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

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

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.