Re: [PATCH][RFC] FPU LWP 0/5: patch description

[Wei Huang <wei.huang2@amd.com>]

Hi Keir,

I ran a quick test to calculate the overhead of __fpu_unlazy_save() and 
__fpu_unlazy_restore(), which are used to save/restore LWP state. Here 
are the results:

(1) tsc_total: total time used for context_switch() in x86/domain.c
(2) tsc_unlazy: total time used for __fpu_unlazy_save() + 
__fpu_unlazy_retore()

One example:
(XEN) tsc_unlazy=0x00000000008ae174
(XEN) tsc_total=0x00000001028b4907

So the overhead is about 0.2% of total time used by context_switch(). Of 
course, this is just one example. I would say the overhead ratio would 
be <1% for most cases.

Thanks,
-Wei



On 04/14/2011 04:09 PM, Keir Fraser wrote:
> On 14/04/2011 21:37, "Wei Huang"<wei.huang2@amd.com>  wrote:
>
>> The following patches support AMD lightweight profiling.
>>
>> Because LWP isn't tracked by CR0.TS bit, we clean up the FPU code to
>> handle lazy and unlazy FPU states differently. Lazy FPU state (such as
>> SSE, YMM) is handled when #NM is triggered. Unlazy state, such as LWP,
>> is saved and restored on each vcpu context switch. To simplify the code,
>> we also add a mask option to xsave/xrstor function.
> How much cost is added to context switch paths in the (overwhelmingly
> likely) case that LWP is not being used by the guest? Is this adding a whole
> lot of unconditional overhead for a feature that noone uses?
>
>   -- Keir
>
>> Thanks,
>> -Wei
>>
>>
>>
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>