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

[Trond Myklebust <trond.myklebust@fys.uio.no>]

On Thu, 2006-08-17 at 13:59 +1000, Neil Brown wrote:
> On Tuesday August 15, akpm@osdl.org wrote:
> > > When Dirty hits 0 (and Writeback is theoretically 80% of RAM)
> > > balance_dirty_pages will no longer be able to flush the full
> > > 'write_chunk' (1.5 times number of recent dirtied pages) and so will
> > > spin in a loop calling blk_congestion_wait(WRITE, HZ/10), so it isn't
> > > a busy loop, but it won't progress.
> > 
> > This assumes that the queues are unbounded.  They're not - they're limited
> > to 128 requests, which is 60MB or so.
> 
> Ahhh... so the limit on the requests-per-queue is an important part of
> write-throttling behaviour.  I didn't know that, thanks.
> 
> fs/nfs doesn't seem to impose a limit.  It will just allocate as many
> as you ask for until you start running out of memory.  I've seen 60%
> of memory (10 out of 16Gig) in writeback for NFS.
> 
> Maybe I should look there to address my current issue, though imposing
> a system-wide writeback limit seems safer.

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).

Cheers,
  Trond