Re: [dpdk-dev] [PATCH 2/2] net/vhost: support mrg-rxbuf disabling

[Matan Azrad <matan@mellanox.com>]

From: Maxime Coquelin
> On 6/26/19 9:50 AM, Matan Azrad wrote:
> > Hi Maxim
> >
> > Any response here?
> >
> > Besides that,
> >
> > Regarding the TSO and this patch:
> > I think we shouldn't be so strict to not take them for this version:
> > 1. The later time was a technical issue with the mailer - a mistake.
> > 2. The patches don't change any default and makes sense - will not hurt
> anyone.
> >
> > So I think we can do it beyond the letter of the law.
> >
> >   From: Maxime Coquelin
> >   > Sent: Thursday, June 20, 2019 10:19 AM
> >   > To: Matan Azrad <matan@mellanox.com>; Noa Ezra
> >   <noae@mellanox.com>
> >   > Cc: dev@dpdk.org
> >   > Subject: Re: [PATCH 2/2] net/vhost: support mrg-rxbuf disabling
> >   >
> >   >
> >   >
> >   > On 6/20/19 8:52 AM, Matan Azrad wrote:
> >   > > Hi all
> >   > >
> >   > >> -----Original Message-----
> >   > >> From: Noa Ezra
> >   > >> Sent: Thursday, June 20, 2019 8:58 AM
> >   > >> To: Maxime Coquelin <maxime.coquelin@redhat.com>
> >   > >> Cc: Matan Azrad <matan@mellanox.com>; dev@dpdk.org
> >   > >> Subject: RE: [PATCH 2/2] net/vhost: support mrg-rxbuf disabling
> >   > >>
> >   > >> Hi Maxime,
> >   > >> Thanks for your comment, please see below.
> >   > >>
> >   > >>> -----Original Message-----
> >   > >>> From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
> >   > >>> Sent: Wednesday, June 19, 2019 12:10 PM
> >   > >>> To: Noa Ezra <noae@mellanox.com>
> >   > >>> Cc: Matan Azrad <matan@mellanox.com>; dev@dpdk.org
> >   > >>> Subject: Re: [PATCH 2/2] net/vhost: support mrg-rxbuf disabling
> >   > >>>
> >   > >>> Hi Noa,
> >   > >>>
> >   > >>> On 6/19/19 8:13 AM, Noa Ezra wrote:
> >   > >>>> Rx mergeable buffers is a virtio feature that allows chaining of
> >   > >>>> multiple virtio descriptors to handle large packet size.
> >   > >>>> This behavior is supported and enabled by default, however in
> >   > >>>> case the user knows that rx mergeable buffers are not needed, he
> >   > >>>> can disable the feature.
> >   > >>>> The user should also set mrg_rxbuf=off in virtual machine's xml.
> >   > >>>
> >   > >>> I'm not sure to understand why it is needed, as the vhost-user
> >   > >>> library supports the feature, it's better to let it being advertised.
> >   > >>>
> >   > >>> As you say, it is up to the user to disable it in the VM's XML.
> >   > >>> Done this way, the feature won't be negotiated.
> >   > >>>
> >   > >> I agree with you, I'll remove this patch from the series.
> >   > >
> >   > > Are you sure that no performance impact exists for redundant
> >   > > merg-rx-buf
> >   > configuration here?
> >   >
> >   > I'm not sure to understand what you mean, could you please elaborate?
> >   >
> >   I guess that if this feature is enabled and the feature actually are not used
> >   (no packets are scattered or merged) it will hurt the performance.
> 
> Well, latest performance measurements does not show a big impact now on
> enabling mergeable buffers feature unconditionaly.
 
Did you test small packets \ big?

> >   So if one of the sides doesn't want to use it because of performance, it
> may
> >   want to disable it.
> 
> And even if there is an impact, the way to disable it is through
> Libvirt/Qemu.

Not sure, as TSO application may decide to not do it in spite of it is configured in Qemu. 
 
> >   > > What if the second side want it and the current side no?
> >   >
> >   > The feature won't be negotiated, assuming it has been disabled in
> QEMU
> >   > cmdline (or via libvirt).
> >   > > It may be that the vhost PMD user may want to disable it to save
> >   > performance from some reasons, no?
> >   > >
> >   >
> >   > Then this user should disable it at QEMU level.
> >   >
> >   So the vhost PMD is not one of the sides to decide?
> >   If so, why do we need the APIs to configure the features?
> 
> Are you talking about the rte_vhost_driver_set_features() and related
> APIs?
 
Yes

> This is used for example by the external backends that support features
> specific to the backend type (e.g. crypto), or also used by OVS-DPDK, to
> disable TSO. So these usages are for functional reasons, not tuning.

Exactly, applications (like OVS) may decide to disable features because a lot of reasons. 

> >   Looks like also the qemu is configured with the feature the VM\host sides
> >   may decide in some cases to disable it.
> 
> For functional reasons, I agree. So I that's why I agree with your tso
> patch as the application has to support it, but that's not the case of
> the mergeable buffers features.

Performance reasons are not good enough?

> Tiwei, what's your opinion on this?
> 
> >   > Regards,
> >   > Maxime
> >