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From: "Michael S. Tsirkin" <mst@redhat.com>
To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: jasowang@redhat.com, Yajun Wu <yajunw@mellanox.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Subject: Re: Any reason VIRTQUEUE_MAX_SIZE is 1024? Can we increase this limit?
Date: Wed, 5 Aug 2020 08:13:29 -0400	[thread overview]
Message-ID: <20200805081144-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20200805121107.GG361702@stefanha-x1.localdomain>

On Wed, Aug 05, 2020 at 01:11:07PM +0100, Stefan Hajnoczi wrote:
> On Thu, Jul 30, 2020 at 07:46:09AM +0000, Yajun Wu wrote:
> > I'm doing iperf test on VIRTIO net through vhost-user(HW VDPA).
> > Find maximal acceptable tx_queue_size/rx_queue_size is 1024.
> > Basically increase queue size can get better RX rate for my case.
> > 
> > Can we increase the limit(VIRTQUEUE_MAX_SIZE) to 8192 to possibly gain better performance?
> 
> Hi,
> The VIRTIO 1.1 specification says the maximum number of descriptors is
> 32768 for both split and packed virtqueues.
> 
> The vhost kernel code seems to support 32768.
> 
> The 1024 limit is an implementation limit in QEMU. Increasing it would
> require QEMU code changes. For example, VIRTQUEUE_MAX_SIZE is used as
> the size of arrays.
> 
> I can't think of a fundamental reason why QEMU needs to limit itself to
> 1024 descriptors. Raising the limit would require fixing up the code and
> ensuring that live migration remains compatible with older versions of
> QEMU.
> 
> Stefan

There's actually a reason for a limit: in theory the vq size
also sets a limit on the number of scatter/gather entries.
both QEMU and vhost can't handle a packet split over > 1k chunks.

We could add an extra limit for s/g size like block and scsi do,
this will need spec, guest and host side work.

-- 
MST



  reply	other threads:[~2020-08-05 12:14 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-30  7:46 Any reason VIRTQUEUE_MAX_SIZE is 1024? Can we increase this limit? Yajun Wu
2020-08-05 12:11 ` Stefan Hajnoczi
2020-08-05 12:13   ` Michael S. Tsirkin [this message]
2020-08-06 12:37     ` Stefan Hajnoczi
2020-08-07  3:35       ` Jason Wang
2020-08-07  9:59         ` Stefan Hajnoczi
2020-08-10  9:49           ` Jason Wang
2020-08-10 14:45       ` Michael S. Tsirkin

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