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".
>
>
This particular server had a cache for WRITE requests on a
per-file basis. It could only write data to the underlying
file on a strictly sequential basis because the underlying
file was a variable length record based file. Out of order
data coming from the client was cached until the correct
data had been received.
>> 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....
I agree with this. We need something and waiting for the final,
perfect solution won't help the situation that I was asked to
look at.
I suspect that we will need some way to control the number of
outstanding WRITE requests, no matter what the underlying
mechanism ends up being. This will need to be at least at the
granularity of a per-file system. System wide will be too
coarse.
Thanx...
ps
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