Date:	Fri, 8 Aug 2014 09:26:35 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 1/4] mm: memcontrol: reduce reclaim invocations for
 higher order requests

On Fri, Aug 08, 2014 at 02:32:58PM +0200, Michal Hocko wrote:
> On Thu 07-08-14 11:31:41, Johannes Weiner wrote:
> > On Thu, Aug 07, 2014 at 03:08:22PM +0200, Michal Hocko wrote:
> > > On Mon 04-08-14 17:14:54, Johannes Weiner wrote:
> > > > Instead of passing the request size to direct reclaim, memcg just
> > > > manually loops around reclaiming SWAP_CLUSTER_MAX pages until the
> > > > charge can succeed.  That potentially wastes scan progress when huge
> > > > page allocations require multiple invocations, which always have to
> > > > restart from the default scan priority.
> > > > 
> > > > Pass the request size as a reclaim target to direct reclaim and leave
> > > > it to that code to reach the goal.
> > > 
> > > THP charge then will ask for 512 pages to be (direct) reclaimed. That
> > > is _a lot_ and I would expect long stalls to achieve this target. I
> > > would also expect quick priority drop down and potential over-reclaim
> > > for small and moderately sized memcgs (e.g. memcg with 1G worth of pages
> > > would need to drop down below DEF_PRIORITY-2 to have a chance to scan
> > > that many pages). All that done for a charge which can fallback to a
> > > single page charge.
> > > 
> > > The current code is quite hostile to THP when we are close to the limit
> > > but solving this by introducing long stalls instead doesn't sound like a
> > > proper approach to me.
> > 
> > THP latencies are actually the same when comparing high limit nr_pages
> > reclaim with the current hard limit SWAP_CLUSTER_MAX reclaim,
> 
> Are you sure about this? I fail to see how they can be same as THP
> allocations/charges are __GFP_NORETRY so there is only one reclaim
> round for the hard limit reclaim followed by the charge failure if
> it is not successful.

I use this test program that faults in anon pages, reports average and
max for every 512-page chunk (THP size), then reports the aggregate at
the end:

memory.max:

avg=18729us max=450625us

real    0m14.335s
user    0m0.157s
sys     0m6.307s

memory.high:

avg=18676us max=457499us

real    0m14.375s
user    0m0.046s
sys     0m4.294s

> > although system time is reduced with the high limit.
> > High limit reclaim with SWAP_CLUSTER_MAX has better fault latency but
> > it doesn't actually contain the workload - with 1G high and a 4G load,
> > the consumption at the end of the run is 3.7G.
> 
> Wouldn't it help to simply fail the charge and allow the charger to
> fallback for THP allocations if the usage is above high limit too
> much? The follow up single page charge fallback would be still
> throttled.

This is about defining the limit semantics in unified hierarchy, and
not really the time or place to optimize THP charge latency.

What are you trying to accomplish here?

> > So what I'm proposing works and is of equal quality from a THP POV.
> > This change is complicated enough when we stick to the facts, let's
> > not make up things based on gut feeling.
> 
> Agreed and I would expect those _facts_ to be part of the changelog.

You made unfounded claims about THP allocation latencies, and I showed
the numbers to refute it, but that doesn't make any of this relevant
for the changelogs of these patches.
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