Re: [PATCH 5/7] mm: vmscan: Do not writeback filesystem pages in kswapd except in high priority

[Mel Gorman <mgorman@suse.de>]

On Thu, Aug 11, 2011 at 06:10:29PM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed, 10 Aug 2011 11:47:18 +0100
> Mel Gorman <mgorman@suse.de> wrote:
> 
> > It is preferable that no dirty pages are dispatched for cleaning from
> > the page reclaim path. At normal priorities, this patch prevents kswapd
> > writing pages.
> > 
> > However, page reclaim does have a requirement that pages be freed
> > in a particular zone. If it is failing to make sufficient progress
> > (reclaiming < SWAP_CLUSTER_MAX at any priority priority), the priority
> > is raised to scan more pages. A priority of DEF_PRIORITY - 3 is
> > considered to be the point where kswapd is getting into trouble
> > reclaiming pages. If this priority is reached, kswapd will dispatch
> > pages for writing.
> > 
> > Signed-off-by: Mel Gorman <mgorman@suse.de>
> > Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
> 
> 
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 

Thanks

> BTW, I'd like to see summary of the effect of priority..
> 

What sort of summary are you looking for? If pressure is high enough,
writes start happening from reclaim. On NUMA, it can be particularly
pronounced. Here is a summary of page writes from reclaim over a range
of tests

512M1P-xfs           Page writes file fsmark                                 8113        74
512M1P-xfs           Page writes file simple-wb                             19895         1
512M1P-xfs           Page writes file mmap-strm                               997        95
512M-xfs             Page writes file fsmark                                12071         9
512M-xfs             Page writes file simple-wb                             31709         1
512M-xfs             Page writes file mmap-strm                            148274      2448
512M-4X-xfs          Page writes file fsmark                                12828         0
512M-4X-xfs          Page writes file simple-wb                             32168         5
512M-4X-xfs          Page writes file mmap-strm                            346460      4405
512M-16X-xfs         Page writes file fsmark                                11566        29
512M-16X-xfs         Page writes file simple-wb                             31935         4
512M-16X-xfs         Page writes file mmap-strm                             38085      4371

With 1 processor (512M1P), very few writes occur as for the most part
flushers are keeping up. With 4x times more processors than there are
CPUs (512M-4X), there are more writes by kswapd..

1024M1P-xfs          Page writes file fsmark                                 3446         1
1024M1P-xfs          Page writes file simple-wb                             11697         6
1024M1P-xfs          Page writes file mmap-strm                              4077       446
1024M-xfs            Page writes file fsmark                                 5159         0
1024M-xfs            Page writes file simple-wb                             12785         5
1024M-xfs            Page writes file mmap-strm                            251153      8108
1024M-4X-xfs         Page writes file fsmark                                 4781         0
1024M-4X-xfs         Page writes file simple-wb                             12486         6
1024M-4X-xfs         Page writes file mmap-strm                           1627122     15000
1024M-16X-xfs        Page writes file fsmark                                 3777         1
1024M-16X-xfs        Page writes file simple-wb                             11856         2
1024M-16X-xfs        Page writes file mmap-strm                              6563      2638
4608M1P-xfs          Page writes file fsmark                                 1497         0
4608M1P-xfs          Page writes file simple-wb                              4305         0
4608M1P-xfs          Page writes file mmap-strm                             17586     10153
4608M-xfs            Page writes file fsmark                                 3380         0
4608M-xfs            Page writes file simple-wb                              5528         0
4608M-4X-xfs         Page writes file fsmark                                 4650         0
4608M-4X-xfs         Page writes file simple-wb                              5621         0
4608M-4X-xfs         Page writes file mmap-strm                            149751     18395
4608M-16X-xfs        Page writes file fsmark                                  388         0
4608M-16X-xfs        Page writes file simple-wb                              5466         0
4608M-16X-xfs        Page writes file mmap-strm                           3349772     19307

This is the same type of tests just with more memory. If enough
processes are running, kswapd will start writing pages as it tries
to reclaim memory.

4096M8N-xfs          Page writes file fsmark                                11571      8163
4096M8N-xfs          Page writes file simple-wb                             28979     11460
4096M8N-xfs          Page writes file mmap-strm                            178999     12181
4096M8N-4X-xfs       Page writes file fsmark                                14421      7487
4096M8N-4X-xfs       Page writes file simple-wb                             26474     10529
4096M8N-4X-xfs       Page writes file mmap-strm                            163770     58765
4096M8N-16X-xfs      Page writes file fsmark                                16726      9265
4096M8N-16X-xfs      Page writes file simple-wb                             28800     11129
4096M8N-16X-xfs      Page writes file mmap-strm                             73303     48267

This is with 8 NUMA nodes, each 512M in size. As the flusher threads are
not targetting a specific ndoe, kswapd writing pages happens more
frequently.

Is this what you are looking for?

-- 
Mel Gorman
SUSE Labs