On Tue, 24 Jan 2017 15:59:08 +0000, Bruce Richardson <bruce.richard...@intel.com> wrote: > On Tue, Jan 24, 2017 at 04:19:25PM +0100, Olivier Matz wrote: > > Based on discussion done in [1], this patchset reorganizes the mbuf. > > > > Hi Olivier, > > thanks for all the work on this. From a quick scan of the patches, and > the description below, it looks like a good set of changes. Comments > below to see about kick-starting some further discussion about some of > the other changes you propose. > > > 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 > > - 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 > > - 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 it would require to change all the drivers, which is > > not an easy task. > > But if we do make this change and update the drivers, some of them > should perform a little better, since they do fewer writes. I don't > think the fastest vector drivers will be affected, since they already > coalesce the writes to these fields with other writes, but others > drivers may well be improved by the change.
Yes, that's something I forgot to say in the cover letter: after this patchset, the Rx path of drivers could be optimized a bit by removing writes to m->next, m->nb_segs and m->refcnt. The patch 4/8 gives an idea of what could be done. Once most drivers are updated, we could reconsider moving nb_segs and refcnt in the second cache line. > > > - remove the m->port field: too much impact on many examples and > > libraries, and some people highlighted they are using it. > > - moving m->next in the 1st cache line: there is not enough room, > > and having it set to NULL for unused mbuf should remove the need > > for it. > > I agree. > > > - merge seqn and timestamp together in a union: we could imagine > > use cases were both are activated. There is no flag indicating the > > presence of seqn, so it looks preferable to keep them separated for > > now. > > What were the use-cases? If we have a timestamp, surely sequence can > be determined from that? Even if you use the TSC as a timestamp per > burst, you can still sequence the packets cheaply by just adding 1 to > each subsequent value. Assuming the timestamp is in nanosecond, it is not a sequence number, so I'm not sure it should be hijacked for this purpose. A timestamp can be used to reorder packets, but having a sequence number is better because you can be sure that when you get packets 1, 3, 2, 0 that no packet is missing between 0 and 3. For that reason, I guess both features could be used at the same time. Regards, Olivier
