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. > - 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. /Bruce
