Messages in this thread | | | Subject | Re: [PATCH -v8][RFC] mutex: implement adaptive spinning | From | Peter Zijlstra <> | Date | Mon, 12 Jan 2009 18:13:08 +0100 |
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On Mon, 2009-01-12 at 18:13 +0200, Avi Kivity wrote:
> One thing that worries me here is that the spinners will spin on a > memory location in struct mutex, which means that the cacheline holding > the mutex (which is likely to be under write activity from the owner) > will be continuously shared by the spinners, slowing the owner down when > it needs to unshare it. One way out of this is to spin on a location in > struct mutex_waiter, and have the mutex owner touch it when it schedules > out.
Yeah, that is what pure MCS locks do -- however I don't think its a feasible strategy for this spin/sleep hybrid.
> So: > - each task_struct has an array of currently owned mutexes, appended to > by mutex_lock()
That's not going to fly I think. Lockdep does this but its very expensive and has some issues. We're currently at 48 max owners, and still some code paths manage to exceed that.
> - mutex waiters spin on mutex_waiter.wait, which they initialize to zero > - when switching out of a task, walk the mutex list, and for each mutex, > bump each waiter's wait variable, and clear the owner array
Which is O(n).
> - when unlocking a mutex, bump the nearest waiter's wait variable, and > remove from the owner array > > Something similar might be done to spinlocks to reduce cacheline > contention from spinners and the owner.
Spinlocks can use 'pure' MCS locks.
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