On Thu, May 06, 2021 at 09:58:57AM +0800, taoyunupt wrote: > 在 2021-05-06 03:26:46,"Ben Pfaff" <b...@ovn.org> 写道: > > >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. > > > I think we could set the initial value to 200,000, and adjust the > increase to 20,000 each time. Can you describe the rationale > algorithm you meationed in detailed ?
I'd expect that whoever is changing it would propose the rationale. I believe that part of the current rationale is to keep the limit at a level such that revalidation takes no more than 1 second. That's an important aspect too. _______________________________________________ discuss mailing list disc...@openvswitch.org https://mail.openvswitch.org/mailman/listinfo/ovs-discuss