Re: [PATCH 02/11] writeback: switch to per-bdi threads forflushing data

[Richard Kennedy]

On Tue, 2009-05-19 at 14:23 +0200, Jens Axboe wrote:
> On Tue, May 19 2009, Richard Kennedy wrote:
> > Jens Axboe wrote:
> > > This gets rid of pdflush for bdi writeout and kupdated style cleaning.
> > > <snip>
> > > index 2296ff4..76269f8 100644
> > > --- a/mm/page-writeback.c
> > > +++ b/mm/page-writeback.c
> > > @@ -541,7 +530,7 @@ static void balance_dirty_pages(struct address_space *mapping)
> > >  		 * been flushed to permanent storage.
> > >  		 */
> > >  		if (bdi_nr_reclaimable) {
> > > -			writeback_inodes(&wbc);
> > > +			generic_sync_bdi_inodes(NULL, &wbc);
> > >  			pages_written += write_chunk - wbc.nr_to_write;
> > >  			get_dirty_limits(&background_thresh, &dirty_thresh,
> > >  				       &bdi_thresh, bdi);
> > > @@ -592,7 +581,7 @@ static void balance_dirty_pages(struct address_space *mapping)
> > >  			(!laptop_mode && (global_page_state(NR_FILE_DIRTY)
> > >  					  + global_page_state(NR_UNSTABLE_NFS)
> > >  					  > background_thresh)))
> > > -		pdflush_operation(background_writeout, 0);
> > > +		bdi_start_writeback(bdi, NULL, 0);
> > >  }
> > >  
> > Hi Jens,
> > 
> > I'm interested in this slight change of behaviour, when over the
> > background dirty limit background_writeout will write any dirty pages
> > while bdi_start_writeout writes only pages for the current bdi. Are
> > there any benefits in making this change?
> > 
> > Thinking about the case of 2 apps writing to different bdis. When app A
> > stops writing, then next time app B goes over the background dirty
> > threshold it will only be able to write its own pages, leaving any from
> > app A dirty until they reach their age limit.
> 
> The function in question balances dirty pages against a specific address
> space, which has a specific mapping. The async part of the background
> writeout could be global as you mention. The whole thing is a bit weird
> in balance_dirty_pages(), for instance it checks for writeout against a
> given queue then proceeds to do a global writeout if not busy. At least
> it's consistent now.
> 
> > So we may be keeping dirty pages for the app that's finished longer than
> > necessary. Keeping pages for a finished app while flushing pages from a
> > running app seems a bit strange. I guess this is an odd corner case and
> > may not be worth worrying about, but I'd be interested to hear what you
> > think.
> 
> The kupdated() initiated background writeout will take care of that, if
> nobody does a sync on that data first. If nobody is dirtying new data on
> the given bdi, then it seems perfectly fine to let normal background
> writeout handle it.
> 
> > Do you think your new code will require any changes to the per bdi dirty
> > limits? It may be informative & interesting to run some tests writing to
> > fast & slow devices at the same time.
> 
> Generally the code should behave fairly closely to the existing pdflush
> based code, so I don't think bdi dirty limit tweaking will be necessary.
> I'd definitely welcome some testing though, particularly slow vs fast as
> you mention. I've mainly been doing benchmarking to make sure we don't
> regress on performance, and that has been for fairly similar hardware.
> Since testing does take a lot of time, it would be nice if someone else
> would gather their own experiences, especially in areas that have been
> problematic in the past (slow vs fast devices, for instance!).

Thanks for the explanation.
I'm definitely going to test this, although I don't have any interesting
hardware, only a basic workstation. But I'll let you know if I turn up
anything useful.

Balance_dirty_pages contains Peter Zijlstra's per bdi write throttling
code and I wonder if it will need tuning for best performance with your
changes, just because some of its assumptions may have changed. I'll run
some tests here and see what happens. Peter may have some insight and
possibly useful test cases.

regards
Richard

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