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

[Rik van Riel <riel@redhat.com>]

On 06/15/2015 02:08 AM, Leon Romanovsky wrote:
> On Mon, Jun 15, 2015 at 8:43 AM, Rik van Riel <riel@redhat.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@gmail.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@gmail.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).

The chosen threshold ensures that the remaining non-resident
4kB pages in a THP are only brought in if 7/8th (or 87.5%) of
the pages are already resident.

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