Messages in this thread | | | Date | Wed, 16 Mar 2022 20:10:07 -0700 | From | Jakub Kicinski <> | Subject | Re: [PATCH net-next] net: set default rss queues num to physical cores / 2 |
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On Tue, 15 Mar 2022 10:18:32 +0100 Íñigo Huguet wrote: > Network drivers can call to netif_get_num_default_rss_queues to get the > default number of receive queues to use. Right now, this default number > is min(8, num_online_cpus()). > > Instead, as suggested by Jakub, use the number of physical cores divided > by 2 as a way to avoid wasting CPU resources and to avoid using both CPU > threads, but still allowing to scale for high-end processors with many > cores. > > As an exception, select 2 queues for processors with 2 cores, because > otherwise it won't take any advantage of RSS despite being SMP capable. > > Tested: Processor Intel Xeon E5-2620 (2 sockets, 6 cores/socket, 2 > threads/core). NIC Broadcom NetXtreme II BCM57810 (10GBps). Ran some > tests with `perf stat iperf3 -R`, with parallelisms of 1, 8 and 24, > getting the following results: > - Number of queues: 6 (instead of 8) > - Network throughput: not affected > - CPU usage: utilized 0.05-0.12 CPUs more than before (having 24 CPUs > this is only 0.2-0.5% higher) > - Reduced the number of context switches by 7-50%, being more noticeable > when using a higher number of parallel threads.
Thanks for following up, Inigo!
Heads up for the maintainers of drivers which use netif_get_num_default_rss_queues() today - please note the above - the default number of Rx queues may change for you starting with the 5.18 kernel.
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