Messages in this thread | | | Date | Thu, 15 Apr 2010 11:18:43 +0300 | From | Avi Kivity <> | Subject | Re: VM performance issue in KVM guests. |
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On 04/15/2010 07:58 AM, Srivatsa Vaddagiri wrote: > On Sun, Apr 11, 2010 at 11:40 PM, Avi Kivity <avi@redhat.com > <mailto:avi@redhat.com>> wrote: > > The current handing of PLE is very suboptimal. With proper > directed yield we should be much better there. > > > > Hi Avi, > By directed yield, do you mean transfer the timeslice of > one thread (which is contending for a lock) to another thread (which > is holding a lock)?
It's a priority transfer (in CFS terms, vruntime) (we don't know who holds the lock, so we pick a co-vcpu at random).
> If at that point in time, the lock-holder thread/VCPU is actually not > running currently, ie it is at the back of the runqueue, would it help > much? In such case, it will take time for the lock holder to run again > and the default timeslice it would have got could have been sufficient > to release the lock?
The idea is to increase the chances to the target vcpu to run, and to decrease the changes of the spinner to run (hopefully they change places).
> > I am also working on a prototype for some other technique here - to > avoid preempting guest threads/VCPUs in the middle of their > (spin-lock) critical section. This requires guest to hint host when > there are in such a section. [1] has shown 33% improvement to an > apache benchmark based on this idea. >
Certainly that has even greater potential for Linux guests. Note that we spin on mutexes now, so we need to prevent preemption while the lock owner is running.
-- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.
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