Re: [dpdk-dev] [PATCH] vhost: add support to large linear mbufs

[Flavio Leitner <fbl@sysclose.org>]

On Tue,  1 Oct 2019 19:19:35 -0300
Flavio Leitner <fbl@sysclose.org> wrote:

> The rte_vhost_dequeue_burst supports two ways of dequeuing data. If
> the data fits into a buffer, then all data is copied and a single
> linear buffer is returned. Otherwise it allocates additional mbufs
> and chains them together to return a multiple segments mbuf.
> 
> While that covers most use cases, it forces applications that need
> to work with larger data sizes to support multiple segments mbufs.
> The non-linear characteristic brings complexity and performance
> implications to the application.
> 
> To resolve the issue, change the API so that the application can
> optionally provide a second mempool containing larger mbufs. If that
> is not provided (NULL), the behavior remains as before the change.
> Otherwise, the data size is checked and the corresponding mempool
> is used to return linear mbufs.
> 
> Signed-off-by: Flavio Leitner <fbl@sysclose.org>

The use case behind the patch is TSO support in DPDK accelerated Open
vSwitch (customers are using ovs-dpdk to send big packets too!). The
effort has been going on for many months now. There is a recent
proposal in OvS dev mailing using the API as is today here:
https://mail.openvswitch.org/pipermail/ovs-dev/2019-September/362573.html

There is also a reply from me in that thread with the patches
implementing a Proof-Of-Concept with the proposed API change:
https://mail.openvswitch.org/pipermail/ovs-dev/2019-September/362881.html

I opted to improve the current API instead of adding a new one, but I
have no strong opinion either way.

I know it's a bit late for v19.11, but since the TSO support effort is
going on for months now and OvS uses only LTS, I wonder if this can be
part of v19.11.

Thanks,
fbl