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

[Yan Vugenfirer]

> On 5 May 2017, at 12:20, Jason Wang <address@hidden> wrote:
> 
> 
> 
> On 2017年05月05日 13:53, Wei Wang wrote:
>> 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?
> 
This limitation assumption is incorrect for Windows. We saw cases (and 
strangely enough with small packets) when Windows returns scatter gather list 
with 32 descriptors or more.

Best regards,
Yan.

> Looks not, see skb_copy_datagram_from_iter().

> 
> Thanks
> 
>> 
>> Best,
>> Wei
>> 
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