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Date: Sun, 30 Aug 2020 10:46:09 +0200 From: Sebastian Gottschall <s.gottschall@...wrt.com> To: Jakub Kicinski <kuba@...nel.org>, Felix Fietkau <nbd@....name> Cc: netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>, Hillf Danton <hdanton@...a.com> Subject: Re: [PATCH v3 1/2] net: add support for threaded NAPI polling Am 22.08.2020 um 03:49 schrieb Jakub Kicinski: > On Fri, 21 Aug 2020 21:01:50 +0200 Felix Fietkau wrote: >> For some drivers (especially 802.11 drivers), doing a lot of work in the NAPI >> poll function does not perform well. Since NAPI poll is bound to the CPU it >> was scheduled from, we can easily end up with a few very busy CPUs spending >> most of their time in softirq/ksoftirqd and some idle ones. >> >> Introduce threaded NAPI for such drivers based on a workqueue. The API is the >> same except for using netif_threaded_napi_add instead of netif_napi_add. >> >> In my tests with mt76 on MT7621 using threaded NAPI + a thread for tx scheduling >> improves LAN->WLAN bridging throughput by 10-50%. Throughput without threaded >> NAPI is wildly inconsistent, depending on the CPU that runs the tx scheduling >> thread. >> >> With threaded NAPI, throughput seems stable and consistent (and higher than >> the best results I got without it). >> >> Based on a patch by Hillf Danton > I've tested this patch on a non-NUMA system with a moderately > high-network workload (roughly 1:6 network to compute cycles) > - and it provides ~2.5% speedup in terms of RPS but 1/6/10% worse > P50/P99/P999 latency. > > I started working on a counter-proposal which uses a pool of threads > dedicated to NAPI polling. It's not unlike the workqueue code but > trying to be a little more clever. It gives me ~6.5% more RPS but at > the same time lowers the p99 latency by 35% without impacting other > percentiles. (I only started testing this afternoon, so hopefully the > numbers will improve further). > > I'm happy for this patch to be merged, it's quite nice, but I wanted > to give the heads up that I may have something that would replace it... > > The extremely rough PoC, less than half-implemented code which is really > too broken to share: > https://git.kernel.org/pub/scm/linux/kernel/git/kuba/linux.git/log/?h=tapi looks interesting. keep going Sebastian >
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