Re: RFC: default group_isolation to 1, remove option

[Andrea Righi]

On Wed, Mar 02, 2011 at 04:28:45PM -0500, Vivek Goyal wrote:
> On Tue, Mar 01, 2011 at 10:44:52AM -0800, Justin TerAvest wrote:
> > On Tue, Mar 1, 2011 at 6:20 AM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> > > On Mon, Feb 28, 2011 at 04:19:43PM -0800, Justin TerAvest wrote:
> > >> Hi Vivek,
> > >>
> > >> I'd like to propose removing the group_isolation setting and changing
> > >> the default to 1. Do we know if anyone is using group_isolation=0 to get
> > >> easy group separation between sequential readers and random readers?
> > >
> > > CCing Corrado.
> > >
> > > I like the idea of setting group_isolation = 1 to default. So far I have
> > > not found anybody trying to use group_isolation=0 and every time I had
> > > to say can you try setting group_isolation to 1 if you are not seeing
> > > service differentiation.
> > >
> > > I think I would not mind removing it completely altogether also. This will
> > > also remove some code from CFQ. The reason we introduced group_isolation
> > > because by default we idle on sync-noidle tree and on fast devices idling on
> > > every syn-noidle tree can be very harmful for throughput, especially on faster
> > > storage like storage arrays.
> > >
> > > One of the soutions for that problem can be that run with slice idle
> > > enabled on SATA disks and run with slice_idle=0 and possibly group_idle=0
> > > also on faster storage. Setting idling to 0 will increase throughput but
> > > at the same time reduce the isolation significantly. But I guess this
> > > is the performance vs isolation trade off.
> > 
> > I agree. Thanks! I'll send a patch shortly, CCing everyone here and we
> > can have any further discussion there.
> > 
> > >
> > >>
> > >> Allowing group_isolation complicates implementing per-cgroup request
> > >> descriptor pools when a queue is moved to the root group. Specifically,
> > >> if we have pools per-cgroup, we would be forced to use request
> > >> descriptors from the pool for the "original" cgroup, while the requests
> > >> are actually being serviced by the root cgroup.
> > >
> > > I think creating per group request pool will complicate the implementation
> > > further. (we have done that once in the past). Jens once mentioned that
> > > he liked number of requests per iocontext limit better than overall queue
> > > limit. So if we implement per iocontext limit, it will get rid of need
> > > of doing anything extra for group infrastructure.
> > 
> > I will go read the discussion history for this, but I am concerned that doing
> > page tracking to look up the iocontext will be more complicated than
> > tracking dirtied pages per-cgroup. I would hope there is a big advantage
> > to per icontext limits.
> 
> Advantage of iocontext limit I think is that we don't have to implement
> per group request descriptor pools or limits.
> 
> Secondly, it also allows some isolation between two processes/iocontexts
> with-in the group.
> 
> So we could think of a page taking a reference on iocontext but the
> real issue would be how to store the pointer of that iocontext in
> page_cgroup. We are already hard pressed for space and cssid consumes
> less bits.
> 
> So may be keeping it per group might turn out to be simpler.
> 
> Also once we were talking (I think at LSF last year) about associating an
> inode to a cgroup. So a bio can point to page and page can point to its inode
> and we get the pointer to the cgroup. This approach should not require tracking
> information in page and at coarse level it should work (as long as
> applications are working on separate inodes).
> 
> I think another advantage of this scheme is that request queue can
> export per cgroup congestion mechanism. So if a flusher thread picks
> an inode for writting back the pages, it can retrieve the cgroup from
> inode and enquire block layer if associated cgroup on the device is
> congested and if it is, flusher thread can move on to next inode.
> 
> In fact if we are just worried about isolating READS from WRITES, then
> we can skip implementing per group congestion idea. Even if flusher
> thread gets blocked because of request descriptors, it does not impact
> reads. I think at last summit people did not like the idea of congestion
> per group per bdi.

I'm not so confident about this approach and I think we should never
block flusher threads. READs are not probably affected, but the
writeback of dirty pages for that particular block device will be
stopped. This means that all the tasks that are generating dirty pages
for that device will soon exceed the dirty_ratio limit and will start to
actively writeback dirty pages.

I would consider the writeback IO performed (only) by the flusher thread
a "good" condition. Or better, if flusher threads are able to maintain
the amount of dirty pages below dirty_background_ratio limit, then they
shouldn't be throttled.

Instead, when normal tasks are forced to writeback dirty pages to disk
via writeback_inodes_wb(), because flusher threads aren't able to
maintain the quota of dirty pages below the limit, well, only in that
case IMHO each task should be forced to writeback the inodes of the
cgroup they belong (when we'll have the inode->cgroup mapping, or kind
of) and in case stop those tasks if blkio limits are exceeded.

> 
> One could do similar things with getting page->cgroup pointer also but
> then flusher threads will also have to go throgh all the pages of
> inode before it can decide whether to dispatch some IO or not and that
> might slow down the writeback.
> 
> So if we are ready to sacrifice some granularity of async writeback
> control and do it per inode basis instead of per page basis, it might
> simplify the implementation and reduce performance overhead.
> 
> CCing Greg and Andrea. They might have thoughts on this.
> 
> Thanks
> Vivek

Thanks,
-Andrea
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