Re: nfsd write throughput

[Olaf Kirch <okir@suse.de>]

On Tue, Aug 03, 2004 at 09:24:46PM +1000, Greg Banks wrote:
> With IRIX clients, which do far fewer COMMITs than (at least 2.4) Linux
> clients, I have seen COMMIT latencies in the order of multiple seconds
> as over a gigabyte of data is written to disk at 180 MB/s.

Yes - commit latencies varies a lot, depending on disk speed, client,
network bandwidth, and the number of clients.

> I imagine they will not be thrilled by the idea.

Probably not :-) And judging by the experiments I did with this, it's
not worth it - now we end up woth all nfsd's stuck in filemap_fdatawait.

> I still think the best approach is to get the page cache to start
> pushing unstable NFS pages to disk more aggresively, after the WRITE
> but before the COMMIT.  This should avoid long waits for disk IO
> with i_sem held; IIRC the page cache will only hold i_sem long enough
> to traverse page lists, allowing another WRITE call to get in soon.

Right.

I looked into how else I could do this, using the normal
background writeout as you suggested. I looked at the way
pdflush_operation(background_writeout) is doing it, but I'm wondering
if this is the right place for us. background_writeout loops over all
dirty inodes in all super blocks, only to call do_writepages in the end
pretty much the same way filemap_flush does - and in the end it
may send out the wrong pages.

So in order to implement what you suggest, we would need to change a lot
of code in page-writeback.c and fs-writeback.c - just for the benefit
of nfsd. Is this worth it?

On the other hand, fadvise and filemap_flush is a perfectly sane way
of telling the kernel to get rid of a specific range of pages we're no
longer interested in, without having to have pdflush do a linear crawl
over all dirty supers and inodes.

So I guess the question I'm asking is - what would be a reasonable
heuristic for nfsd_write that improves streaming writes without hurting
random ones, and that doesn't cause fragmentation etc?

Olaf
-- 
Olaf Kirch     |  The Hardware Gods hate me.
okir@suse.de   |
---------------+ 


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