Re: [PATCH v2] flow control for WRITE requests

Peter Staubach
2009-05-27T19:18:44Z

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J. Bruce Fields wrote:
> On Tue, Mar 24, 2009 at 03:31:50PM -0400, Peter Staubach wrote:
>
>> Hi.
>>
>> Attached is a patch which implements some flow control for the
>> NFS client to control dirty pages. The flow control is
>> implemented on a per-file basis and causes dirty pages to be
>> written out when the client can detect that the application is
>> writing in a serial fashion and has dirtied enough pages to
>> fill a complete over the wire transfer.
>>
>> This work was precipitated by working on a situation where a
>> server at a customer site was not able to adequately handle
>> the behavior of the Linux NFS client. This particular server
>> required that all data to the file written to the file be
>> written in a strictly serial fashion. It also had problems
>> handling the Linux NFS client semantic of caching a large
>> amount of data and then sending out that data all at once.
>>
>> The sequential ordering problem was resolved by a previous
>> patch which was submitted to the linux-nfs list. This patch
>> addresses the capacity problem.
>>
>> The problem is resolved by sending WRITE requests much
>> earlier in the process of the application writing to the file.
>> The client keeps track of the number of dirty pages associated
>> with the file and also the last offset of the data being
>> written. When the client detects that a full over the wire
>> transfer could be constructed and that the application is
>> writing sequentially, then it generates an UNSTABLE write to
>> server for the currently dirty data.
>>
>> The client also keeps track of the number of these WRITE
>> requests which have been generated. It flow controls based
>> on a configurable maximum. This keeps the client from
>> completely overwhelming the server.
>>
>> A nice side effect of the framework is that the issue of
>> stat()'ing a file being written can be handled much more
>> quickly than before. The amount of data that must be
>> transmitted to the server to satisfy the "latest mtime"
>> requirement is limited. Also, the application writing to
>> the file is blocked until the over the wire GETATTR is
>> completed. This allows the GETATTR to be send and the
>> response received without competing with the data being
>> written.
>>
>> No performance regressions were seen during informal
>> performance testing.
>>
>> As a side note -- the more natural model of flow control
>> would seem to be at the client/server level instead of
>> the per-file level. However, that level was too coarse
>> with the particular server that was required to be used
>> because its requirements were at the per-file level.
>>
>
> I don't understand what you mean by "its requirements were at the
> per-file level".
>
>
>> The new functionality in this patch is controlled via the
>> use of the sysctl, nfs_max_outstanding_writes. It defaults
>> to 0, meaning no flow control and the current behaviors.
>> Setting it to any non-zero value enables the functionality.
>> The value of 16 seems to be a good number and aligns with
>> other NFS and RPC tunables.
>>
>> Lastly, the functionality of starting WRITE requests sooner
>> to smooth out the i/o pattern should probably be done by the
>> VM subsystem. I am looking into this, but in the meantime
>> and to solve the immediate problem, this support is proposed.
>>
>
> It seems unfortunate if we add a sysctl to work around a problem that
> ends up being fixed some other way a version or two later.
>
> Would be great to have some progress on these problems, though....
>
> --b.
>

Hi.

I have attached a new testcase which exhibits this particular
situation. One script writes out 6 ~1GB files in parallel,
while the other script is simultaneously running an "ls -l"
in the directory.

When run on a system large enough to store all ~6GB of data,
the dd processes basically write(2) all of their data into
memory very quickly and then spend most of their time in the
close(2) system call flushing the page cache due to the close
to open processing.

The current flow control support in the NFS client does not work
well for this situation. It was designed to catch the process
filling memory and to block it while the page cache flush is
being done by the process doing the stat(2).

The problem with this approach is that there could potentially be
gigabytes of page cache which needs to be flushed to the server
during the stat(2) processing. This blocks the application
doing the stat(2) for potentially a very long time, based on the
amount of data which was cached, the speed of the network, and
the speed of the server.

The solution is to limit the amount of data that must be flushed
during the stat(2) call. This can be done by starting i/o when
the application has filled enough pages to fill an entire wsize'd
transfer and by limiting the number of these transfers which are
outstanding so as not to overwhelm the server.

-----------

While it seems that it would be good to have this done by the
VM itself, the current architecture of the VM does not seem to
yield itself easily to doing this. It seems like doing something
like a per-file bdi would do the trick, however the system is
not scalable to the number of bdi's that that would require.

I am open to suggestions for alternate solutions, but in the
meantime, this support does seem to address the situation. In
my test environment, it also increases, significantly,
performance when sequentially writing large files. My throughput
when dd'ing /dev/sda1 to an NFS mounted file went from ~22MB/s
to ~38MB/s. (I do this for image backups for my laptop.) Your
mileage may vary however. :-)

So, we can consider taking this so that we can address some
customer needs?

Thanx...

ps

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