Re: [Qemu-devel] [virtio-dev] Re: virtio-net: configurable TX queue size

[Wei Wang <wei.w.wang@intel.com>]

On 05/05/2017 10:27 AM, Jason Wang wrote:
>
>
> On 2017年05月04日 18:58, Wang, Wei W wrote:
>> Hi,
>>
>> I want to re-open the discussion left long time ago:
>> https://lists.gnu.org/archive/html/qemu-devel/2015-11/msg06194.html
>> , and discuss the possibility of changing the hardcoded (256) TX  queue
>> size to be configurable between 256 and 1024.
>
> Yes, I think we probably need this.

That's great, thanks.

>
>>
>> The reason to propose this request is that a severe issue of packet 
>> drops in
>> TX direction was observed with the existing hardcoded 256 queue size,
>> which causes performance issues for packet drop sensitive guest
>> applications that cannot use indirect descriptor tables. The issue 
>> goes away
>> with 1K queue size.
>
> Do we need even more, what if we find 1K is even not sufficient in the 
> future? Modern nics has size up to ~8192.

Yes. Probably, we can also set the RX queue size to 8192 (currently it's 
1K) as well.

>
>>
>> The concern mentioned in the previous discussion (please check the link
>> above) is that the number of chained descriptors would exceed
>> UIO_MAXIOV (1024) supported by the Linux.
>
> We could try to address this limitation but probably need a new 
> feature bit to allow more than UIO_MAXIOV sgs.

I think we should first discuss whether it would be an issue below.

>
>>
>>  From the code,  I think the number of the chained descriptors is 
>> limited to
>> MAX_SKB_FRAGS + 2 (~18), which is much less than UIO_MAXIOV.
>
> This is the limitation of #page frags for skb, not the iov limitation.

I think the number of page frags are filled into the same number of 
descriptors
in the virtio-net driver (e.g. use 10 descriptors for 10 page frags). On 
the other
side, the virtio-net backend uses the same number of iov for the 
descriptors.

Since the number of page frags is limited to 18, I think there wouldn't 
be more
than 18 iovs to be passed to writev, right?

Best,
Wei