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

[Andrew Morton <akpm@osdl.org>]

On Thu, 17 Aug 2006 09:21:51 -0400
Trond Myklebust <trond.myklebust@fys.uio.no> wrote:

> 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).
> 

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.

Perhaps.  The top-level poll-the-superblocks writeback loop might tend to
prevent that from happening.  But if applications were doing a lot of
superblock-specific writeback (fdatasync,
sync_file_range(SYNC_FILE_RANGE_WRITE), etc) then unfairness might occur.