> From: dev [mailto:dev-boun...@dpdk.org] On Behalf Of Slava Ovsiienko > Sent: Tuesday, 28 September 2021 11.01 > > Hi, > > I've re-read the entire thread. > If I understand correctly, the root problem was (in initial patch): > > > m1 = rte_pktmbuf_alloc(mp); > > rte_pktmbuf_append(m1, 500); > > m2 = rte_pktmbuf_alloc(mp); > > rte_pktmbuf_append(m2, 500); > > rte_pktmbuf_chain(m1, m2); > > m0 = rte_pktmbuf_alloc(mp); > > rte_pktmbuf_append(m0, 500); > > rte_pktmbuf_chain(m0, m1); > > > > As rte_pktmbuf_chain() does not reset nb_seg in the initial m1 > segment > > (this is not required), after this code the mbuf chain have 3 > > segments: > > - m0: next=m1, nb_seg=3 > > - m1: next=m2, nb_seg=2 > > - m2: next=NULL, nb_seg=1 > > > The proposed fix was to ALWAYS set next and nb_seg fields on > mbuf_free(), > regardless next field content. That would perform unconditional write > to mbuf, and might affect the configurations, where are no multi- > segment > packets at al. mbuf_free() is "backbone" API, it is used by all cases, > all > scenaries are affected. > > As far as I know, the current approach for nb_seg field - it contains > other > value than 1 only in the first mbuf , for the following segments, it > should > not be considered at all (only the first segment fields are valid), and > it is > supposed to contain 1, as it was initially allocated from the pool. > > In the example above the problem was introduced by > rte_pktmbuf_chain(). Could we consider fixing the rte_pktmbuf_chain() > (used in potentially fewer common sceneries) instead of touching > the extremely common rte_mbuf_free() ? > > With best regards, > Slava
Great idea, Slava! Changing the invariant for 'nb_segs', so it must be 1, except in the first segment of a segmented packet. Thinking further about it, perhaps we can achieve even higher performance by a minor additional modification: Use 0 instead of 1? Or offset 'nb_segs' by -1, so it reflects the number of additional segments? And perhaps combining the invariants for 'nb_segs' and 'next' could provide even more performance improvements. I don't know, just sharing a thought. Anyway, I vote for fixing the bug. One way or the other! -Morten > > > -----Original Message----- > > From: Thomas Monjalon <tho...@monjalon.net> > > Sent: Tuesday, September 28, 2021 11:29 > > > > Follow-up again: > > We have added a note in 21.08, we should fix it in 21.11. > > If there are no counter proposal, I suggest applying this patch, no > matter the > > performance regression. > > > > > > 30/07/2021 16:54, Thomas Monjalon: > > > 30/07/2021 16:35, Morten Brørup: > > > > > From: Olivier Matz [mailto:olivier.m...@6wind.com] > > > > > Sent: Friday, 30 July 2021 14.37 > > > > > > > > > > Hi Thomas, > > > > > > > > > > On Sat, Jul 24, 2021 at 10:47:34AM +0200, Thomas Monjalon > wrote: > > > > > > What's the follow-up for this patch? > > > > > > > > > > Unfortunatly, I still don't have the time to work on this topic > yet. > > > > > > > > > > In my initial tests, in our lab, I didn't notice any > performance > > > > > regression, but Ali has seen an impact (0.5M PPS, but I don't > know > > > > > how much in percent). > > > > > > > > > > > > > > > > 19/01/2021 15:04, Slava Ovsiienko: > > > > > > > Hi, All > > > > > > > > > > > > > > Could we postpose this patch at least to rc2? We would like > to > > > > > conduct more investigations? > > > > > > > > > > > > > > With best regards, Slava > > > > > > > > > > > > > > From: Olivier Matz <olivier.m...@6wind.com> > > > > > > > > On Mon, Jan 18, 2021 at 05:52:32PM +0000, Ali Alnubani > wrote: > > > > > > > > > Hi, > > > > > > > > > (Sorry had to resend this to some recipients due to > mail > > > > > > > > > server > > > > > problems). > > > > > > > > > > > > > > > > > > Just confirming that I can still reproduce the > regression > > > > > > > > > with > > > > > single core and > > > > > > > > 64B frames on other servers. > > > > > > > > > > > > > > > > Many thanks for the feedback. Can you please detail what > is > > > > > > > > the > > > > > amount of > > > > > > > > performance loss in percent, and confirm the test case? > (I > > > > > suppose it is > > > > > > > > testpmd io forward). > > > > > > > > > > > > > > > > Unfortunatly, I won't be able to spend a lot of time on > this > > > > > > > > soon > > > > > (sorry for > > > > > > > > that). So I see at least these 2 options: > > > > > > > > > > > > > > > > - postpone the patch again, until I can find more time to > analyze > > > > > > > > and optimize > > > > > > > > - apply the patch if the performance loss is acceptable > > > > > > > > compared > > > > > to > > > > > > > > the added value of fixing a bug > > > > > > > > > > > > > > [...] > > > > > > > > > > Statu quo... > > > > > > > > > > Olivier > > > > > > > > > > > > > The decision should be simple: > > > > > > > > Does the DPDK project support segmented packets? > > > > If yes, then apply the patch to fix the bug! > > > > > > > > If anyone seriously cares about the regression it introduces, > optimization > > patches are welcome later. We shouldn't wait for it. > > > > > > You're right, but the regression is flagged to a 4-years old patch, > > > that's why I don't consider it as urgent. > > > > > > > If the patch is not applied, the documentation must be updated to > > mention that we are releasing DPDK with a known bug: that segmented > > packets are handled incorrectly in the scenario described in this > patch. > > > > > > Yes, would be good to document the known issue, no matter how old > it > > > is. > > > > > > > Generally, there could be some performance to gain by not > supporting > > segmented packets at all, as a compile time option. But that is a > different > > discussion. > > > > > > >
