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

From: Trond Myklebust
Date: Thu Aug 17 2006 - 12:16:35 EST

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@xxxxxxxxxx> 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.

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.


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