Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx> · Wed, 26 Jun 2013 23:24:33 +0530

On 06/26/2013 09:41 PM, Gleb Natapov wrote:
On Wed, Jun 26, 2013 at 07:10:21PM +0530, Raghavendra K T wrote:
On 06/26/2013 06:22 PM, Gleb Natapov wrote:
On Wed, Jun 26, 2013 at 01:37:45PM +0200, Andrew Jones wrote:
On Wed, Jun 26, 2013 at 02:15:26PM +0530, Raghavendra K T wrote:
On 06/25/2013 08:20 PM, Andrew Theurer wrote:
On Sun, 2013-06-02 at 00:51 +0530, Raghavendra K T wrote:
This series replaces the existing paravirtualized spinlock mechanism
with a paravirtualized ticketlock mechanism. The series provides
implementation for both Xen and KVM.

Changes in V9:
- Changed spin_threshold to 32k to avoid excess halt exits that are
    causing undercommit degradation (after PLE handler improvement).
- Added  kvm_irq_delivery_to_apic (suggested by Gleb)
- Optimized halt exit path to use PLE handler

V8 of PVspinlock was posted last year. After Avi's suggestions to look
at PLE handler's improvements, various optimizations in PLE handling
have been tried.

Sorry for not posting this sooner.  I have tested the v9 pv-ticketlock
patches in 1x and 2x over-commit with 10-vcpu and 20-vcpu VMs.  I have
tested these patches with and without PLE, as PLE is still not scalable
with large VMs.

Hi Andrew,

Thanks for testing.

System: x3850X5, 40 cores, 80 threads

1x over-commit with 10-vCPU VMs (8 VMs) all running dbench:
----------------------------------------------------------
						Total
Configuration				Throughput(MB/s)	Notes

3.10-default-ple_on			22945			5% CPU in host kernel, 2% spin_lock in guests
3.10-default-ple_off			23184			5% CPU in host kernel, 2% spin_lock in guests
3.10-pvticket-ple_on			22895			5% CPU in host kernel, 2% spin_lock in guests
3.10-pvticket-ple_off			23051			5% CPU in host kernel, 2% spin_lock in guests
[all 1x results look good here]

Yes. The 1x results look too close

2x over-commit with 10-vCPU VMs (16 VMs) all running dbench:
-----------------------------------------------------------
						Total
Configuration				Throughput		Notes

3.10-default-ple_on			 6287			55% CPU  host kernel, 17% spin_lock in guests
3.10-default-ple_off			 1849			2% CPU in host kernel, 95% spin_lock in guests
3.10-pvticket-ple_on			 6691			50% CPU in host kernel, 15% spin_lock in guests
3.10-pvticket-ple_off			16464			8% CPU in host kernel, 33% spin_lock in guests

I see 6.426% improvement with ple_on
and 161.87% improvement with ple_off. I think this is a very good sign
  for the patches

[PLE hinders pv-ticket improvements, but even with PLE off,
  we still off from ideal throughput (somewhere >20000)]

Okay, The ideal throughput you are referring is getting around atleast
80% of 1x throughput for over-commit. Yes we are still far away from
there.

1x over-commit with 20-vCPU VMs (4 VMs) all running dbench:
----------------------------------------------------------
						Total
Configuration				Throughput		Notes

3.10-default-ple_on			22736			6% CPU in host kernel, 3% spin_lock in guests
3.10-default-ple_off			23377			5% CPU in host kernel, 3% spin_lock in guests
3.10-pvticket-ple_on			22471			6% CPU in host kernel, 3% spin_lock in guests
3.10-pvticket-ple_off			23445			5% CPU in host kernel, 3% spin_lock in guests
[1x looking fine here]

I see ple_off is little better here.

2x over-commit with 20-vCPU VMs (8 VMs) all running dbench:
----------------------------------------------------------
						Total
Configuration				Throughput		Notes

3.10-default-ple_on			 1965			70% CPU in host kernel, 34% spin_lock in guests		
3.10-default-ple_off			  226			2% CPU in host kernel, 94% spin_lock in guests
3.10-pvticket-ple_on			 1942			70% CPU in host kernel, 35% spin_lock in guests
3.10-pvticket-ple_off			 8003			11% CPU in host kernel, 70% spin_lock in guests
[quite bad all around, but pv-tickets with PLE off the best so far.
  Still quite a bit off from ideal throughput]

This is again a remarkable improvement (307%).
This motivates me to add a patch to disable ple when pvspinlock is on.
probably we can add a hypercall that disables ple in kvm init patch.
but only problem I see is what if the guests are mixed.

  (i.e one guest has pvspinlock support but other does not. Host
supports pv)

How about reintroducing the idea to create per-kvm ple_gap,ple_window
state. We were headed down that road when considering a dynamic window at
one point. Then you can just set a single guest's ple_gap to zero, which
would lead to PLE being disabled for that guest. We could also revisit
the dynamic window then.

Can be done, but lets understand why ple on is such a big problem. Is it
possible that ple gap and SPIN_THRESHOLD are not tuned properly?

The one obvious reason I see is commit awareness inside the guest. for
under-commit there is no necessity to do PLE, but unfortunately we do.

atleast we return back immediately in case of potential undercommits,
but we still incur vmexit delay.
But why do we? If SPIN_THRESHOLD will be short enough (or ple windows
long enough) to not generate PLE exit we will not go into PLE handler
at all, no?

Yes. you are right. dynamic ple window was an attempt to solve it.

Probelm is, reducing the SPIN_THRESHOLD is resulting in excess halt
exits in under-commits and increasing ple_window may be sometimes
counter productive as it affects other busy-wait constructs such as
flush_tlb AFAIK.
So if we could have had a dynamically changing SPIN_THRESHOLD too, that
would be nice.

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