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

[Avi Kivity <avi@redhat.com>]

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.

> I also got perf top output to analyse the difference. Difference comes
> because of flushtlb (and also spinlock).

That's in the guest, yes?

> 
> 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?




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