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

[Bernd Schubert <bschubert@ddn.com>]

On 10/21/22 08:57, Miklos Szeredi wrote:
> On Tue, 13 Sept 2022 at 10:44, Bernd Schubert <bschubert@ddn.com> wrote:
>>
>>
>>
>> On 6/17/22 14:43, Miklos Szeredi wrote:
>>> On Fri, 17 Jun 2022 at 11:25, Bernd Schubert <bernd.schubert@fastmail.fm> wrote:
>>>>
>>>> Hi Miklos,
>>>>
>>>> On 6/17/22 09:36, Miklos Szeredi wrote:
>>>>> On Fri, 17 Jun 2022 at 09:10, Dharmendra Singh <dharamhans87@gmail.com> 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
> 

Thank you!