[dpdk-dev] [PATCH 2/2] net/vhost: support mrg-rxbuf disabling
Maxime Coquelin
maxime.coquelin at redhat.com
Wed Jun 26 12:27:35 CEST 2019
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 at mellanox.com>; Noa Ezra
> <noae at mellanox.com>
> > Cc: dev at 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 at redhat.com>
> > >> Cc: Matan Azrad <matan at mellanox.com>; dev at 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 at redhat.com]
> > >>> Sent: Wednesday, June 19, 2019 12:10 PM
> > >>> To: Noa Ezra <noae at mellanox.com>
> > >>> Cc: Matan Azrad <matan at mellanox.com>; dev at 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.
> 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.
> > > 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?
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.
> 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.
Tiwei, what's your opinion on this?
> > Regards,
> > Maxime
>
More information about the dev
mailing list