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Date: Wed, 26 Jun 2013 14:15:26 +0530
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Organization: IBM
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Subject: Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks
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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)

/me thinks

>
> In summary, I would state that the pv-ticket is an overall win, but the
> current PLE handler tends to "get in the way" on these larger guests.
>
> -Andrew
>

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