| Message ID | 20200416180132.GB3352@xps-13 (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v3] mm: swap: properly update readahead statistics in unuse_pte_range() | expand |
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. It's more important to describe why we need this patch in the patch description. So, please add some information about your use case. And please focus on the technical part instead of the business part. > 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 > > Signed-off-by: "Huang, Ying" <ying.huang@intel.com> Thanks! But you don't need to do this. You can add my Reviewed-by after we have finished the work on patch description. Best Regards, Huang, Ying
On Fri, 17 Apr 2020 11:01:22 +0800 "Huang\, Ying" <ying.huang@intel.com> wrote: > 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. > > It's more important to describe why we need this patch in the patch > description. So, please add some information about your use case. And > please focus on the technical part instead of the business part. Confused. I thought the changelog was quite good. If "business part" means "end user effect of the patch" then that's a very important thing. > Thanks! But you don't need to do this. You can add my Reviewed-by after > we have finished the work on patch description. Can you be more specific about how you want this changed?
Andrew Morton <akpm@linux-foundation.org> writes: > On Fri, 17 Apr 2020 11:01:22 +0800 "Huang\, Ying" <ying.huang@intel.com> wrote: > >> 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. >> >> It's more important to describe why we need this patch in the patch >> description. So, please add some information about your use case. And >> please focus on the technical part instead of the business part. > > Confused. I thought the changelog was quite good. If "business part" > means "end user effect of the patch" then that's a very important > thing. Previously, Andrea has described their use case in the cloud environment to hiberate the guest and swapoff after resuming. So swapoff performance is important for them. I think that should be included. For the business part, I mean something like "Ubuntu used in AWS EC2", I think that isn't important for the patch description. >> Thanks! But you don't need to do this. You can add my Reviewed-by after >> we have finished the work on patch description. > > Can you be more specific about how you want this changed? Please use Reviewed-by: "Huang, Ying" <ying.huang@intel.com> Best Regards, Huang, Ying
On Fri, Apr 17, 2020 at 01:18:37PM +0800, Huang, Ying wrote: > Andrew Morton <akpm@linux-foundation.org> writes: > > > On Fri, 17 Apr 2020 11:01:22 +0800 "Huang\, Ying" <ying.huang@intel.com> wrote: > > > >> 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. > >> > >> It's more important to describe why we need this patch in the patch > >> description. So, please add some information about your use case. And > >> please focus on the technical part instead of the business part. > > > > Confused. I thought the changelog was quite good. If "business part" > > means "end user effect of the patch" then that's a very important > > thing. > > Previously, Andrea has described their use case in the cloud environment > to hiberate the guest and swapoff after resuming. So swapoff > performance is important for them. I think that should be included. > For the business part, I mean something like "Ubuntu used in AWS EC2", I > think that isn't important for the patch description. I just sent a v4 of this patch adding "conclusion" section in the description to better explain the purpose of this patch. Let me know if you have any comment on that. Thanks, -Andrea
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;
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 Signed-off-by: "Huang, Ying" <ying.huang@intel.com> Signed-off-by: Andrea Righi <andrea.righi@canonical.com> --- Changes in v3: - properly update swap readahead statistics instead of forcing a fixed-size readahead mm/swapfile.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-)