Messages in this thread | | | Date | Fri, 24 Mar 2023 18:57:03 +0100 | From | Felix Fietkau <> | Subject | Re: [PATCH net-next] net/core: add optional threading for backlog processing |
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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@nbd.name/
- Felix
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