On 04/15/2010 07:58 AM, Srivatsa Vaddagiri wrote:
On Sun, Apr 11, 2010 at 11:40 PM, Avi Kivity <avi@xxxxxxxxxx
<mailto:avi@xxxxxxxxxx>> 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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