On Wed, Aug 31, 2016 at 08:17:24AM -0700, Huang, Ying wrote:

Mel Gorman [off-list ref] writes:

quoted

On Tue, Aug 30, 2016 at 10:28:09AM -0700, Huang, Ying wrote:

quoted

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_page

I didn't see anything wrong with the patch but it's worth highlighting
that this hunk means we are now out of GFP bits.

Sorry, I don't know whether I understand your words.  It is something
about,

__GFP_BITS_SHIFT == 26

So remainning bits in mapping_flags is 6.  And now the latest bit is
used for the flag introduced in the patch?

__GFP_BITS_SHIFT + 5 (AS_NO_WRITEBACK_TAGS) = 31

mapping->flags is a combination of AS and GFP flags so increasing
__GFP_BITS_SHIFT overflows mapping->flags on 32-bit as gfp_t is an
unsigned int.

-- 
Mel Gorman
SUSE Labs

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