linux-kernel - Re: [RFC PATCH 0/2] Removal of lumpy reclaim

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Date:	Fri, 6 Apr 2012 12:34:39 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC PATCH 0/2] Removal of lumpy reclaim

On Wed, 28 Mar 2012 17:06:21 +0100
Mel Gorman <mgorman@...e.de> wrote:

> (cc'ing active people in the thread "[patch 68/92] mm: forbid lumpy-reclaim
> in shrink_active_list()")
> 
> In the interest of keeping my fingers from the flames at LSF/MM, I'm
> releasing an RFC for lumpy reclaim removal.

I grabbed them, thanks.

>
> ...
>
> MMTests Statistics: vmstat
> Page Ins                                     5426648     2840348     2695120
> Page Outs                                    7206376     7854516     7860408
> Swap Ins                                       36799           0           0
> Swap Outs                                      76903           4           0
> Direct pages scanned                           31981       43749      160647
> Kswapd pages scanned                        26658682     1285341     1195956
> Kswapd pages reclaimed                       2248583     1271621     1178420
> Direct pages reclaimed                          6397       14416       94093
> Kswapd efficiency                                 8%         98%         98%
> Kswapd velocity                            22134.225    1127.205    1051.316
> Direct efficiency                                20%         32%         58%
> Direct velocity                               26.553      38.367     141.218
> Percentage direct scans                           0%          3%         11%
> Page writes by reclaim                       6530481           4           0
> Page writes file                             6453578           0           0
> Page writes anon                               76903           4           0
> Page reclaim immediate                        256742       17832       61576
> Page rescued immediate                             0           0           0
> Slabs scanned                                1073152      971776      975872
> Direct inode steals                                0      196279      205178
> Kswapd inode steals                           139260       70390       64323
> Kswapd skipped wait                            21711           1           0
> THP fault alloc                                    1         126         143
> THP collapse alloc                               324         294         224
> THP splits                                        32           8          10
> THP fault fallback                                 0           0           0
> THP collapse fail                                  5           6           7
> Compaction stalls                                364        1312        1324
> Compaction success                               255         343         366
> Compaction failures                              109         969         958
> Compaction pages moved                        265107     3952630     4489215
> Compaction move failure                         7493       26038       24739
>
> ...
>
> Success rates are completely hosed for 3.4-rc1 which is almost certainly
> due to [fe2c2a10: vmscan: reclaim at order 0 when compaction is enabled]. I
> expected this would happen for kswapd and impair allocation success rates
> (https://lkml.org/lkml/2012/1/25/166) but I did not anticipate this much
> a difference: 95% less scanning, 43% less reclaim by kswapd
> 
> In comparison, reclaim/compaction is not aggressive and gives up easily
> which is the intended behaviour. hugetlbfs uses __GFP_REPEAT and would be
> much more aggressive about reclaim/compaction than THP allocations are. The
> stress test above is allocating like neither THP or hugetlbfs but is much
> closer to THP.

We seem to be thrashing around a bit with the performance, and we
aren't tracking this closely enough.

What is kswapd efficiency?  pages-relclaimed/pages-scanned?  Why did it
increase so much?  Are pages which were reclaimed via prune_icache_sb()
included?  If so, they can make a real mess of the scanning efficiency
metric.

The increase in PGINODESTEAL is remarkable.  It seems to largely be a
transfer from kswapd inode stealing.  Bad from a latency POV, at least.
What would cause this change?
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