Re: RFC - how to balance Dirty+Writeback in the face of slow writeback.

[Andrew Morton <akpm@osdl.org>]

On Thu, 17 Aug 2006 12:18:52 -0400
Trond Myklebust <trond.myklebust@fys.uio.no> wrote:

> On Thu, 2006-08-17 at 08:30 -0700, Andrew Morton wrote:
> > On Thu, 17 Aug 2006 09:21:51 -0400
> > Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> > > Exactly how would a request limit help? All that boils down to is having
> > > the VM monitor global_page_state(NR_FILE_DIRTY) versus monitoring
> > > global_page_state(NR_FILE_DIRTY)+global_page_state(NR_WRITEBACK).
> > > 
> > 
> > I assume that if NFS is not limiting its NR_WRITEBACK consumption and block
> > devices are doing so, we could get in a situation where NFS hogs all of the
> > fixed-size NR_DIRTY+NR_WRITEBACK resource at the expense of concurrent
> > block-device-based writeback.
> 
> Since NFS has no control over NR_DIRTY, how does controlling
> NR_WRITEBACK help? The only resource that NFS shares with the block
> device writeout queues is memory.

Block devices have a limit on the amount of IO which they will queue.  NFS
doesn't.

> IOW: The resource that needs to be controlled is the dirty pages, not
> the write-out queue. Unless you can throttle back on the creation of
> dirty NFS pages in the first place, then the potential for unfairness
> will exist.

Please read the whole thread - we're violently agreeing.