Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE handler

From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
To: Avi Kivity <avi@redhat.com>
Cc: Rik van Riel <riel@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Srikar <srikar@linux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com>,
	KVM <kvm@vger.kernel.org>, Jiannan Ouyang <ouyang@cs.pitt.edu>,
	chegu vinod <chegu_vinod@hp.com>,
	"Andrew M. Theurer" <habanero@linux.vnet.ibm.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>,
	Gleb Natapov <gleb@redhat.com>
Subject: Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE handler
Date: Thu, 04 Oct 2012 16:19:15 +0530	[thread overview]
Message-ID: <506D69AB.7020400@linux.vnet.ibm.com> (raw)
In-Reply-To: <506C7057.6000102@redhat.com>

On 10/03/2012 10:35 PM, Avi Kivity wrote:
> On 10/03/2012 02:22 PM, Raghavendra K T wrote:
>>> So I think it's worth trying again with ple_window of 20000-40000.
>>>
>>
>> Hi Avi,
>>
>> I ran different benchmarks increasing ple_window, and results does not
>> seem to be encouraging for increasing ple_window.
>
> Thanks for testing! Comments below.
>
>> Results:
>> 16 core PLE machine with 16 vcpu guest.
>>
>> base kernel = 3.6-rc5 + ple handler optimization patch
>> base_pleopt_8k = base kernel + ple window = 8k
>> base_pleopt_16k = base kernel + ple window = 16k
>> base_pleopt_32k = base kernel + ple window = 32k
>>
>>
>> Percentage improvements of benchmarks w.r.t base_pleopt with ple_window = 4096
>>
>> 		base_pleopt_8k	base_pleopt_16k	base_pleopt_32k
>> -----------------------------------------------------------------			
>> kernbench_1x	-5.54915	-15.94529	-44.31562
>> kernbench_2x	-7.89399	-17.75039	-37.73498
>
> So, 44% degradation even with no overcommit?  That's surprising.

Yes. Kernbench was run with #threads = #vcpu * 2 as usual. Is it
spending 8 times the original ple_window cycles for 16 vcpus
significant?

>
>> I also got perf top output to analyse the difference. Difference comes
>> because of flushtlb (and also spinlock).
>
> That's in the guest, yes?

Yes. Perf is in guest.

>
>>
>> Ebizzy run for 4k ple_window
>> -  87.20%  [kernel]  [k] arch_local_irq_restore
>>     - arch_local_irq_restore
>>        - 100.00% _raw_spin_unlock_irqrestore
>>           + 52.89% release_pages
>>           + 47.10% pagevec_lru_move_fn
>> -   5.71%  [kernel]  [k] arch_local_irq_restore
>>     - arch_local_irq_restore
>>        + 86.03% default_send_IPI_mask_allbutself_phys
>>        + 13.96% default_send_IPI_mask_sequence_phys
>> -   3.10%  [kernel]  [k] smp_call_function_many
>>       smp_call_function_many
>>
>>
>> Ebizzy run for 32k ple_window
>>
>> -  91.40%  [kernel]  [k] arch_local_irq_restore
>>     - arch_local_irq_restore
>>        - 100.00% _raw_spin_unlock_irqrestore
>>           + 53.13% release_pages
>>           + 46.86% pagevec_lru_move_fn
>> -   4.38%  [kernel]  [k] smp_call_function_many
>>       smp_call_function_many
>> -   2.51%  [kernel]  [k] arch_local_irq_restore
>>     - arch_local_irq_restore
>>        + 90.76% default_send_IPI_mask_allbutself_phys
>>        + 9.24% default_send_IPI_mask_sequence_phys
>>
>
> Both the 4k and the 32k results are crazy.  Why is
> arch_local_irq_restore() so prominent?  Do you have a very high
> interrupt rate in the guest?

How to measure if I have high interrupt rate in guest?
 From /proc/interrupt numbers I am not able to judge :(

I went back and got the results on a 32 core machine with 32 vcpu guest.
Strangely, I got result supporting the claim that increasing ple_window 
helps for non-overcommitted scenario.

32 core 32 vcpu guest 1x scenarios.

ple_gap = 0
kernbench: Elapsed Time 38.61
ebizzy: 7463 records/s

ple_window = 4k
kernbench: Elapsed Time 43.5067
ebizzy:    2528 records/s

ple_window = 32k
kernebench : Elapsed Time 39.4133
ebizzy: 7196 records/s

perf top for ebizzy for above:
ple_gap = 0
-  84.74%  [kernel]  [k] arch_local_irq_restore
    - arch_local_irq_restore
       - 100.00% _raw_spin_unlock_irqrestore
          + 50.96% release_pages
          + 49.02% pagevec_lru_move_fn
-   6.57%  [kernel]  [k] arch_local_irq_restore
    - arch_local_irq_restore
       + 92.54% default_send_IPI_mask_allbutself_phys
       + 7.46% default_send_IPI_mask_sequence_phys
-   1.54%  [kernel]  [k] smp_call_function_many
      smp_call_function_many

ple_window = 32k
-  84.47%  [kernel]  [k] arch_local_irq_restore
    + arch_local_irq_restore
-   6.46%  [kernel]  [k] arch_local_irq_restore
    - arch_local_irq_restore
       + 93.51% default_send_IPI_mask_allbutself_phys
       + 6.49% default_send_IPI_mask_sequence_phys
-   1.80%  [kernel]  [k] smp_call_function_many
    - smp_call_function_many
       + 99.98% native_flush_tlb_others

ple_window = 4k
-  91.35%  [kernel]  [k] arch_local_irq_restore
    - arch_local_irq_restore
       - 100.00% _raw_spin_unlock_irqrestore
          + 53.19% release_pages
          + 46.81% pagevec_lru_move_fn
-   3.90%  [kernel]  [k] smp_call_function_many
      smp_call_function_many
-   2.94%  [kernel]  [k] arch_local_irq_restore
    - arch_local_irq_restore
       + 93.12% default_send_IPI_mask_allbutself_phys
       + 6.88% default_send_IPI_mask_sequence_phys

Let me know if I can try something here..
/me confused :(