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
