On 07/07/2015 07:24 AM, Peter Zijlstra wrote:
On Mon, Jul 06, 2015 at 11:43:06AM -0400, Waiman Long wrote:
Lock waiting in the qrwlock uses the spinlock (qspinlock for x86)
as the waiting queue. This is slower than using MCS lock directly
because of the extra level of indirection causing more atomics to
be used as well as 2 waiting threads spinning on the lock cacheline
instead of only one.
This needs a better explanation. Didn't we find with the qspinlock thing
that the pending spinner improved performance on light loads?
Taking it out seems counter intuitive, we could very much like these two
the be the same.
Yes, for lightly loaded case, using raw_spin_lock should have an
advantage. It is a different matter when the lock is highly contended.
In this case, having the indirection in qspinlock will make it slower. I
struggle myself as to whether to duplicate the locking code in qrwlock.
So I send this patch out to test the water. I won't insist if you think
this is not a good idea, but I do want to get the previous 2 patches in
which should not be controversial.
Cheers,
Longman
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