On Fri, 2023-03-24 at 18:57 +0100, Felix Fietkau wrote: > On 24.03.23 18:47, Jakub Kicinski wrote: > > On Fri, 24 Mar 2023 18:35:00 +0100 Felix Fietkau wrote: > > > I'm primarily testing this on routers with 2 or 4 CPUs and limited > > > processing power, handling routing/NAT. RPS is typically needed to > > > properly distribute the load across all available CPUs. When there is > > > only a small number of flows that are pushing a lot of traffic, a static > > > RPS assignment often leaves some CPUs idle, whereas others become a > > > bottleneck by being fully loaded. Threaded NAPI reduces this a bit, but > > > CPUs can become bottlenecked and fully loaded by a NAPI thread alone. > > > > The NAPI thread becomes a bottleneck with RPS enabled? > > The devices that I work with often only have a single rx queue. That can > easily become a bottleneck. > > > > Making backlog processing threaded helps split up the processing work > > > even more and distribute it onto remaining idle CPUs. > > > > You'd want to have both threaded NAPI and threaded backlog enabled? > > Yes > > > > It can basically be used to make RPS a bit more dynamic and > > > configurable, because you can assign multiple backlog threads to a set > > > of CPUs and selectively steer packets from specific devices / rx queues > > > > Can you give an example? > > > > With the 4 CPU example, in case 2 queues are very busy - you're trying > > to make sure that the RPS does not end up landing on the same CPU as > > the other busy queue? > > In this part I'm thinking about bigger systems where you want to have a > group of CPUs dedicated to dealing with network traffic without > assigning a fixed function (e.g. NAPI processing or RPS target) to each > one, allowing for more dynamic processing. > > > > to them and allow the scheduler to take care of the rest. > > > > You trust the scheduler much more than I do, I think :) > > In my tests it brings down latency (both avg and p99) considerably in > some cases. I posted some numbers here: > https://lore.kernel.org/netdev/e317d5bc-cc26-8b1b-ca4b-66b5328683c4@xxxxxxxx/ It's still not 110% clear to me why/how this additional thread could reduce latency. What/which threads are competing for the busy CPU[s]? I suspect it could be easier/cleaner move away the others (non RPS) threads. Cheers, Paolo
