Bueso <davidlohr@...com>,
Waiman Long <Waiman.Long@...com>, Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Rik van Riel <riel@...hat.com>,
Peter Hurley <peter@...leysoftware.com>,
Davidlohr Bueso <davidlohr.bueso@...com>,
Alex Shi <alex.shi@...el.com>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Andrea Arcangeli <aarcange@...hat.com>,
Matthew R Wilcox <matthew.r.wilcox@...el.com>,
Dave Hansen <dave.hansen@...el.com>,
Michel Lespinasse <walken@...gle.com>,
Andi Kleen <andi@...stfloor.org>,
"Chandramouleeswaran, Aswin" <aswin@...com>,
"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path
* Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> [...]
>
> And your numbers for Ingo's patch:
>
> > After testing Ingo's anon-vma rwlock_t conversion (v2) on a 8 socket,
> > 80 core system with aim7, I am quite surprised about the numbers -
> > considering the lack of queuing in rwlocks. A lot of the tests didn't
> > show hardly any difference, but those that really contend this lock
> > (with high amounts of users) benefited quite nicely:
> >
> > Alltests: +28% throughput after 1000 users and runtime was reduced from
> > 7.2 to 6.6 secs.
> >
> > Custom: +61% throughput after 100 users and runtime was reduced from 7
> > to 4.9 secs.
> >
> > High_systime: +40% throughput after 1000 users and runtime was reduced
> > from 19 to 15.5 secs.
> >
> > Shared: +30.5% throughput after 100 users and runtime was reduced from
> > 6.5 to 5.1 secs.
> >
> > Short: Lots of variance in the numbers, but avg of +29% throughput -
> > no particular performance degradation either.
>
> Are just overwhelming, in my opinion. The conversion *from* a spinlock
> never had this kind of support behind it.
Agreed. Especially given how primitive rwlock_t is especially on 80 cores,
this is really a no-brainer conversion.
I have to say I am surprised by the numbers - after so many years it's
still amazing how powerful the "get work done and don't interrupt it"
batching concept is in computing...
> Btw, did anybody run Ingo's patch with lockdep and the spinlock sleep
> debugging code to verify that we haven't introduced any problems wrt
> sleeping since the lock was converted into a rw-semaphore?
>
> Because quite frankly, considering these kinds of numbers, I really
> don't see how we could possibly make excuses for keeping that
> rw-semaphore unless there is some absolutely _horrible_ latency issue?
Given that there's only about a dozen critical sections that this lock
covers I simply cannot imagine any latency problem that couldn't be fixed
in some other fashion. (shrinking the critical section, breaking up a bad
loop, etc.)
[ Btw., if PREEMPT_RT goes upstream we might not even need to break
latencies all that much: people whose usecase values scheduling latency
above throughput would run such a critical section preemptible anyway. ]
Thanks,
Ingo
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