linux-kernel - Re: [PATCH 0/3 v2] mm: Batch page reclamation under shink_page

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Date:	Wed, 12 Sep 2012 12:27:58 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Mel Gorman <mel@....ul.ie>, Minchan Kim <minchan@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Michal Hocko <mhocko@...e.cz>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>, linux-mm@...ck.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Alex Shi <alex.shi@...el.com>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [PATCH 0/3 v2] mm: Batch page reclamation under shink_page_list

On Mon, 10 Sep 2012 09:19:20 -0700
Tim Chen <tim.c.chen@...ux.intel.com> wrote:

> This is the second version of the patch series. Thanks to Matthew Wilcox 
> for many valuable suggestions on improving the patches.
> 
> To do page reclamation in shrink_page_list function, there are two
> locks taken on a page by page basis.  One is the tree lock protecting
> the radix tree of the page mapping and the other is the
> mapping->i_mmap_mutex protecting the mapped
> pages.  I try to batch the operations on pages sharing the same lock
> to reduce lock contentions.  The first patch batch the operations protected by
> tree lock while the second and third patch batch the operations protected by 
> the i_mmap_mutex.
> 
> I managed to get 14% throughput improvement when with a workload putting
> heavy pressure of page cache by reading many large mmaped files
> simultaneously on a 8 socket Westmere server.

That sounds good, although more details on the performance changes
would be appreciated - after all, that's the entire point of the
patchset.

And we shouldn't only test for improvements - we should also test for
degradation.  What workloads might be harmed by this change?  I'd suggest

- a single process which opens N files and reads one page from each
  one, then repeats.  So there are no contiguous LRU pages which share
  the same ->mapping.  Get some page reclaim happening, measure the
  impact.

- The batching means that we now do multiple passes over pageframes
  where we used to do things in a single pass.  Walking all those new
  page lists will be expensive if they are lengthy enough to cause L1
  cache evictions.

  What would be a test for this?  A simple, single-threaded walk
  through a file, I guess?

Mel's review comments were useful, thanks.
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