On Fri 30-10-15 16:01:40, Minchan Kim wrote:
> When I test below piece of code with 12 processes(ie, 512M * 12 = 6G
> consume) on my (3G ram + 12 cpu + 8G swap, the madvise_free is siginficat
> slower (ie, 2x times) than madvise_dontneed.
>
> loop = 5;
> mmap(512M);
> while (loop--) {
> memset(512M);
> madvise(MADV_FREE or MADV_DONTNEED);
> }
>
> The reason is lots of swapin.
>
> 1) dontneed: 1,612 swapin
> 2) madvfree: 879,585 swapin
>
> If we find hinted pages were already swapped out when syscall is called,
> it's pointless to keep the swapped-out pages in pte.
> Instead, let's free the cold page because swapin is more expensive
> than (alloc page + zeroing).
>
> With this patch, it reduced swapin from 879,585 to 1,878 so elapsed time
>
> 1) dontneed: 6.10user 233.50system 0:50.44elapsed
> 2) madvfree: 6.03user 401.17system 1:30.67elapsed
> 2) madvfree + below patch: 6.70user 339.14system 1:04.45elapsed
>
> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
Yes this makes a lot of sense.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
One nit below.
> ---
> mm/madvise.c | 26 +++++++++++++++++++++++++-
> 1 file changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 640311704e31..663bd9fa0ae0 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -270,6 +270,8 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> spinlock_t *ptl;
> pte_t *pte, ptent;
> struct page *page;
> + swp_entry_t entry;
This could go into !pte_present if block
> + int nr_swap = 0;
>
> split_huge_page_pmd(vma, addr, pmd);
> if (pmd_trans_unstable(pmd))
> @@ -280,8 +282,22 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> for (; addr != end; pte++, addr += PAGE_SIZE) {
> ptent = *pte;
>
> - if (!pte_present(ptent))
> + if (pte_none(ptent))
> continue;
> + /*
> + * If the pte has swp_entry, just clear page table to
> + * prevent swap-in which is more expensive rather than
> + * (page allocation + zeroing).
> + */
> + if (!pte_present(ptent)) {
> + entry = pte_to_swp_entry(ptent);
> + if (non_swap_entry(entry))
> + continue;
> + nr_swap--;
> + free_swap_and_cache(entry);
> + pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> + continue;
> + }
>
> page = vm_normal_page(vma, addr, ptent);
> if (!page)
> @@ -313,6 +329,14 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> set_pte_at(mm, addr, pte, ptent);
> tlb_remove_tlb_entry(tlb, pte, addr);
> }
> +
> + if (nr_swap) {
> + if (current->mm == mm)
> + sync_mm_rss(mm);
> +
> + add_mm_counter(mm, MM_SWAPENTS, nr_swap);
> + }
> +
> arch_leave_lazy_mmu_mode();
> pte_unmap_unlock(pte - 1, ptl);
> cond_resched();
> --
> 1.9.1
--
Michal Hocko
SUSE Labs
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