[Devel] Re: [PATCH 1/9] io-throttle documentation

[Vivek Goyal]

On Tue, Apr 21, 2009 at 10:23:05AM -0400, Vivek Goyal wrote:
> On Tue, Apr 21, 2009 at 10:37:03AM +0200, Andrea Righi wrote:
> > On Mon, Apr 20, 2009 at 09:08:46PM -0400, Vivek Goyal wrote:
> > > On Tue, Apr 21, 2009 at 12:05:12AM +0200, Andrea Righi wrote:
> > > 
> > > [..]
> > > > > > > Are we not already controlling submission of request (at crude level).
> > > > > > > If application is doing writeout at high rate, then it hits vm_dirty_ratio
> > > > > > > hits and this application is forced to do write out and hence it is slowed
> > > > > > > down and is not allowed to submit writes at high rate.
> > > > > > > 
> > > > > > > Just that it is not a very fair scheme right now as during right out
> > > > > > > a high prio/high weight cgroup application can start writing out some
> > > > > > > other cgroups' pages.
> > > > > > > 
> > > > > > > For this we probably need to have some combination of solutions like
> > > > > > > per cgroup upper limit on dirty pages. Secondly probably if an application
> > > > > > > is slowed down because of hitting vm_drity_ratio, it should try to
> > > > > > > write out the inode it is dirtying first instead of picking any random
> > > > > > > inode and associated pages. This will ensure that a high weight
> > > > > > > application can quickly get through the write outs and see higher
> > > > > > > throughput from the disk.
> > > > > > 
> > > > > > For the first, I submitted a patchset some months ago to provide this
> > > > > > feature in the memory controller:
> > > > > > 
> > > > > > https://lists.linux-foundation.org/pipermail/containers/2008-September/013140.html
> > > > > > 
> > > > > > We focused on the best interface to use for setting the dirty pages
> > > > > > limit, but we didn't finalize it. I can rework on that and repost an
> > > > > > updated version. Now that we have the dirty_ratio/dirty_bytes to set the
> > > > > > global limit I think we can use the same interface and the same semantic
> > > > > > within the cgroup fs, something like:
> > > > > > 
> > > > > >   memory.dirty_ratio
> > > > > >   memory.dirty_bytes
> > > > > > 
> > > > > > For the second point something like this should be enough to force tasks
> > > > > > to write out only the inode they're actually dirtying when they hit the
> > > > > > vm_dirty_ratio limit. But it should be tested carefully and may cause
> > > > > > heavy performance regressions.
> > > > > > 
> > > > > > Signed-off-by: Andrea Righi <righi.andrea at gmail.com>
> > > > > > ---
> > > > > >  mm/page-writeback.c |    2 +-
> > > > > >  1 files changed, 1 insertions(+), 1 deletions(-)
> > > > > > 
> > > > > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > > > > > index 2630937..1e07c9d 100644
> > > > > > --- a/mm/page-writeback.c
> > > > > > +++ b/mm/page-writeback.c
> > > > > > @@ -543,7 +543,7 @@ static void balance_dirty_pages(struct address_space *mapping)
> > > > > >  		 * been flushed to permanent storage.
> > > > > >  		 */
> > > > > >  		if (bdi_nr_reclaimable) {
> > > > > > -			writeback_inodes(&wbc);
> > > > > > +			sync_inode(mapping->host, &wbc);
> > > > > >  			pages_written += write_chunk - wbc.nr_to_write;
> > > > > >  			get_dirty_limits(&background_thresh, &dirty_thresh,
> > > > > >  				       &bdi_thresh, bdi);
> > > > > 
> > > > > This patch seems to be helping me a bit in getting more service
> > > > > differentiation between two writer dd of different weights. But strangely
> > > > > it is helping only for ext3 and not ext4. Debugging is on.
> > > > 
> > > > Are you explicitly mounting ext3 with data=ordered?
> > > 
> > > Yes. Still using 29-rc8 and data=ordered was the default then.
> > > 
> > > I got two partitions on same disk and created one ext3 filesystem on each
> > > partition (just to take journaling intereference out of two dd threads 
> > > for the time being).
> > > 
> > > Two dd threads doing writes to each partition. 
> > 
> > ...and if you're using data=writeback with ext4 sync_inode() should sync
> > the metadata only. If this is the case, could you check data=ordered
> > also for ext4?
> 
> No, even data=ordered mode with ext4 is also not helping. It has to be
> something else.
> 

Ok, with data=ordered mode with ext4, now I can get significant service
differentiation between two dd processes. I had to tweak cfq a bit.

- Instead of 40ms slice for async queue, do 20ms at a time (tunable).
- change cfq quantum to 1 from 4 to not dispatch a bunch of requests at
  one go.

Above changes help a bit in making sure two continuously backlogged queues
at IO scheduler so that IO scheduler can offer more disk time to higher
weight process.

Thanks
Vivek

> BTW, with the above patch, what happens if the address space being dirtied
> does not have sufficient dirty pages to write back (more than write_chunk).
> Will the process not be in loop until the number of dirty pages come down
> (hopefully due to writeout by pdflush or by other processes?)
> 
> Thanks
> Vivek
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