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 ?
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