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

[Leon Romanovsky]

On Mon, Jun 15, 2015 at 8:43 AM, Rik van Riel <riel@xxxxxxxxxx> wrote:
> On 06/15/2015 01:40 AM, Leon Romanovsky wrote:
>> On Sun, Jun 14, 2015 at 6:04 PM, Ebru Akagunduz
>> <ebru.akagunduz@xxxxxxxxx> 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@xxxxxxxxx>
>
>>> @@ -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@xxxxxxx
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