Re: [PATCH -v2] mm: Don't use radix tree writeback tags for pages in swap cache
From: Mel Gorman <hidden>
Date: 2016-08-31 09:15:06
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On Tue, Aug 30, 2016 at 10:28:09AM -0700, Huang, Ying wrote:
From: Huang Ying <redacted>
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. 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.
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. The flag is set for swap caches. It 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. 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
\ | \
2506952 +- 2% +28.1% 3212076 +- 7% vm-scalability.throughput
1207402 +- 7% +22.3% 1476578 +- 6% vmstat.swap.so
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_swap.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_inactive_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_writeback.page_endio.pmem_rw_pageI didn't see anything wrong with the patch but it's worth highlighting that this hunk means we are now out of GFP bits.
quoted hunk ↗ jump to hunk
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h index 66a1260..2f5a65dd 100644 --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h@@ -25,6 +25,8 @@ enum mapping_flags { AS_MM_ALL_LOCKS = __GFP_BITS_SHIFT + 2, /* under mm_take_all_locks() */ AS_UNEVICTABLE = __GFP_BITS_SHIFT + 3, /* e.g., ramdisk, SHM_LOCK */ AS_EXITING = __GFP_BITS_SHIFT + 4, /* final truncate in progress */ + /* writeback related tags are not used */ + AS_NO_WRITEBACK_TAGS = __GFP_BITS_SHIFT + 5, };
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