Re: [PATCH v4] mm: swap: properly update readahead statistics in unuse_pte_range()

["Huang\, Ying" <ying.huang@intel.com>]

Andrea Righi <andrea.righi@canonical.com> 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@intel.com>
> Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

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;