Messages in this thread | | | Date | Tue, 14 Jul 2015 22:45:07 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 6/7] locking/qspinlock: A fairer queued unfair lock |
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On Tue, Jul 14, 2015 at 02:47:16PM -0400, Waiman Long wrote: > On 07/12/2015 04:21 AM, Peter Zijlstra wrote: > >On Sat, Jul 11, 2015 at 04:36:57PM -0400, Waiman Long wrote: > >>For a virtual guest with the qspinlock patch, a simple unfair byte lock > >>will be used if PV spinlock is not configured in or the hypervisor > >>isn't either KVM or Xen. > >Why do we care about this case enough to add over 300 lines of code? > > From my testing, I found the queued unfair lock to be superior to both the > byte lock or the PV qspinlock when the VM is overcommitted. My current > opinion is to use PV qspinlock for VMs that are not likely to run into the > overcommited problem. For other VMs that are overcommitted, it will be > better to use the queued unfair lock. However, this is a choice that the > system administrators have to made. That is also the reason why I sent out > another patch to add a KVM command line option to disable PV spinlock like > what Xen already has. In this way, depending on how the kernel is booted, we > can choose either PV qspinlock or queued unfair lock.
No, we're not going to add another 300 line lock implementation and a knob.
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