On Tue, Aug 30, 2016 at 10:28:09AM -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. 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.
> 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,
> };
>
--
Mel Gorman
SUSE Labs
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