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Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode |
Date: |
Mon, 14 Nov 2016 17:06:11 +0000 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
On Mon, Nov 14, 2016 at 04:29:49PM +0100, Paolo Bonzini wrote:
> 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.
Can you spell out which syscall you mean? Reading the ioeventfd?
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.
Thanks,
Stefan
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- Re: [Qemu-devel] [RFC 1/3] aio-posix: add aio_set_poll_handler(), (continued)
- Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode, Fam Zheng, 2016/11/16
- Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode, Karl Rister, 2016/11/14
- Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode, Karl Rister, 2016/11/14
- Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode, Paolo Bonzini, 2016/11/14
- Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode, Stefan Hajnoczi, 2016/11/15
- Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode, Karl Rister, 2016/11/15
Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode, no-reply, 2016/11/13