On Fri, Apr 30, 2021 at 06:10:43PM +0800, taoyunupt wrote: > > > > At 2021-04-29 06:39:11, "Ben Pfaff" <b...@ovn.org> wrote: > >On Wed, Apr 28, 2021 at 08:12:06PM +0800, taoyunupt wrote: > >> Hi， > >> Recently I encountered a TCP connection performance problem, the test > >> tool is Apache benchmark. > >> The OVS in my environment is set for hardware offload solution. > >> The "Requests per second" is about 6000/s， it closed to non-offload > >> solution. > >> > >> > >> "flow-lmit" has a dynamic balance in udpif_revalidator, it will > >> modify by the OVS condition（which is pind to "duration"）. In the > >> revalidate function, when the number of flows is greater than twice the > >> "flow-limit" , the delete flow operation will be triggered to delete all > >> flows; when the number of flows is greater than the "flow-limit", the > >> aging time will be adjusted to 0.1s, Slowly delete flow. > >> > >> > >> > >> I found that the reason for the poor performance is that when the > >> number of flows in the datapath increases and the processing power of OVS > >> decreases, a large number of flow deletions are generated. > >> As we know, In the hardware offloading scenario, although there are a > >> lot of flows, in fact, apart from the first packet, there is no need to > >> process subsequent packets. > >> In my opinion, the dynamic balance mechanism is very necessary, but > >> we need to increase the value of “duration”, or provide some new switches > >> for some high-performance scenarios, such as hardware offloading. > >> Do we still need to restrict the number of flows so strictly? By the > >> way, do you have another solution to resolve this? > > > >It's been a long time since I worked on this, but I recall two reasons > >for the flow limit. First, each flow takes up memory. Second, each > >flow must be revalidated periodically, meaning that it uses CPU as > >well. > > > >I don't, off-hand, remember the real reasons why the logic for deleting > >flows works as it does. It might be in the comments or the commit > >messages. But, I suspect, it is because above the flow-limit we want to > >try to reduce the amount of memory and CPU time dedicated to the cache > >and, if we arrive at twice the flow limit, we conclude that that try > >failed and that we must have a large number of very short flows so that > >caching is not very valuable anyhow. > > > >In a hardware offload scenario, we get rid of some costs (the cost of > >processing and forwarding packets and perhaps the memory cost in the > >datapath) but we still have the cost of revalidating them. When there > >are many flows, we add the extra cost of balancing flows between > >software and the offload hardware. > > > >Because of the remaining cost and the added ones when there is hardware > >offload, it's not obvious to me that we can stop limiting the number of > >flows. I think that experimentation and measurements would be needed. > >Perhaps this would be an adjustment to the dynamic algorithm, rather > > >than a removal of it. > > > I think we can increase the init `flow_limit` in udpif_create,10000 is a > small number for current server and OS, and if 'duration' is small ,we should > increase faster by a lager number not `flow_limit += 1000;`. > I have not better idea for this situation. Do you have some suggestion? I am > very glad to do this change.
What kind of number are you thinking about? I'd like to come up with a rationale for choosing it. It might be even better to come up with an algorithm or a heuristic for choosing it. _______________________________________________ discuss mailing list disc...@openvswitch.org https://mail.openvswitch.org/mailman/listinfo/ovs-discuss
