Re: [PATCH v5] virtio-net: clarify coalescing parameters settings

Michael S. Tsirkin
2024-06-10T14:50:14Z

On Mon, Jun 10, 2024 at 09:35:45PM +0800, Heng Qi wrote:
> On Mon, 10 Jun 2024 14:46:02 +0200, Halil Pasic <pasic@linux.ibm.com> wrote:
> > On Sat, 8 Jun 2024 10:34:22 +0800
> > Heng Qi <hengqi@linux.alibaba.com> wrote:
> >
> > > On Fri, 7 Jun 2024 22:02:46 +0200, Halil Pasic <pasic@linux.ibm.com> wrote:
> > > > On Tue, 28 May 2024 12:47:02 +0800
> > > > Heng Qi <hengqi@linux.alibaba.com> wrote:
> > > >
> > > > > The device can set any initial coalescing parameters (0 or non-zero)
> > > > > for the receive/send queue before the setting command is executed,
> > > > > not just 0, enhancing device performance even without DIM enabled.
> > > > >
> > > > > So we need to clarify descriptions that don't fit the behavior.
> > > >
> > > > Sorry I'm late to the party -- again! Just for my understanding: how/why
> > > > is this a clarification and not just a (basically incompatible) change?
> > >
> > > In my opinion, "clarification" means that something may have been described
> > > incorrectly before, and we now need to discuss, explain clearly, and correct
> > > the possibly incorrect description.
> > >
> >
> > I figure the difference in perceived semantics of the word
> > "clarification" is at the root of my confusion. Let us have a look at
> > https://dictionary.cambridge.org/de/worterbuch/englisch/clarification
> >
> > According to my understanding a "clarification", while an improvement in
> > ease of understanding and/or decrease of ambiguity (possibly to no
> > ambiguity at all) implies that what receiving a clarification is not
> > outright wrong.
> >
> > When rectifying something that is outright incorrect or wrong, I would
> > refer to that with words like "correction", "fix", "erratum" or
> > "corrigendum".
> >
> > > >
> > > > I mean if I read this correctly, before the driver had the guaranty
> > > > that if the parameters are not set by the driver, negotiating the
> > > > feature does not introduce any coalescing. After this in theory
> > > > the device could just pick some max value and potentially introduce
> > > > maximal latency in certain scenarios.
> > >
> > > "maximum latency" also means "throughput improvement".
> > >
> >
> > Under certain assumptions. But not necessarily. Again my concern is
> > mostly the type of change. The virtio standard maintain a revision
> > history appendix, and I would like to avoid the nature of this change
> > being misrepresented there. If Connie and/or Michael think it is worth
> > fixing, I believe it can be fixed with an editorial change.
> >
> > AFAIU VIRTIO_NET_F_NOTF_COAL and VIRTIO_NET_F_VQ_NOTF_COAL are about to
> > land with virtio-1.3, i.e. there is no released/standardized virtio
> > version where the "initialize to 0" is released. In that sense it looks
> > like we are still on time to change this. But I am not 100% certain. In
> > any case I don't think this as a huge impact and I'm fine going ahead
> > with the change.
>
> Sorry for the late reply, I'm on vacation.
>
> I agree with this, and I prefer to release this patch as a correction for
> virtio1.3 instead of a new patch for virtio1.4, because if devices support moderation coalescing based on virtio1.3,
> and after virtio1.4 is released, these devices need to be updated again for a
> more reasonable coalescing parameters.
>
> Cornelia and Michael, what do you think?

The TC is just voting to start the public review process.
We can defer that by a couple more weeks if there is
a known issue to address.

For that I expect we want a final patch and a couple of acks
on list from TC members by end of the ballot, June 13.

> >
> > > >
> > > > I understand that it is probably in the best interest of the devices to
> > > > not pick stupid defaults. But it is also probably in the best interest
> > > > of the driver to set those params, and if the driver is going to set its
> > > > values, the devices defaults are moot unless we assume that those may end
> > > > up being used by the driver as a hint when deciding which parameter
> > > > values to choose.
> > >
> > > "Any values" is compatible with "0 for max-usecs and max-frames", and the device
> > > can choose "no coalescing".
> > >
> > > "No coalescing" means "latency friendly", but it also means "a lot of
> > > interruptions and throughput unfriendly".
> >
> > To what extent does virtio's "normal" notification suppression
> > (VIRTIO_F_EVENT_IDX) would alleviate that in practice? At least in
> > theory the interruption suppression could save us there right?
> >
>
> Interruption suppression is useful, but many scenes still have a lot of
> interruptions.
>
> >
> > > If the device chooses a stupid
> > > maximum value, it is his choice (spec should give more devices choices instead of
> > > forcing them to choose "0" which is not the best practice). We can't talk about
> > > performance for drivers when the devices tend to choose any "stupid" designs.
> > >
> > > We need relaxe the restrictions and makes the spec more reasonable.
> > >
> >
> > Hm, I see Linux virtio-net changes have landed with v6.0 and if I read
> > those correctly the driver -- contrary to my initial expectation --
> > negotiates the feature, but does not set the parameters explicitly and
>
> How does the driver know what parameters to set? The parameters should be
> exposed by each device.
>
> > thus keeps the defaults (until userspace decides to set the parameters).
> > So it does matter whether the defaults are guaranteed to be 0 or not,
> > and if not it does matter what defaults are chosen by the device.
>
> Didn't follow this. More below.
>
> >
> > One could even argue that those patches have been reviewed under the
> > assumption that the device needs to use 0 as the default parameter value.
>
> The default value should not be explicitly specified in the spec, because one
> size does not fit multiple devices. The source of this problem is that we are
> missing fields like default_{rx, tx}_coalesicng_params that indicate the device
> capabilities. No?
>
> Thanks.
>
> >
> > Well no strong opinions here. If the community is fine with it, I'm fine
> > with it as well.
> >
> > Thank you!
> >
> > Regards,
> > Halil