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.

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.

    Thanx...

        ps

Signed-off-by: Peter Staubach <staubach@redhat.com>