linux-kernel - Re: [PATCH -v2 -mm] add extra free kbytes tunable

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Message-ID: <65795E11DBF1E645A09CEC7EAEE94B9CAFE00221@USINDEVS02.corp.hds.com>
Date:	Wed, 14 Sep 2011 23:33:34 -0400
From:	Satoru Moriya <satoru.moriya@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>
CC:	Randy Dunlap <rdunlap@...otime.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"lwoodman@...hat.com" <lwoodman@...hat.com>,
	Seiji Aguchi <saguchi@...hat.com>,
	"hughd@...gle.com" <hughd@...gle.com>,
	"hannes@...xchg.org" <hannes@...xchg.org>
Subject: Re: [PATCH -v2 -mm] add extra free kbytes tunable

On 09/02/2011 12:31 PM, Satoru Moriya wrote:
> On 09/01/2011 05:58 PM, Andrew Morton wrote:
>> On Thu, 1 Sep 2011 15:26:50 -0400
>> Rik van Riel <riel@...hat.com> wrote:
> Anyway, now I'm testing this patch and will report a test result later.

Sorry for late reply. Here is my test result.

I ran some sample workloads and measure memory allocation latency
(latency of __alloc_page_nodemask()).
The test is like following:

 - CPU: 1 socket, 4 core
 - Memory: 4GB

 - Background load:
   $ dd if=/dev/zero of=/tmp/tmp1
   $ dd if=/dev/zero of=/tmp/tmp2
   $ dd if=/dev/zero of=/tmp/tmp3

 - Main load:
   $ mapped-file-stream 1 $((1024 * 1024 * 640))  --(*)

 (*) This is made by Johannes Weiner
     https://lkml.org/lkml/2010/8/30/226

     It allocates/access 640MByte memory at a burst.

The result is follwoing:

                               |         |  extra   |
                               | default |  kbytes  |
--------------------------------------------------------------
min_free_kbytes                |    8113 |   8113   |
extra_free_kbytes              |       0 | 640*1024 | (KB)
--------------------------------------------------------------
worst latency                  | 517.762 |  20.775  | (usec)
--------------------------------------------------------------
vmstat result                  |         |          |
 nr_vmscan_write               |       0 |      0   |
 pgsteal_dma                   |       0 |      0   |
 pgsteal_dma32                 |  143667 | 144882   |
 pgsteal_normal                |   31486 |  27001   |
 pgsteal_movable               |       0 |      0   |
 pgscan_kswapd_dma             |       0 |      0   |
 pgscan_kswapd_dma32           |  138617 | 156351   |
 pgscan_kswapd_normal          |   30593 |  27955   |
 pgscan_kswapd_movable         |       0 |      0   |
 pgscan_direct_dma             |       0 |      0   |
 pgscan_direct_dma32           |    5050 |      0   |
 pgscan_direct_normal          |     896 |      0   |
 pgscan_direct_movable         |       0 |      0   |
 kswapd_steal                  |  169207 | 171883   |
 kswapd_inodesteal             |       0 |      0   |
 kswapd_low_wmark_hit_quickly  |      43 |     45   |
 kswapd_high_wmark_hit_quickly |       1 |      0   |
 allocstall                    |      32 |      0   |


As you can see, in the default case there were 32 direct reclaim (allocstall)
and its worst latency was 517.762 usecs. This value may be larger if
a process would sleep or issue I/O in the direct reclaim path. OTOH,
ii the other case where I add extra free bytes, there were no direct
reclaim and its worst latency was 20.775 usecs.

In this test case, we can avoid direct reclaim and keep a latency low.

Tested-by: Satoru Moriya <satoru.moriya@....com>
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