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

[Gleb Natapov <gleb@redhat.com>]

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?

--
			Gleb.