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

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Message-ID: <20160831153908.GA8119@techsingularity.net>
Date:   Wed, 31 Aug 2016 16:39:08 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, tim.c.chen@...el.com,
        dave.hansen@...el.com, andi.kleen@...el.com, aaron.lu@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>, Shaohua Li <shli@...nel.org>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
        Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [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@...hsingularity.net> writes:
> 
> > On Tue, Aug 30, 2016 at 10:28:09AM -0700, Huang, Ying wrote:
> >> From: Huang Ying <ying.huang@...el.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