Re: [PATCH 0/2] Batch writes to MMIO

[Avi Kivity <avi@qumranet.com>]

Laurent Vivier wrote:
> Le mercredi 23 avril 2008 à 17:05 +0300, Avi Kivity a écrit :
>   
>> Laurent Vivier wrote:
>>     
>>> These two patches allow to batch writes to MMIO.
>>>
>>> When kernel has to send MMIO writes to userspace, it stores them
>>> in memory until it has to pass the hand to userspace for another
>>> reason. This avoids to have too many context switches on operations
>>> that can wait.
>>>
>>>   
>>>       
>> Did you obtain any measurable performance benefit?
>>     
>
> Well, the problem is how to measure it. Really, I don't know.
>
> But when I add traces I saw MMIO writes are batched: by group of 170
> (this is the max in a page) at the beginning,

That's the lovely animation. You can time XP startup, or look at system 
time in 'top' once it stabilizes.  Also compare total host_state_reloads 
to boot in kvm_stat (each costs 5 usec, after discount).

>  and by group of 10 with XP
> when we move a window.
>   

That's probably programming the cirrus blitter, also a candidate for 
batching, but probably not measurable.  Maybe one of those Windows 
graphics benchmarks which draw tons of rectangles very fast and cause 
epilepsy seizures.

I guess most interesting is e1000, if we need to do more than one mmio 
to start transmitting.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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