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SubjectRe: [PATCH -v2] mm: Don't use radix tree writeback tags for pages in swap cache
On Wed, Aug 31, 2016 at 08:17:24AM -0700, Huang, Ying wrote:
> Mel Gorman <mgorman@techsingularity.net> writes:
>
> > On Tue, Aug 30, 2016 at 10:28:09AM -0700, Huang, Ying wrote:
> >> From: Huang Ying <ying.huang@intel.com>
> >>
> >> 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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