Re: [PATCH v5 1/1] Allow non-extending parallel direct writes on the same file.

[Miklos Szeredi]

On Tue, 13 Sept 2022 at 10:44, Bernd Schubert <bschubert@xxxxxxx> wrote:
>
>
>
> On 6/17/22 14:43, Miklos Szeredi wrote:
> > On Fri, 17 Jun 2022 at 11:25, Bernd Schubert <bernd.schubert@xxxxxxxxxxx> wrote:
> >>
> >> Hi Miklos,
> >>
> >> On 6/17/22 09:36, Miklos Szeredi wrote:
> >>> On Fri, 17 Jun 2022 at 09:10, Dharmendra Singh <dharamhans87@xxxxxxxxx> wrote:
> >>>
> >>>> This patch relaxes the exclusive lock for direct non-extending writes
> >>>> only. File size extending writes might not need the lock either,
> >>>> but we are not entirely sure if there is a risk to introduce any
> >>>> kind of regression. Furthermore, benchmarking with fio does not
> >>>> show a difference between patch versions that take on file size
> >>>> extension a) an exclusive lock and b) a shared lock.
> >>>
> >>> I'm okay with this, but ISTR Bernd noted a real-life scenario where
> >>> this is not sufficient.  Maybe that should be mentioned in the patch
> >>> header?
> >>
> >>
> >> the above comment is actually directly from me.
> >>
> >> We didn't check if fio extends the file before the runs, but even if it
> >> would, my current thinking is that before we serialized n-threads, now
> >> we have an alternation of
> >>          - "parallel n-1 threads running" + 1 waiting thread
> >>          - "blocked  n-1 threads" + 1 running
> >>
> >> I think if we will come back anyway, if we should continue to see slow
> >> IO with MPIIO. Right now we want to get our patches merged first and
> >> then will create an updated module for RHEL8 (+derivatives) customers.
> >> Our benchmark machines are also running plain RHEL8 kernels - without
> >> back porting the modules first we don' know yet what we will be the
> >> actual impact to things like io500.
> >>
> >> Shall we still extend the commit message or are we good to go?
> >
> > Well, it would be nice to see the real workload on the backported
> > patch.   Not just because it would tell us if this makes sense in the
> > first place, but also to have additional testing.
>
>
> Sorry for the delay, Dharmendra and me got busy with other tasks and
> Horst (in CC) took over the patches and did the MPIIO benchmarks on 5.19.
>
> Results with https://github.com/dchirikov/mpiio.git
>
>                 unpatched    patched      patched
>                 (extending) (extending)  (non-extending)
> ----------------------------------------------------------
>                  MB/s        MB/s            MB/s
> 2 threads     2275.00      2497.00       5688.00
> 4 threads     2438.00      2560.00      10240.00
> 8 threads     2925.00      3792.00      25600.00
> 16 threads    3792.00     10240.00      20480.00
>
>
> (Patched-nonextending is a manual operation on the file to extend the
> size, mpiio does not support that natively, as far as I know.)
>
>
>
> Results with IOR (HPC quasi standard benchmark)
>
> ior -w -E -k -o /tmp/test/home/hbi/test/test.1 -a mpiio -s 1280 -b 8m -t 8m
>
>
>                 unpatched       patched
>                 (extending)     (extending)
> -------------------------------------------
>                    MB/s           MB/s
> 2 threads       2086.10         2027.76
> 4 threads       1858.94         2132.73
> 8 threads       1792.68         4609.05
> 16 threads      1786.48         8627.96
>
>
> (IOR does not allow manual file extension, without changing its code.)
>
> We can see that patched non-extending gives the best results, as
> Dharmendra has already posted before, but results are still
> much better with the patches in extending mode. My assumption is here
> instead serializing N-writers, there is an alternative
> run of
>         - 1 thread extending, N-1 waiting
>         - N-1 writing, 1 thread waiting
> in the patched version.
>

Okay, thanks for the heads up.

I queued the patch up for v6.2

Thanks,
Miklos


>
>
> Thanks,
> Bernd