Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode

[Paolo Bonzini]

On 14/11/2016 16:26, Stefan Hajnoczi wrote:
> On Fri, Nov 11, 2016 at 01:59:25PM -0600, Karl Rister wrote:
>> QEMU_AIO_POLL_MAX_NS      IOPs
>>                unset    31,383
>>                    1    46,860
>>                    2    46,440
>>                    4    35,246
>>                    8    34,973
>>                   16    46,794
>>                   32    46,729
>>                   64    35,520
>>                  128    45,902
> 
> The environment variable is in nanoseconds.  The range of values you
> tried are very small (all <1 usec).  It would be interesting to try
> larger values in the ballpark of the latencies you have traced.  For
> example 2000, 4000, 8000, 16000, and 32000 ns.
> 
> Very interesting that QEMU_AIO_POLL_MAX_NS=1 performs so well without
> much CPU overhead.

That basically means "avoid a syscall if you already know there's
something to do", so in retrospect it's not that surprising.  Still
interesting though, and it means that the feature is useful even if you
don't have CPU to waste.

Paolo

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