Re: nfsd write throughput

[Greg Banks <gnb@sgi.com>]

On Tue, Aug 03, 2004 at 10:09:13AM +0200, Olaf Kirch wrote:
> On Tue, Aug 03, 2004 at 05:55:06PM +1000, Greg Banks wrote:
> > I think another useful approach would be to writeback pages which
> > have been written by NFS unstable writes at a faster rate than pages
> > written by local applications, i.e. add a new /proc/vm/ sysctl like
> > nfs_dirty_writeback_centisecs and a per-page flag.
> 
> The problem with this approach is that we have no access to the
> pages. nfsd_write goes through writev.

I understand that the IRIX approach involves passing a special
flag through the equivalent of vfs_writev().  For Linux you
could probably do this with a magic flag in the struct file.
Obviously this is non-trivial.

> > > That may be a useful solution, too. My patch basically does what
> > > fadvise(WONTNEED) does.
> > 
> > Sure, the key question is when and for how many pages.  You don't
> > really have enough information in nfsd_write() to tell that safely.
> 
> Well, for streaming writes "everything we've written so far" is a
> reasonable approximation. Random writes may receive a penalty, I
> admit.

Also reverse writes, and writes of many complete small (too small
to kick in the streaming heuristic) files.  Doing stuff in the page
cache has a better chance of handling those cases.

> > It writes every time `offset' is a multiple of 64 times `cnt' and
> > `cnt' is a multiple of 1024.  At this point `cnt' is the length
> > of the data received in the WRITE call, which has only a vague
> > relationship to the client page size.
> 
> Okay, I should have been more precise: the test tries to make
> sure we're seeing a full wsize worth of data, which is usually a
> good indication of writes being streamed, and tries to lump
> enough of them together to allow the file system to make an
> intelligent decision.

The two problems are:

1.  the heuristic is too simple and can be fooled by a number of
    non-streaming access patterns which won't benefit from the
    early flush.  You could fix this by keeping per-file state
    like the readahead state, but...

2.  even when the heuristic does detect a streaming write it may
    be too early to usefully flush data.

In both cases the page cache has a better chance of getting it right.

> I'm not claiming this is god's wisdom - I'm trying out ideas :)

Sure.

Greg.
-- 
Greg Banks, R&D Software Engineer, SGI Australian Software Group.
I don't speak for SGI.


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