On Tue, 5 Apr 2022 at 16:44, Jens Axboe <axboe@xxxxxxxxx> wrote:
>
> On 4/5/22 1:45 AM, Miklos Szeredi wrote:
> > On Sat, 2 Apr 2022 at 03:17, Jens Axboe <axboe@xxxxxxxxx> wrote:
> >>
> >> On 4/1/22 10:21 AM, Jens Axboe wrote:
> >>> On 4/1/22 10:02 AM, Miklos Szeredi wrote:
> >>>> On Fri, 1 Apr 2022 at 17:36, Jens Axboe <axboe@xxxxxxxxx> wrote:
> >>>>
> >>>>> I take it you're continually reusing those slots?
> >>>>
> >>>> Yes.
> >>>>
> >>>>> If you have a test
> >>>>> case that'd be ideal. Agree that it sounds like we just need an
> >>>>> appropriate breather to allow fput/task_work to run. Or it could be the
> >>>>> deferral free of the fixed slot.
> >>>>
> >>>> Adding a breather could make the worst case latency be large. I think
> >>>> doing the fput synchronously would be better in general.
> >>>
> >>> fput() isn't sync, it'll just offload to task_work. There are some
> >>> dependencies there that would need to be checked. But we'll find a way
> >>> to deal with it.
> >>>
> >>>> I test this on an VM with 8G of memory and run the following:
> >>>>
> >>>> ./forkbomb 14 &
> >>>> # wait till 16k processes are forked
> >>>> for i in `seq 1 100`; do ./procreads u; done
> >>>>
> >>>> You can compare performance with plain reads (./procreads p), the
> >>>> other tests don't work on public kernels.
> >>>
> >>> OK, I'll check up on this, but probably won't have time to do so before
> >>> early next week.
> >>
> >> Can you try with this patch? It's not complete yet, there's actually a
> >> bunch of things we can do to improve the direct descriptor case. But
> >> this one is easy enough to pull off, and I think it'll fix your OOM
> >> case. Not a proposed patch, but it'll prove the theory.
> >
> > Sorry for the delay..
> >
> > Patch works like charm.
>
> OK good, then it is the issue I suspected. Thanks for testing!
Tested with v5.18-rc3 and performance seems significantly worse than
with the test patch:
test patch:
avg min max stdev
real 0.205 0.190 0.266 0.011
user 0.017 0.007 0.029 0.004
sys 0.374 0.336 0.503 0.022
5.18.0-rc3-00016-gb253435746d9:
avg min max stdev
real 0.725 0.200 18.090 2.279
user 0.019 0.005 0.046 0.006
sys 0.454 0.241 1.022 0.199
Thanks,
Miklos
