Re: [RFC PATCH 0/3] generic hypercall support

Gregory Haskins
2009-05-13T10:53:54Z

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Anthony Liguori wrote:
> Gregory Haskins wrote:
>>
>> So, yes, the delta from PIO to HC is 350ns. Yes, this is a ~1.4%
>> improvement. So what? Its still an improvement. If that improvement
>> were for free, would you object? And we all know that this change isn't
>> "free" because we have to change some code (+128/-0, to be exact). But
>> what is it specifically you are objecting to in the first place? Adding
>> hypercall support as an pv_ops primitive isn't exactly hard or complex,
>> or even very much code.
>>
>
> Where does 25us come from? The number you post below are 33us and 66us.

<snip>

The 25us is approximately the max from an in-kernel harness strapped
directly to the driver gathered informally during testing. The 33us is
from formally averaging multiple runs of a userspace socket app in
preparation for publishing. I consider the 25us the "target goal" since
there is obviously overhead that a socket application deals with that
theoretically a guest bypasses with the tap-device. Note that the
socket application itself often sees < 30us itself...this was just a
particularly "slow" set of runs that day.

Note that this is why I express the impact as "approximately" (e.g.
"~4%"). Sorry for the confusion.

-Greg

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