[gmx-developers] Re: flushing files

[Roland Schulz]

On Wed, Oct 13, 2010 at 2:20 AM, Erik Lindahl <lindahl at cbr.su.se> wrote:

> Hi,
>
> On Oct 13, 2010, at 7:53 AM, Roland Schulz wrote:
>
>
>
> On Wed, Oct 13, 2010 at 1:02 AM, Erik Lindahl <lindahl at cbr.su.se> wrote:
>
>> Hi,
>>
>> File flushing has been a huge issue to get working properly on AFS and
>> other systems that have an extra layer of network disk cache. We also want
>> to make sure the files are available e.g. on the frontend node of a cluster
>> while the simulation is still running.
>>
> Do we want to guarantee that it is available sooner than at each checkpoint
> (thus by default 15min)?
>
>
> It's not only a matter of "being available", but making sure you don't lose
> all that data in the disk cache layer of the node crashes and you (for some
> reason) disabled checkpointing.
>
Well but if you disabled checkpointing than it's your own fault ;-)

>
> Basically, when a frame has been "written", it is reasonable for the user
> to expect that it is actually on disk. The default behavior should be safe,
> IMHO.
>
I'm not sure whether the user necessarily assumes that. Their are well known
cases where the behavior of the cache is exposed to the user (e.g. writing
files to USB sticks). Currently GROMACS only does a fflush not a fsync after
each frame. Thus, it is not guaranteed that it is immediate on the disk
because it can still be in the kernel buffers. Already now, a fsync is only
done after each checkpoint.

The problem is that MPI-IO doesn't have this distinction. Their is only a
MPI_File_sync (and no MPI_File_flush). And a sync can be *very* expensive.

>  I think the proper solution is rather to have a separate IO thread so the
>> disk operation can take all the latency in the world without delaying the
>> run.
>>
> This won't solve it for all cases. Depending on the write frequency (e.g.
> every 10 frames) the flushing time can take longer than computing the frames
> while the actually writing time (measured as the writing time with only
> infrequent flush) is fast enough to not cause significant overhead. In those
> situations the simulation would still wait on the IO thread.
>
> Also this adds additional complexity. Not all systems like oversubscribing
> threads as far as I know. I know that older versions of Cray had problems
> and I heard their are also problems with BlueGene. Thus we would need to
> make the IO thread functionality optional which would add yet
> another duplication of code (both with and without IO thread).
>
>
> The difference is that an IO thread would virtually never run though; it
> would instantly block waiting for the filesystem, and in the mean time the
> real threads would get control back?
>
Yes but you can only have one IO thread per file (otherwise
the synchronization becomes quite difficult). Thus if the overhead is larger
than the time between writes than your are still waiting. The time for
MPI_File_sync can be * extremely* long (compared to fflush).

Roland

> ----------------------------------------------------------
>
> Erik Lindahl <lindahl at cbr.su.se>
> Professor, Computational Structural Biology
> Center for Biomembrane Research & Swedish e-Science Research Center
> Department of Biochemistry & Biophysics, Stockholm University
> Tel: +468164675 Cell: +46703844534
>
>


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