Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks
From: Raghavendra K T
Date: Wed Jul 10 2013 - 07:47:21 EST
On 07/10/2013 05:11 PM, Gleb Natapov wrote:
On Wed, Jul 10, 2013 at 04:54:12PM +0530, Raghavendra K T wrote:
Ingo, Gleb,
From the results perspective, Andrew Theurer, Vinod's test results are
pro-pvspinlock.
Could you please help me to know what will make it a mergeable
candidate?.
I need to spend more time reviewing it :) The problem with PV interfaces
is that they are easy to add but hard to get rid of if better solution
(HW or otherwise) appears.
Infact Avi had acked the whole V8 series, but delayed for seeing how
PLE improvement would affect it.
I see that Ingo was happy with it too.
The only addition from that series has been
1. tuning the SPIN_THRESHOLD to 32k (from 2k)
and
2. the halt handler now calls vcpu_on_spin to take the advantage of
PLE improvements. (this can also go as an independent patch into
kvm)
The rationale for making SPIN_THERSHOLD 32k needs big explanation.
Before PLE improvements, as you know,
kvm undercommit scenario was very worse in ple enabled cases.
(compared to ple disabled cases).
pvspinlock patches behaved equally bad in undercommit. Both had
similar reason so at the end there was no degradation w.r.t base.
The reason for bad performance in PLE case was unneeded vcpu
iteration in ple handler resulting in high yield_to calls and double
run queue locks.
With pvspinlock applied, same villain role was played by excessive
halt exits.
But after ple handler improved, we needed to throttle unnecessary halts
in undercommit for pvspinlock to be on par with 1x result.
Make sense. I will review it ASAP. BTW the latest version is V10 right?
Yes. Thank you.
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