Hi Morten, On Tue, 21 Feb 2017 15:20:23 +0100, Morten Brørup <m...@smartsharesystems.com> wrote: > Comments at the end. > > > > -----Original Message----- > > From: dev [mailto:dev-boun...@dpdk.org] On Behalf Of Olivier Matz > > Sent: Thursday, February 16, 2017 5:14 PM > > To: Bruce Richardson > > Cc: Ananyev, Konstantin; dev@dpdk.org > > Subject: Re: [dpdk-dev] [RFC 0/8] mbuf: structure reorganization > > > > On Thu, 16 Feb 2017 15:46:19 +0000, Bruce Richardson > > <bruce.richard...@intel.com> wrote: > > > On Thu, Feb 16, 2017 at 02:48:07PM +0100, Olivier Matz wrote: > > > > Hi Konstantin, > > > > > > > > Thanks for the feedback. > > > > Comments inline. > > > > > > > > > > > > On Mon, 6 Feb 2017 18:41:27 +0000, "Ananyev, Konstantin" > > > > <konstantin.anan...@intel.com> wrote: > > > > > Hi Olivier, > > > > > Looks good in general, some comments from me below. > > > > > Thanks > > > > > Konstantin > > > > > > > > > > > > > > > > > The main changes are: > > > > > > - reorder structure to increase vector performance on some > > > > > > non-ia platforms. > > > > > > - add a 64bits timestamp field in the 1st cache line > > > > > > > > > > Wonder why it deserves to be in first cache line? > > > > > How it differs from seqn below (pure SW stuff right now). > > > > > > > > In case the timestamp is set from a NIC value, it is set in the > > > > Rx path. So that's why I think it deserve to be located in the > > > > 1st cache line. > > > > > > > > As you said, the seqn is a pure sw stuff right: it is set in a > > > > lib, not in a PMD rx path. > > > > > > > > > > - m->next, m->nb_segs, and m->refcnt are always initialized > > > > > > for mbufs in the pool, avoiding the need of setting > > > > > > m->next > > (located > > > > > > in the 2nd cache line) in the Rx path for mono-segment > > > > > > packets. > > > > > > - change port and nb_segs to 16 bits > > > > > > > > > > Not that I am completely against it, but changing nb_segs to > > > > > 16 bits seems like an overkill to me. > > > > > I think we can keep and extra 8bits for something more useful > > > > > in future. > > > > > > > > In my case, I use the m->next field to chain more than 256 > > > > segments for L4 socket buffers. It also updates nb_seg that can > > > > overflow. It's not a big issue since at the end, nb_seg is > > > > decremented for each segment. On the other hand, if I enable > > > > some sanity checks on mbufs, it complains because the number of > > > > segments is not equal to nb_seg. > > > > > > > > There is also another use case with fragmentation as discussed > > > > recently: http://dpdk.org/dev/patchwork/patch/19819/ > > > > > > > > Of course, dealing with a long mbuf list is not that efficient, > > > > but the application can maintain another structure to > > > > accelerate the access to the middle/end of the list. > > > > > > > > Finally, we have other ideas to get additional 8 bits if > > > > required > > in > > > > the future, so I don't think it's really a problem. > > > > > > > > > > > > > > > > > > > - move seqn in the 2nd cache line > > > > > > > > > > > > Things discussed but not done in the patchset: > > > > > > - move refcnt and nb_segs to the 2nd cache line: many > > > > > > drivers sets them in the Rx path, so it could introduce a > > > > > > performance regression, or > > > > > > > > > > I wonder can refcnt only be moved into the 2-nd cacheline? > > > > > As I understand thanks to other change (from above) > > > > > m->refcnt > > will > > > > > already be initialized, so RX code don't need to touch it. > > > > > Though yes, it still would require changes in all PMDs. > > > > > > > > Yes, I agree, some fields could be moved in the 2nd cache line > > > > once all PMDs stop to write them in RX path. I propose to issue > > > > some guidelines to PMD maintainers at the same time the > > > > patchset is pushed. Then we can consider changing it in a > > > > future version, in case we need more room in the 1st mbuf cache > > > > line. > > > > > > If we are changing things, we should really do all that now, > > > rather than storing up future breaks to mbuf. Worst case, we > > > should plan for it immediately after the release where we make > > > these changes. Have > > two > > > releases that break mbuf immediately after each other - and > > > flagged > > as > > > such, but keep it stable thereafter. I don't like having technical > > > debt on mbuf just after we supposedly "fix" it. > > > > I think there is no need to do this change now. And I don't feel > > good with the idea of having a patchset that updates all the PMDs > > to remove the access to a field because it moved to the 2nd cache > > line (especially thinking about vector PMDs). > > > > That's why I think the plan could be: > > - push an updated version of this patchset quickly > > - advertise to PMD maintainers "you don't need to set the m->next, > > m->refcnt, and m->nb_segs in the RX path, please update your > > drivers" > > - later, if we need more room in the 1st cache line of the mbuf, we > > can move refcnt and nb_seg, probably without impacting the > > performance. > > > > > > Olivier > > I suppose you mean that PMDs don't need to /initialize/ m->next, > m->refcnt and m->nb_segs. > > Forgive my ignorance, and this is wild speculation, but: Would a PMD > not need to set m->next and m->nb_segs if it receives a jumbogram > larger than an mbuf packet buffer? And if this is a realistic use > case, these fields actually do belong in the 1st cache line. PMD > developers please chime in.
Nothing to add to Bruce's answer :) > > And I tend to agree with Bruce about making all these mbuf changes in > one go, rather than postponing some of them to later. Especially > because the postponement also closes and reopens the whole discussion > and decision process! (Not initializing a few fields in a PMD cannot > require a lot of work by the PMD developers. Moving the fields to the > 2nd cache line will in the worst case degrade the performance of the > non-updated PMDs.) > > A two step process makes good sense for the developers of DPDK, but > both steps should be taken within the same release, so they are > transparent to the users of DPDK. I don't think this is doable, knowing the submission dead line is in less than 2 weeks. On my side, honestly, I don't want to dive in the code of into all PMDs. I feel this would be more risky than letting the PMD maintainers update their own PMD code. Olivier
