Re: [Qemu-devel] Can we increase vring size over 1024?

[Michael S. Tsirkin]

On Fri, Sep 02, 2016 at 06:55:35AM +0000, Gonglei (Arei) wrote:
> Hi Michael & all,
> 
> Michael, you made a presentation about the virto 1.1's new features in KVM 
> Forum last week.
> That's wonderful!
> 
> And I'd like to know can we increase vring size over 1024, such as 4096 or 
> 8192?
> 
> My colleage had asked the same question in 2014, but she didn't get a 
> definite answare,
> So, I want to rewake up the dissusstion about this. Becase for the 
> virtio-crypto device,
> I also need to increase the vring size to get better performance and 
> thoughput, but the Qemu
> side limit the thought as VIRTQUEUE_MAX_SIZE is 1024.
> 
>  [QA-virtio]:Why vring size is limited to 1024?
> http://qemu.11.n7.nabble.com/QA-virtio-Why-vring-size-is-limited-to-1024-td292450.html
> 
> Avi Kivity said that google cloud exposed the vring size to 16k.
> 
> Regards,
> -Gonglei

Fundamentally, the reason is that the ring size
currently also sets the max s/g list length, and linux
hosts can't support bigger lists.

We should fix that in 1.1.

-- 
MST