From: Stefan Hajnoczi <stefanha@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Nir Soffer <nsoffer@redhat.com>,
qemu-devel@nongnu.org, qemu-stable@nongnu.org
Subject: Re: [PATCH 2/6] virtio-scsi: don't waste CPU polling the event virtqueue
Date: Sat, 30 Apr 2022 06:52:45 +0100 [thread overview]
Message-ID: <YmzOrb/zE/UgljtS@stefanha-x1.localdomain> (raw)
In-Reply-To: <ca28d5bf-5bd6-e002-c420-ed9b98989555@redhat.com>
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On Fri, Apr 29, 2022 at 01:17:05AM +0200, Paolo Bonzini wrote:
> On 4/27/22 16:35, Stefan Hajnoczi wrote:
> > This is typical for rx virtqueues where the device uses buffers when
> > some event occurs (e.g. a packet is received, an error condition
> > happens, etc).
> >
> > Polling non-empty virtqueues wastes CPU cycles. We are not waiting for
> > new buffers to become available, we are waiting for an event to occur,
> > so it's a misuse of CPU resources to poll for buffers.
>
> Shouldn't polling wait for _used_ buffers, rather than available ones?
>
> I agree that it's generally useless to poll the event queue, but not because
> it doesn't empty the virtqueue.
This is device emulation code, not driver code. It's the device that
uses buffers and the driver that waits for used buffers. So the device
shouldn't poll for used buffers.
Stefan
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next prev parent reply other threads:[~2022-04-30 5:55 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-27 14:35 [PATCH 0/6] virtio-scsi: fix 100% CPU consumption in IOThread Stefan Hajnoczi
2022-04-27 14:35 ` [PATCH 1/6] virtio-scsi: fix ctrl and event handler functions in dataplane mode Stefan Hajnoczi
2022-04-27 19:47 ` Michael Tokarev
2022-04-28 9:05 ` Stefan Hajnoczi
2022-04-28 23:18 ` Paolo Bonzini
2022-04-27 14:35 ` [PATCH 2/6] virtio-scsi: don't waste CPU polling the event virtqueue Stefan Hajnoczi
2022-04-27 20:12 ` Nir Soffer
2022-04-28 9:05 ` Stefan Hajnoczi
2022-04-28 23:17 ` Paolo Bonzini
2022-04-30 5:52 ` Stefan Hajnoczi [this message]
2022-04-27 14:35 ` [PATCH 3/6] virtio-scsi: clean up virtio_scsi_handle_event_vq() Stefan Hajnoczi
2022-04-28 23:17 ` Paolo Bonzini
2022-04-27 14:35 ` [PATCH 4/6] virtio-scsi: clean up virtio_scsi_handle_ctrl_vq() Stefan Hajnoczi
2022-04-28 23:17 ` Paolo Bonzini
2022-04-27 14:35 ` [PATCH 5/6] virtio-scsi: clean up virtio_scsi_handle_cmd_vq() Stefan Hajnoczi
2022-04-28 23:17 ` Paolo Bonzini
2022-04-27 14:35 ` [PATCH 6/6] virtio-scsi: move request-related items from .h to .c Stefan Hajnoczi
2022-04-28 23:17 ` Paolo Bonzini
2022-05-09 9:45 ` [PATCH 0/6] virtio-scsi: fix 100% CPU consumption in IOThread Stefan Hajnoczi
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