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

[Jakub Kicinski <kuba@kernel.org>]

On Tue, 29 Sep 2020 13:16:59 -0700 Wei Wang wrote:
> On Tue, Sep 29, 2020 at 12:19 PM Jakub Kicinski <kuba@kernel.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.

FWIW this is the data based on which I concluded the nice -20 actually
makes things worse here:

      threded: -1.50%
 threded p-20: -5.67%
     thr poll:  2.93%
thr poll p-20:  2.22%

Annoyingly relative performance change varies day to day and this test
was run a while back (over the weekend I was getting < 2% improvement
with this set).

> I am not sure what the benchmark is doing

Not a benchmark, real workload :)

> 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.

Yeah... If I do pinning or isolation I can get to 15% RPS improvement
for this application.. no threaded NAPI needed. The point for me is to
not have to do such tuning per app x platform x workload of the day.

> 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.

Isn't the fundamental problem that scheduler works at ms scale while
where we're talking about 100us at most? And AFAICT scheduler doesn't
have a knob to adjust migration cost per process? :(

I just reached out to the kernel experts @FB for their input.

Also let me re-run with a normal prio WQ.

> > 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?