From: Maxime Coquelin <maxime.coquelin@redhat.com>
To: Matan Azrad <matan@mellanox.com>, Noa Ezra <noae@mellanox.com>
Cc: "dev@dpdk.org" <dev@dpdk.org>, Tiwei Bie <tiwei.bie@intel.com>
Subject: Re: [dpdk-dev] [PATCH 2/2] net/vhost: support mrg-rxbuf disabling
Date: Wed, 26 Jun 2019 14:05:52 +0200 [thread overview]
Message-ID: <81735ee1-645d-286b-4516-64755fd07c94@redhat.com> (raw)
In-Reply-To: <AM0PR0502MB4019CBF6166854662AA0844DD2E20@AM0PR0502MB4019.eurprd05.prod.outlook.com>
On 6/26/19 1:18 PM, Matan Azrad wrote:
>
>
> 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?
64B packets, in non-vector mode on Virtio PMD side.
>
>>> 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?
No, that's not what I mean.
I mean that the application should be able to disable a feature when it
does not meet the functional requirement.
For performance tuning, the qemu way is available, and enough.
>
>> Tiwei, what's your opinion on this?
>>
>>> > Regards,
>>> > Maxime
>>>
next prev parent reply other threads:[~2019-06-26 12:05 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-19 6:13 [dpdk-dev] [PATCH 0/2] support tso and " Noa Ezra
2019-06-19 6:13 ` [dpdk-dev] [PATCH 1/2] net/vhost: support TSO disabling Noa Ezra
2019-06-19 9:53 ` Maxime Coquelin
2019-06-20 2:26 ` Tiwei Bie
2019-06-20 6:08 ` Matan Azrad
2019-06-20 7:25 ` Maxime Coquelin
2019-08-30 8:44 ` Maxime Coquelin
2019-09-30 9:02 ` Maxime Coquelin
2019-06-19 6:13 ` [dpdk-dev] [PATCH 2/2] net/vhost: support mrg-rxbuf disabling Noa Ezra
2019-06-19 9:10 ` Maxime Coquelin
2019-06-20 5:57 ` Noa Ezra
2019-06-20 6:52 ` Matan Azrad
2019-06-20 7:19 ` Maxime Coquelin
2019-06-20 7:55 ` Matan Azrad
2019-06-26 7:50 ` Matan Azrad
2019-06-26 10:27 ` Maxime Coquelin
2019-06-26 11:18 ` Matan Azrad
2019-06-26 12:05 ` Maxime Coquelin [this message]
2019-06-26 13:24 ` Matan Azrad
2019-06-26 14:17 ` Maxime Coquelin
2019-06-27 5:04 ` Matan Azrad
2019-08-30 8:48 ` Maxime Coquelin
2019-06-20 7:27 ` Maxime Coquelin
2019-09-30 9:04 ` Maxime Coquelin
2019-06-19 14:20 ` [dpdk-dev] [PATCH 0/2] support tso and " Aaron Conole
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