Messages in this thread | | | Date | Tue, 06 Apr 2010 16:30:35 +0300 | From | Avi Kivity <> | Subject | Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning |
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On 04/06/2010 02:15 AM, Darren Hart wrote: > Avi Kivity wrote: > >>>> 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), > > It can, but there is a futex value per lock. If the task_struct had a > list of held futex locks (as it does for pi futex locks) the > deschedule() path could walk that and mark the FUTEX_OWNER_SLEEPING bit. >
You don't want the context switch path to walk a list whose length is user controlled.
>> 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. > > This appears to be 1 bit per task instead of 1 bit per lock.
Yes. O(1) on context switch instead of O(n).
> Also, the value is thread-specific... so how would a potential waiter > be able to determine if the owner of a particular lock was running or > not with this method? ... maybe I'm missing some core bit about > TLS... are you talking about pthread_key_create() and > pthread_getspecific() ?
The lock would need to contain a pointer to the owning task. Could be set with cmpxchg{8,16}b on x86.
-- error compiling committee.c: too many arguments to function
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