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

[Paolo Bonzini]

On 14/11/2016 18:06, Stefan Hajnoczi wrote:
>>> > > 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.
> Can you spell out which syscall you mean?  Reading the ioeventfd?

I mean ppoll.  If ppoll succeeds without ever going to sleep, you can
achieve the same result with QEMU_AIO_POLL_MAX_NS=1, but cheaper.

Paolo

> The benchmark uses virtio-blk dataplane and iodepth=1 so there shouldn't
> be much IOThread event loop activity besides the single I/O request.
> 
> The reason this puzzles me is that I wouldn't expect poll to succeed
> with QEMU_AIO_POLL_MAX_NS and iodepth=1.

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