Andrea Righi <andrea.righi@xxxxxxxxxxxxx> writes:
> In unuse_pte_range() we blindly swap-in pages without checking if the
> swap entry is already present in the swap cache.
>
> By doing this, the hit/miss ratio used by the swap readahead heuristic
> is not properly updated and this leads to non-optimal performance during
> swapoff.
>
> Tracing the distribution of the readahead size returned by the swap
> readahead heuristic during swapoff shows that a small readahead size is
> used most of the time as if we had only misses (this happens both with
> cluster and vma readahead), for example:
>
> r::swapin_nr_pages(unsigned long offset):unsigned long:$retval
> COUNT EVENT
> 36948 $retval = 8
> 44151 $retval = 4
> 49290 $retval = 1
> 527771 $retval = 2
>
> Checking if the swap entry is present in the swap cache, instead, allows
> to properly update the readahead statistics and the heuristic behaves in
> a better way during swapoff, selecting a bigger readahead size:
>
> r::swapin_nr_pages(unsigned long offset):unsigned long:$retval
> COUNT EVENT
> 1618 $retval = 1
> 4960 $retval = 2
> 41315 $retval = 4
> 103521 $retval = 8
>
> In terms of swapoff performance the result is the following:
>
> Testing environment
> ===================
>
> - Host:
> CPU: 1.8GHz Intel Core i7-8565U (quad-core, 8MB cache)
> HDD: PC401 NVMe SK hynix 512GB
> MEM: 16GB
>
> - Guest (kvm):
> 8GB of RAM
> virtio block driver
> 16GB swap file on ext4 (/swapfile)
>
> Test case
> =========
> - allocate 85% of memory
> - `systemctl hibernate` to force all the pages to be swapped-out to the
> swap file
> - resume the system
> - measure the time that swapoff takes to complete:
> # /usr/bin/time swapoff /swapfile
>
> Result (swapoff time)
> ======
> 5.6 vanilla 5.6 w/ this patch
> ----------- -----------------
> cluster-readahead 22.09s 12.19s
> vma-readahead 18.20s 15.33s
>
> Conclusion
> ==========
>
> The specific use case this patch is addressing is to improve swapoff
> performance in cloud environments when a VM has been hibernated, resumed
> and all the memory needs to be forced back to RAM by disabling swap.
>
> This change allows to better exploits the advantages of the readahead
> heuristic during swapoff and this improvement allows to to speed up the
> resume process of such VMs.
>
> Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Signed-off-by: Andrea Righi <andrea.righi@xxxxxxxxxxxxx>
Looks good to me, Thanks!
Best Regards,
Huang, Ying
> ---
> Changes in v4:
> - added a "conclusion" section in the patch description to better
> explain the importance of this change (no functional changes)
>
> mm/swapfile.c | 12 ++++++++----
> 1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 5871a2aa86a5..f8bf926c9c8f 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1937,10 +1937,14 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>
> pte_unmap(pte);
> swap_map = &si->swap_map[offset];
> - vmf.vma = vma;
> - vmf.address = addr;
> - vmf.pmd = pmd;
> - page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE, &vmf);
> + page = lookup_swap_cache(entry, vma, addr);
> + if (!page) {
> + vmf.vma = vma;
> + vmf.address = addr;
> + vmf.pmd = pmd;
> + page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
> + &vmf);
> + }
> if (!page) {
> if (*swap_map == 0 || *swap_map == SWAP_MAP_BAD)
> goto try_next;
