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

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Message-ID: <1370974231.9844.212.camel@gandalf.local.home>
Date:	Tue, 11 Jun 2013 14:10:31 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Davidlohr Bueso <davidlohr.bueso@...com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.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 Tue, 2013-06-11 at 10:53 -0700, Davidlohr Bueso wrote:

> 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:

This doesn't surprise me as the spin lock now contains a function call
on any contention. Not to mention the added i$ pressure on the embedded
spinlock code having to setup a function call.

Even if the queues are not used, it adds a slight overhead to all
spinlocks, due to the code size increase as well as a function call on
all contention, which will also have an impact on i$ and branch
prediction.


> 
> * 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

Perhaps this actually started using the queues?

> 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.

Perhaps short work loads have a cold cache, and the impact on cache is
not as drastic?

It would be interesting to see what perf reports on these runs.

-- Steve


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