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

[Vivek Goyal]

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.

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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