Re: [dpdk-dev] [RFC 0/8] mbuf: structure reorganization

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Feb 21, 2017 at 04:04:40PM +0100, Olivier MATZ wrote:
> Hi Morten,
> 
> On Tue, 21 Feb 2017 15:20:23 +0100, Morten Brørup
> <mb@smartsharesystems.com> wrote:
> > Comments at the end.
> > 
> > 
> > > -----Original Message-----
> > > From: dev [mailto:dev-bounces@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.richardson@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.ananyev@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.
> 
I, sadly, have to agree here. I think undertaking rework of all PMDs is
a huge job, that probably needs to be shared among the PMD authors.

Regards,
/Bruce