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

[Mel Gorman]

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