linux-kernel - Re: [PATCH 2/2] locking/qrwlock: Don't contend with readers when setting _QW

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Date:	Tue, 09 Jun 2015 11:23:06 -0400
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH 2/2] locking/qrwlock: Don't contend with readers when
 setting _QW_WAITING

On 06/09/2015 08:04 AM, Peter Zijlstra wrote:
> On Mon, Jun 08, 2015 at 06:20:44PM -0400, Waiman Long wrote:
>> The current cmpxchg() loop in setting the _QW_WAITING flag for writers
>> in queue_write_lock_slowpath() will contend with incoming readers
>> causing possibly extra cmpxchg() operations that are wasteful. This
>> patch changes the code to do a byte cmpxchg() to eliminate contention
>> with new readers.
> This is very narrow, would not the main cost still be the cacheline
> transfers?
>
> Do you have any numbers to back this? I would feel much better about
> this if there's real numbers attached.

I have just sent out a v2 patch with the microbenchmark data for the 2nd 
patch. The extra cmpxchg() because of reader contention should have 
about the same cost of a cacheline miss. The performance gain depends on 
how often this kind of reader contention happens.

Regards,
Longman
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