On Thu, 2016-08-25 at 12:27 -0700, Huang, Ying wrote:
> File pages use a set of radix tags (DIRTY, TOWRITE, WRITEBACK, etc.)
> to
> accelerate finding the pages with a specific tag in the radix tree
> during inode writeback. But for anonymous pages in the swap cache,
> there is no inode writeback. So there is no need to find the
> pages with some writeback tags in the radix tree. It is not
> necessary
> to touch radix tree writeback tags for pages in the swap cache.
>
> With this patch, the swap out bandwidth improved 22.3% (from ~1.2GB/s
> to
> ~ 1.48GBps) in the vm-scalability swap-w-seq test case with 8
> processes.
> The test is done on a Xeon E5 v3 system. The swap device used is a
> RAM
> simulated PMEM (persistent memory) device. The improvement comes
> from
> the reduced contention on the swap cache radix tree lock. To test
> sequential swapping out, the test case uses 8 processes, which
> sequentially allocate and write to the anonymous pages until RAM and
> part of the swap device is used up.
>
> Details of comparison is as follow,
>
> base base+patch
> ---------------- --------------------------
> %stddev %change %stddev
> \ | \
> 1207402 ± 7% +22.3% 1476578 ± 6% vmstat.swap.so
> 2506952 ± 2% +28.1% 3212076 ± 7% vm-
> scalability.throughput
> 10.86 ± 12% -23.4% 8.31 ± 16% perf-profile.cycles-
> pp._raw_spin_lock_irq.__add_to_swap_cache.add_to_swap_cache.add_to_sw
> ap.shrink_page_list
> 10.82 ± 13% -33.1% 7.24 ± 14% perf-profile.cycles-
> pp._raw_spin_lock_irqsave.__remove_mapping.shrink_page_list.shrink_in
> active_list.shrink_zone_memcg
> 10.36 ± 11% -100.0% 0.00 ± -1% perf-profile.cycles-
> pp._raw_spin_lock_irqsave.__test_set_page_writeback.bdev_write_page._
> _swap_writepage.swap_writepage
> 10.52 ± 12% -100.0% 0.00 ± -1% perf-profile.cycles-
> pp._raw_spin_lock_irqsave.test_clear_page_writeback.end_page_writebac
> k.page_endio.pmem_rw_page
>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Shaohua Li <shli@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> ---
> mm/page-writeback.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 82e7252..599d2f9 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2728,7 +2728,8 @@ int test_clear_page_writeback(struct page
> *page)
> int ret;
>
> lock_page_memcg(page);
> - if (mapping) {
> + /* Pages in swap cache don't use writeback tags */
> + if (mapping && !PageSwapCache(page)) {
I wonder if that should be a mapping_uses_tags(mapping)
macro or similar, and a per-mapping flag?
I suspect there will be another case coming up soon
where we have a page cache radix tree, but no need
for dirty/writeback/... tags.
That use case would be DAX filesystems, where we do
use a struct page, but that struct page points at
persistent storage, and the tags are not necessary.
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