linux-kernel - Re: [RFC 2/3] mm: make optimistic check for swapin readahead

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Date:	Mon, 15 Jun 2015 09:08:02 +0300
From:	Leon Romanovsky <leon@...n.nu>
To:	Rik van Riel <riel@...hat.com>
Cc:	Ebru Akagunduz <ebru.akagunduz@...il.com>,
	Linux-MM <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"kirill.shutemov" <kirill.shutemov@...ux.intel.com>,
	n-horiguchi <n-horiguchi@...jp.nec.com>,
	aarcange <aarcange@...hat.com>,
	"iamjoonsoo.kim" <iamjoonsoo.kim@....com>,
	Xiexiuqi <xiexiuqi@...wei.com>, gorcunov <gorcunov@...nvz.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, rientjes <rientjes@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	"aneesh.kumar" <aneesh.kumar@...ux.vnet.ibm.com>,
	hughd <hughd@...gle.com>, Johannes Weiner <hannes@...xchg.org>,
	mhocko <mhocko@...e.cz>, boaz <boaz@...xistor.com>,
	raindel <raindel@...lanox.com>
Subject: Re: [RFC 2/3] mm: make optimistic check for swapin readahead

On Mon, Jun 15, 2015 at 8:43 AM, Rik van Riel <riel@...hat.com> wrote:
> On 06/15/2015 01:40 AM, Leon Romanovsky wrote:
>> On Sun, Jun 14, 2015 at 6:04 PM, Ebru Akagunduz
>> <ebru.akagunduz@...il.com> wrote:
>>> This patch makes optimistic check for swapin readahead
>>> to increase thp collapse rate. Before getting swapped
>>> out pages to memory, checks them and allows up to a
>>> certain number. It also prints out using tracepoints
>>> amount of unmapped ptes.
>>>
>>> Signed-off-by: Ebru Akagunduz <ebru.akagunduz@...il.com>
>
>>> @@ -2639,11 +2640,11 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>>>  {
>>>         pmd_t *pmd;
>>>         pte_t *pte, *_pte;
>>> -       int ret = 0, none_or_zero = 0;
>>> +       int ret = 0, none_or_zero = 0, unmapped = 0;
>>>         struct page *page;
>>>         unsigned long _address;
>>>         spinlock_t *ptl;
>>> -       int node = NUMA_NO_NODE;
>>> +       int node = NUMA_NO_NODE, max_ptes_swap = HPAGE_PMD_NR/8;
>> Sorry for asking, my knoweldge of THP is very limited, but why did you
>> choose this default value?
>> From the discussion followed by your patch
>> (https://lkml.org/lkml/2015/2/27/432), I got an impression that it is
>> not necessary right value.
>
> I believe that Ebru's main focus for this initial version of
> the patch series was to get the _mechanism_ (patch 3) right,
> while having a fairly simple policy to drive it.
>
> Any suggestions on when it is a good idea to bring in pages
> from swap, and whether to treat resident-in-swap-cache pages
> differently from need-to-be-paged-in pages, and what other
> factors should be examined, are very welcome...
My concern with these patches that they deal with specific
load/scenario (most of the application returned back from swap). In
scenario there only 10% of data will be required, it theoretically can
bring upto 80% data (70% waste).

>
> --
> All rights reversed



-- 
Leon Romanovsky | Independent Linux Consultant
        www.leon.nu | leon@...n.nu
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