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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: |
Tue, 15 Nov 2016 10:36:02 +0000 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
On Mon, Nov 14, 2016 at 06:15:46PM +0100, Paolo Bonzini wrote:
>
>
> 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.
It's not obvious to me that ioeventfd or Linux AIO will become ready
with QEMU_AIO_POLL_MAX_NS=1.
This benchmark is iodepth=1 so there's just a single request. Fam
suggested that maybe Linux AIO is ready immediately but AFAIK this NVMe
device should still take a few microseconds to complete a request
whereas our polling time is 1 nanosecond.
Tracing would reveal what is going on here.
Stefan
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- [Qemu-devel] [RFC 2/3] virtio: poll virtqueues for new buffers, (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
Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode, Christian Borntraeger, 2016/11/14
Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode, Christian Borntraeger, 2016/11/14