> From: Ferruh Yigit [mailto:ferruh.yi...@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)" <lihuis...@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. :-)
