Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptivespinning

From: Avi Kivity
Date: Tue Apr 06 2010 - 09:31:19 EST

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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