Re: [dpdk-dev] [PATCH 2/3] vhost: optimize dequeue for small packets

["Xie, Huawei" <huawei.xie@intel.com>]

On 6/1/2016 2:41 PM, Yuanhan Liu wrote:
> On Wed, Jun 01, 2016 at 06:24:18AM +0000, Xie, Huawei wrote:
>> On 5/3/2016 8:42 AM, Yuanhan Liu wrote:
>>> Both current kernel virtio driver and DPDK virtio driver use at least
>>> 2 desc buffer for Tx: the first for storing the header, and the others
>>> for storing the data.
>> Tx could prepend some space for virtio net header whenever possible, so
>> that it could use only one descriptor.
> In such case, it will work as well: it will goto the "else" code path
> then.

It works, but the problem is the statement.

>> Another thing is this doesn't reduce the check because you also add a check.
> Actually, yes, it does save check. Before this patch, we have:
>
> 	while (not done yet) {
>  	       if (desc_is_drain)
>  	               ...;
>
>  	       if (mbuf_is_full)
>  	               ...;
>
>  	       COPY();
> 	}
>
> Note that the "while" check will be done twice, therefore, it's 4
> checks in total. After this patch, it would be:
>
> 	if (virtio_net_hdr_takes_one_desc)
> 		...
>
> 	while (1) {
> 		COPY();
>
> 		if (desc_is_drain) {
> 			break if done;
> 			...;
> 		}
>
> 		if (mbuf_is_full {
> 			/* small packets will bypass this check */
> 			....;
> 		}
> 	}
>
> So, for small packets, it takes 2 checks only, which actually saves
> 2 checks.
>
> 	--yliu
>

For small packets, we have three checks totally. It worth we use space
to save checks, so would ack this patch, but note that there is one
assumption that rte_memcpy API could handle zero length copy.