linux-kernel - Re: [PATCH RFC ticketlock] Auto-queued ticketlock

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Date:	Tue, 11 Jun 2013 10:53:06 -0700
From:	Davidlohr Bueso <davidlohr.bueso@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	赖江山 <laijs@...fujitsu.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>, niv@...ibm.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Darren Hart <darren@...art.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	sbw@....edu
Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock

On Mon, 2013-06-10 at 17:51 -0700, Linus Torvalds wrote:
> On Mon, Jun 10, 2013 at 5:44 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > OK, I haven't found a issue here yet, but youss are beiing trickssy! We
> > don't like trickssy, and we must find precccciouss!!!
> 
> .. and I personally have my usual reservations. I absolutely hate
> papering over scalability issues, and historically whenever people
> have ever thought that we want complex spinlocks, the problem has
> always been that the locking sucks.
> 
> So reinforced by previous events, I really feel that code that needs
> this kind of spinlock is broken and needs to be fixed, rather than
> actually introduce tricky spinlocks.
> 
> So in order to merge something like this, I want (a) numbers for real
> loads and (b) explanations for why the spinlock users cannot be fixed.

I hate to be the bearer of bad news but I got some pretty bad aim7
performance numbers with this patch on an 8-socket (80 core) 256 Gb
memory DL980 box against a vanilla 3.10-rc4 kernel:

* shared workload: 
10-100 users is in the noise area.
100-2000 users: -13% throughput.

* high_systime workload: 
10-700 users is in the noise area.
700-2000 users: -55% throughput.

* disk:
10-100 users -57% throughput.
100-1000 users: -25% throughput
1000-2000 users: +8% throughput (this patch only benefits when we have a
lot of concurrency).

* custom:
10-100 users: -33% throughput.
100-2000 users: -46% throughput.

* alltests:
10-1000 users is in the noise area.
1000-2000 users: -10% throughput.

One notable exception is the short workload where we actually see
positive numbers:
10-100 users: +40% throughput.
100-2000 users: +69% throughput.

Thanks,
Davidlohr

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