linux-kernel - Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning

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Date:	Tue, 06 Apr 2010 16:28:26 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Darren Hart <dvhltc@...ibm.com>
CC:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"Peter W. Morreale" <pmorreale@...ell.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Sven-Thorsten Dietrich <sdietrich@...ell.com>,
	Chris Mason <chris.mason@...cle.com>,
	John Cooper <john.cooper@...rd-harmonic.com>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive
 spinning

On 04/06/2010 01:59 AM, Darren Hart wrote:
>
>
>>>> I'd be interested in seeing runs where the average number of 
>>>> waiters is 0.2, 0.5, 1, and 2, corresponding to moderate-to-bad 
>>>> contention.
>>>> 25 average waiters on compute bound code means the application 
>>>> needs to be rewritten, no amount of mutex tweaking will help it.
>>>
>>> Perhaps something NR_CPUS threads would be of more interest? 
>>
>> That seems artificial.
>
> How so? Several real world applications use one thread per CPU to 
> dispatch work to, wait for events, etc.

Yes, but that's the best case for spinning.  You could simply use a 
userspace spinlock in this case.


>
>>>> Does the wakeup code select the spinning waiter, or just a random 
>>>> waiter?
>>>
>>> The wakeup code selects the highest priority task in fifo order to 
>>> wake-up - however, under contention it is most likely going to go 
>>> back to sleep as another waiter will steal the lock out from under 
>>> it. This locking strategy is unashamedly about as "unfair" as it gets.
>>
>> Best to avoid the wakeup if we notice the lock was stolen.
>
> You really can't do this precisely. You can read the futex value at 
> various points along the wakeup path, but at some point you have to 
> commit to waking a task, and you still have a race between the time 
> you wake_up_task() and when it is scheduled and attempts the cmpxchg 
> itself.
>

The race is not a problem as it's just an optimization.  If you lose it 
you get a spurious wakeup, if you win you save some cpu cycles.


-- 
error compiling committee.c: too many arguments to function

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