netdev - Re: [RFC PATCH net-next 0/6] implement kthread based napi poll

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Date:   Tue, 29 Sep 2020 13:16:59 -0700
From:   Wei Wang <weiwan@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Felix Fietkau <nbd@....name>
Subject: Re: [RFC PATCH net-next 0/6] implement kthread based napi poll

On Tue, Sep 29, 2020 at 12:19 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 28 Sep 2020 19:43:36 +0200 Eric Dumazet wrote:
> > Wei, this is a very nice work.
> >
> > Please re-send it without the RFC tag, so that we can hopefully merge it ASAP.
>
> The problem is for the application I'm testing with this implementation
> is significantly slower (in terms of RPS) than Felix's code:
>
>               |        L  A  T  E  N  C  Y       |  App   |     C P U     |
>        |  RPS |   AVG  |  P50  |   P99  |   P999 | Overld |  busy |  PSI  |
> thread | 1.1% | -15.6% | -0.3% | -42.5% |  -8.1% | -83.4% | -2.3% | 60.6% |
> work q | 4.3% | -13.1% |  0.1% | -44.4% |  -1.1% |   2.3% | -1.2% | 90.1% |
> TAPI   | 4.4% | -17.1% | -1.4% | -43.8% | -11.0% | -60.2% | -2.3% | 46.7% |
>
> thread is this code, "work q" is Felix's code, TAPI is my hacks.
>
> The numbers are comparing performance to normal NAPI.
>
> In all cases (but not the baseline) I configured timer-based polling
> (defer_hard_irqs), with around 100us timeout. Without deferring hard
> IRQs threaded NAPI is actually slower for this app. Also I'm not
> modifying niceness, this again causes application performance
> regression here.
>

If I remember correctly, Felix's workqueue code uses HIGHPRIO flag
which by default uses -20 as the nice value for the workqueue threads.
But the kthread implementation leaves nice level as 20 by default.
This could be 1 difference.
I am not sure what the benchmark is doing, but one thing to try is to
limit the CPUs that run the kthreads to a smaller # of CPUs. This
could bring up the kernel cpu usage to a higher %, e.g. > 80%, so the
scheduler is less likely to schedule user threads on these CPUs, thus
providing isolations between kthreads and the user threads, and
reducing the scheduling overhead. This could help if the throughput
drop is caused by higher scheduling latency for the user threads.
Another thing to try is to raise the scheduling class of the kthread
from SCHED_OTHER to SCHED_FIFO. This could help if the throughput drop
is caused by the kthreads experiencing higher scheduling latency.

> 1 NUMA node. 18 NAPI instances each is around 25% of a single CPU.
>
> I was initially hoping that TAPI would fit nicely as an extension
> of this code, but I don't think that will be the case.
>
> Are there any assumptions you're making about the configuration that
> I should try to replicate?