linux-kernel - Re: [RFC PATCH V5] mm readahead: Fix readahead fail for no local memory and limit readahead pages

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Date:	Thu, 13 Feb 2014 12:37:22 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	David Cohen <david.a.cohen@...ux.intel.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Damien Ramonda <damien.ramonda@...el.com>,
	Jan Kara <jack@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V5] mm readahead: Fix readahead fail for no local
 memory and limit readahead pages

On 02/11/2014 03:05 AM, David Rientjes wrote:
> On Mon, 10 Feb 2014, Raghavendra K T wrote:
>
>> So I understood that you are suggesting implementations like below
>>
>> 1) I do not have problem with the below approach, I could post this in
>> next version.
>> ( But this did not include 4k limit Linus mentioned to apply)
>>
>> unsigned long max_sane_readahead(unsigned long nr)
>> {
>>          unsigned long local_free_page;
>>          int nid;
>>
>>          nid = numa_mem_id();
>>
>>          /*
>>           * We sanitize readahead size depending on free memory in
>>           * the local node.
>>           */
>>          local_free_page = node_page_state(nid, NR_INACTIVE_FILE)
>>                            + node_page_state(nid, NR_FREE_PAGES);
>>          return min(nr, local_free_page / 2);
>> }
>>
>> 2) I did not go for below because Honza (Jan Kara) had some
>> concerns for 4k limit for normal case, and since I am not
>> the expert, I was waiting for opinions.
>>
>> unsigned long max_sane_readahead(unsigned long nr)
>> {
>>          unsigned long local_free_page, sane_nr;
>>          int nid;
>>
>>          nid = numa_mem_id();
>> 	/* limit the max readahead to 4k pages */
>> 	sane_nr = min(nr, MAX_REMOTE_READAHEAD);
>>
>>          /*
>>           * We sanitize readahead size depending on free memory in
>>           * the local node.
>>           */
>>          local_free_page = node_page_state(nid, NR_INACTIVE_FILE)
>>                            + node_page_state(nid, NR_FREE_PAGES);
>>          return min(sane_nr, local_free_page / 2);
>> }
>>
>
> I have no opinion on the 4KB pages, either of the above is just fine.
>

I was able to test (1) implementation on the system where readahead 
problem occurred. Unfortunately it did not help.

Reason seem to be that CONFIG_HAVE_MEMORYLESS_NODES dependency of
numa_mem_id(). The PPC machine I am facing problem has topology like
this:

numactl -H
---------
available: 2 nodes (0-1)
node 0 cpus: 0 1 2 3 4 5 6 7 12 13 14 15 16 17 18 19 20 21 22 23 24 25
...
node 0 size: 0 MB
node 0 free: 0 MB
node 1 cpus: 8 9 10 11 32 33 34 35 ...
node 1 size: 8071 MB
node 1 free: 2479 MB
node distances:
node   0   1
   0:  10  20
   1:  20  10

So it seems numa_mem_id() does not help for all the configs..
Am I missing something ?

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