Re: [PATCH -v2] mm: Don't use radix tree writeback tags for pages in swap cache

[Rik van Riel]

On Tue, 2016-08-30 at 10:28 -0700, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> File pages use a set of radix tree tags (DIRTY, TOWRITE, WRITEBACK,
> etc.) to accelerate finding the pages with a specific tag in the
> radix
> tree during inode writeback.ÂÂBut for anonymous pages in the swap
> cache, there is no inode writeback.ÂÂSo there is no need to find the
> pages with some writeback tags in the radix tree.ÂÂIt is not
> necessary
> to touch radix tree writeback tags for pages in the swap cache.
> 
> Per Rik van Riel's suggestion, a new flag AS_NO_WRITEBACK_TAGS is
> introduced for address spaces which don't need to update the
> writeback
> tags.ÂÂThe flag is set for swap caches.ÂÂIt may be used for DAX file
> systems, etc.
> 
> 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.ÂÂThe swap device used is a
> RAM
> simulated PMEM (persistent memory) device.ÂÂThe improvement comes
> from
> the reduced contention on the swap cache radix tree lock.ÂÂTo 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,
> 
> baseÂÂÂÂÂÂÂÂÂÂÂÂÂbase+patch
> ---------------- --------------------------
> ÂÂÂÂÂÂÂÂÂ%stddevÂÂÂÂÂ%changeÂÂÂÂÂÂÂÂÂ%stddev
> ÂÂÂÂÂÂÂÂÂÂÂÂÂ\ÂÂÂÂÂÂÂÂÂÂ|ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ\
> ÂÂÂ2506952 ÂÂÂ2%ÂÂÂÂÂ+28.1%ÂÂÂÂ3212076 ÂÂÂ7%ÂÂvm-
> scalability.throughput
> ÂÂÂ1207402 ÂÂÂ7%ÂÂÂÂÂ+22.3%ÂÂÂÂ1476578 ÂÂÂ6%ÂÂvmstat.swap.so
> ÂÂÂÂÂ10.86 Â 12%ÂÂÂÂÂ-23.4%ÂÂÂÂÂÂÂ8.31 Â 16%ÂÂperf-profile.cycles-
> pp._raw_spin_lock_irq.__add_to_swap_cache.add_to_swap_cache.add_to_sw
> ap.shrink_page_list
> ÂÂÂÂÂ10.82 Â 13%ÂÂÂÂÂ-33.1%ÂÂÂÂÂÂÂ7.24 Â 14%ÂÂperf-profile.cycles-
> pp._raw_spin_lock_irqsave.__remove_mapping.shrink_page_list.shrink_in
> active_list.shrink_zone_memcg
> ÂÂÂÂÂ10.36 Â 11%ÂÂÂÂ-100.0%ÂÂÂÂÂÂÂ0.00 Â -1%ÂÂperf-profile.cycles-
> pp._raw_spin_lock_irqsave.__test_set_page_writeback.bdev_write_page._
> _swap_writepage.swap_writepage
> ÂÂÂÂÂ10.52 Â 12%ÂÂÂÂ-100.0%ÂÂÂÂÂÂÂ0.00 Â -1%ÂÂperf-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>
> 
Acked-by: Rik van Riel <riel@xxxxxxxxxx>

-- 
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