RE: [PATCH v3 0/3] introduce maximum Rx buffer size

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Ferruh Yigit [mailto:ferruh.yigit@amd.com]
> Sent: Thursday, 2 November 2023 17.24
> 
> On 11/2/2023 1:59 AM, lihuisong (C) wrote:
> >
> > 在 2023/11/2 0:08, Stephen Hemminger 写道:
> >> On Wed, 1 Nov 2023 10:36:07 +0800
> >> "lihuisong (C)" <lihuisong@huawei.com> wrote:
> >>
> >>>> Do we need to report this size? It's a common feature for all
> PMDs.
> >>>> It would make sense then to have max_rx_bufsize set to 16K by
> default
> >>>> in ethdev, and PMD could then raise/lower based on hardware.
> >>> It is not appropriate to set to 16K by default in ethdev layer.
> >>> Because I don't see any check for the upper bound in some driver,
> like
> >>> axgbe, enetc and so on.
> >>> I'm not sure if they have no upper bound.
> >>> And some driver's maximum buffer size is "16384(16K) - 128"
> >>> So it's better to set to UINT32_MAX by default.
> >>> what do you think?
> >> The goal is always giving application a working upper bound, and
> >> enforcing
> >> that as much as possible in ethdev layer. It doesnt matter which
> pattern
> >> does that.  Fortunately, telling application an incorrect answer is
> >> not fatal.
> >> If over estimated, application pool would be wasting space.
> >> If under estimated, application will get more fragmented packets.
> > I know what you mean.
> > If we set UINT32_MAX, it just means that driver don't report this
> upper
> > bound.
> > This is also a very common way of handling. And it has no effect on
> the
> > drivers that doesn't report this value.
> > On the contrary, if we set a default value (like 16K) in ethdev, user
> > may be misunderstood and confused by that, right?
> > After all, this isn't the real upper bound of all drivers. And this
> > fixed default value may affect the behavior of some driver that I
> didn't
> > find their upper bound.
> > So I'd like to keep it as UINT32_MAX.
> >
> 
> 
> Hi Stephen, Morten,
> 
> I saw scattered Rx mentioned, there may be some misalignment,
> the purpose of the patch is not to enable application to set as big as
> possible mbuf size, so that application can escape from parsing
> multi-segment mbufs.
> Indeed application can provide a large mbuf anyway, to have same
> result,
> without knowing this information.
> 
> Main motivation is other way around, device may have restriction on
> buffer size that a single descriptor can address, independent from
> scattered Rx used, if mbuf size is bigger than this device limit, each
> mbuf will have some unused space.
> Patch has intention to inform this max per mbuf/descriptor buffer size,
> so that application doesn't allocate bigger mbuf and waste memory.

Good point!

Let's categorize this patch series as a memory optimization for applications that support jumbo frames, but are trying to avoid (or reduce) scattered RX. :-)