Re: [PATCH 6/7] mm: page_alloc: Only account batch allocations requests that are eligible

[Mel Gorman <mgorman@suse.de>]

On Tue, Dec 17, 2013 at 10:43:51AM -0500, Johannes Weiner wrote:
> On Tue, Dec 17, 2013 at 11:20:07AM +0000, Mel Gorman wrote:
> > On Mon, Dec 16, 2013 at 03:52:37PM -0500, Johannes Weiner wrote:
> > > On Fri, Dec 13, 2013 at 02:10:06PM +0000, Mel Gorman wrote:
> > > > Not signed off. Johannes, was the intent really to decrement the batch
> > > > counts regardless of whether the policy was being enforced or not?
> > > 
> > > Yes.  Bursts of allocations for which the policy does not get enforced
> > > will still create memory pressure and affect cache aging on a given
> > > node.  So even if we only distribute page cache, we want to distribute
> > > it in a way that all allocations on the eligible zones equal out.
> > 
> > This means that allocations for page table pages affects the distribution of
> > page cache pages. An adverse workload could time when it faults anonymous
> > pages (to allocate anon and page table pages) in batch sequences and then
> > access files to force page cache pages to be allocated from a single node.
> > 
> > I think I know what your response will be. It will be that the utilisation of
> > the zone for page table pages and anon pages means that you want more page
> > cache pages to be allocated from the other zones so the reclaim pressure
> > is still more or less even. If this is the case or there is another reason
> > then it could have done with a comment because it's a subtle detail.
> 
> Yes, that was the idea, that the cache placement compensates for pages
> that still are always allocated on the preferred zone first, so that
> the end result is approximately as if round-robin had been applied to
> everybody.
> 

Ok, understood. I wanted to be sure that was the thinking behind it.

> This should be documented as part of the patch that first diverges
> between the allocations that are counted and the allocations that are
> round-robined:
> 
>   mm: page_alloc: exclude unreclaimable allocations from zone fairness policy
> 
> I'm updating my tree.

I'll leave it alone in mine then. We'll figure out how to sync up later.

-- 
Mel Gorman
SUSE Labs