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SubjectRe: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning
On 04/06/2010 12:54 AM, Darren Hart wrote:
> Avi Kivity wrote:
>> On 04/05/2010 11:23 PM, Darren Hart wrote:
>>> In-Reply-To:
>>>
>>> NOT FOR INCLUSION
>>>
>>> The following patch series implements a new experimental kernel side
>>> futex mutex
>>> via new FUTEX_LOCK and FUTEX_LOCK_ADAPTIVE futex op codes. The
>>> adaptive spin
>>> follows the kernel mutex model of allowing one spinner until the
>>> lock is
>>> released or the owner is descheduled. The patch currently allows the
>>> user to
>>> specify if they want no spinning, a single adaptive spinner, or
>>> multiple
>>> spinners (aggressive adaptive spinning, or aas... which I have
>>> mistyped as "ass"
>>> enough times to realize a better term is indeed required :-).
>>
>> An interesting (but perhaps difficult to achieve) optimization would
>> be to spin in userspace.
>
> I couldn't think of a lightweight way to determine when the owner has
> been scheduled out in userspace. Kernel assistance is required. You
> could do this on the schedule() side of things, but I figured I'd get
> some strong pushback if I tried to add a hook into descheduling that
> flipped a bit in the futex value stating the owner was about to
> deschedule(). Still, that might be something to explore.

In the futex value it's hopeless (since a thread can hold many locks),
but I don't think it's unreasonable to set a bit in the thread local
storage area. The futex format would then need to be extended to
contain a pointer to this bit.


>
>>
>> How many cores (or hardware threads) does this machine have?
>
> Sorry, I meant to include that. I tested on an 8 CPU (no hardware
> threads) 2.6 GHz Opteron 2218 (2 QuadCore CPUs) system.
>
> > At 10%
>> duty cycle you have 25 waiters behind the lock on average. I don't
>> think this is realistic, and it means that spinning is invoked only
>> rarely.
>
> Perhaps some instrumentation is in order, it seems to get invoked
> enough to achieve some 20% increase in lock/unlock iterations. Perhaps
> another metric would be of more value - such as average wait time?

Why measure an unrealistic workload?

>
>> 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.

> At 10% that's about .8 and at 25% the 2 of your upper limit. I could
> add a few more duty-cycle points and make 25% the max. I'll kick that
> off and post the results... probably tomorrow, 10M iterations takes a
> while, but makes the results relatively stable.

Thanks. But why not vary the number of threads as well?

>
>> 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.


--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.



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