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Date: Wed, 08 Apr 2015 16:29:22 +0800
From: Jason Wang <jasowang@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH V5 16/18] virtio-pci: increase the maximum
 number of virtqueues to 513
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To: Alexander Graf <agraf@suse.de>
Cc: "Michael S. Tsirkin" <mst@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, qemu-ppc@nongnu.org, cornelia.huck@de.ibm.com, Paolo Bonzini <pbonzini@redhat.com>, Luigi Rizzo <rizzo@iet.unipi.it>, Richard Henderson <rth@twiddle.net>



On Wed, Apr 8, 2015 at 2:33 AM, Alexander Graf <agraf@suse.de> wrote:
> On 04/07/2015 08:06 PM, Luigi Rizzo wrote:
>> 
>> 
>> On Tue, Apr 7, 2015 at 6:54 PM, Alexander Graf <agraf@suse.de> wrote:
>>> On 04/01/2015 10:15 AM, Jason Wang wrote:
>>>> This patch increases the maximum number of virtqueues for pci from 
>>>> 64
>>>> to 513. This will allow booting a virtio-net-pci device with 256 
>>>> queue
>>>> pairs.
>>>> ... 
>>>>    * configuration space */
>>>>   #define VIRTIO_PCI_CONFIG_SIZE(dev)     
>>>> VIRTIO_PCI_CONFIG_OFF(msix_enabled(dev))
>>>>   -#define VIRTIO_PCI_QUEUE_MAX 64
>>>> +#define VIRTIO_PCI_QUEUE_MAX 513
>>> 
>>> 513 is an interesting number. Any particular reason for it? Maybe 
>>> this was mentioned before and I just missed it ;)
>>> 
>> 
>> quite large, too. I thought multiple queue pairs were useful
>> to split the load for multicore machines, but targeting VMs with
>> up to 256 cores (and presumably an equal number in the host)
>> seems really forward looking.
> 
> They can also be useful in case your host tap queue is full, so going 
> higher than the host core count may make sense for throughput.
> 
> However, I am in doubt that there is a one-size-fits-all answer to 
> this. Could we maybe make the queue size configurable via a qdev 
> property?

We can do this on top but I'm not sure I understand the question. Do 
you mean a per-device limitation?

> 
> 
> Alex
>