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Gmane
From: Mel Gorman <mgorman <at> suse.de>
Subject: [PATCH 0/7] Reduce filesystem writeback from page reclaim v3
Newsgroups: gmane.linux.kernel
Date: Wednesday 10th August 2011 10:47:13 UTC (over 5 years ago)
Changelog since V2
  o Drop patch eliminating all writes from kswapd until such time as
    particular pages can be prioritised for writeback. Eliminating
    all writes led to stalls on NUMA
  o Lumpy synchronous reclaim now waits for pages currently under
    writeback but can no longer queue pages itself
  o Dropped btrfs warning when filesystems are called from direct
    reclaim. The fallback method for migration looks indistinguishable
    from direct reclaim.
  o Throttle based on pages writeback rather than pages dirty. Throttling
    based on just dirty is too aggressive and can end up trying to stall
    even when the underlying device is not congested

Changelog since v1
  o Drop prio-inode patch. There is now a dependency that the flusher
    threads find these dirty pages quickly.
  o Drop nr_vmscan_throttled counter
  o SetPageReclaim instead of deactivate_page which was wrong
  o Add warning to main filesystems if called from direct reclaim context
  o Add patch to completely disable filesystem writeback from reclaim

Testing from the XFS folk revealed that there is still too much
I/O from the end of the LRU in kswapd. Previously it was considered
acceptable by VM people for a small number of pages to be written
back from reclaim with testing generally showing about 0.3% of pages
reclaimed were written back (higher if memory was low). That writing
back a small number of pages is ok has been heavily disputed for
quite some time and Dave Chinner explained it well;

	It doesn't have to be a very high number to be a problem. IO
	is orders of magnitude slower than the CPU time it takes to
	flush a page, so the cost of making a bad flush decision is
	very high. And single page writeback from the LRU is almost
	always a bad flush decision.

To complicate matters, filesystems respond very differently to requests
from reclaim according to Christoph Hellwig;

	xfs tries to write it back if the requester is kswapd
	ext4 ignores the request if it's a delayed allocation
	btrfs ignores the request

As a result, each filesystem has different performance characteristics
when under memory pressure and there are many pages being dirties. In
some cases, the request is ignored entirely so the VM cannot depend
on the IO being dispatched.

The objective of this series is to reduce writing of filesystem-backed
pages from reclaim, play nicely with writeback that is already in
progress and throttle reclaim appropriately when writeback pages are
encountered. The assumption is that the flushers will always write
pages faster than if reclaim issues the IO. The new problem is that
reclaim has very little control over how long before a page in a
particular zone or container is cleaned which is discussed later. A
secondary goal is to avoid the problem whereby direct reclaim splices
two potentially deep call stacks together.

Patch 1 disables writeback of filesystem pages from direct reclaim
	entirely. Anonymous pages are still written.

Patch 2 removes dead code in lumpy reclaim as it is no longer able
	to synchronously write pages. This hurts lumpy reclaim but
	there is an expectation that compaction is used for hugepage
	allocations these days and lumpy reclaims days are numbered.

Patches 3-4 add warnings to XFS and ext4 if called from
	direct reclaim. With patch 1, this "never happens" and is
	intended to catch regressions in this logic in the future.

Patch 5 disables writeback of filesystem pages from kswapd unless
	the priority is raised to the point where kswapd is considered
	to be in trouble.

Patch 6 throttles reclaimers if too many dirty pages are being
	encountered and the zones or backing devices are congested.

Patch 7 invalidates dirty pages found at the end of the LRU so they
	are reclaimed quickly after being written back rather than
	waiting for a reclaimer to find them

I consider this series to be orthogonal to the writeback work but
it is worth noting that the writeback work affects the viability of
patch 8 in particular.

I tested this on ext4 and xfs using fs_mark, a simple writeback test
based on dd and a micro benchmark that does a streaming write to a
large mapping (exercises use-once LRU logic) followed by streaming
writes to a mix of anonymous and file-backed mappings. The command
line for fs_mark when botted with 512M looked something like

./fs_mark -d  /tmp/fsmark-2676  -D  100  -N  150  -n  150  -L  25  -t  1 
-S0  -s  10485760

The number of files was adjusted depending on the amount of available
memory so that the files created was about 3xRAM. For multiple threads,
the -d switch is specified multiple times.

The test machine is x86-64 with an older generation of AMD processor
with 4 cores. The underlying storage was 4 disks configured as RAID-0
as this was the best configuration of storage I had available. Swap
is on a separate disk. Dirty ratio was tuned to 40% instead of the
default of 20%.

Testing was run with and without monitors to both verify that the
patches were operating as expected and that any performance gain was
real and not due to interference from monitors.

Here is a summary of results based on testing XFS.

512M1P-xfs           Files/s  mean                 32.69 ( 0.00%)     34.44
( 5.08%)
512M1P-xfs           Elapsed Time fsmark                    51.41     48.29
512M1P-xfs           Elapsed Time simple-wb                114.09    108.61
512M1P-xfs           Elapsed Time mmap-strm                113.46    109.34
512M1P-xfs           Kswapd efficiency fsmark                 62%       63%
512M1P-xfs           Kswapd efficiency simple-wb              56%       61%
512M1P-xfs           Kswapd efficiency mmap-strm              44%       42%
512M-xfs             Files/s  mean                 30.78 ( 0.00%)     35.94
(14.36%)
512M-xfs             Elapsed Time fsmark                    56.08     48.90
512M-xfs             Elapsed Time simple-wb                112.22     98.13
512M-xfs             Elapsed Time mmap-strm                219.15    196.67
512M-xfs             Kswapd efficiency fsmark                 54%       56%
512M-xfs             Kswapd efficiency simple-wb              54%       55%
512M-xfs             Kswapd efficiency mmap-strm              45%       44%
512M-4X-xfs          Files/s  mean                 30.31 ( 0.00%)     33.33
( 9.06%)
512M-4X-xfs          Elapsed Time fsmark                    63.26     55.88
512M-4X-xfs          Elapsed Time simple-wb                100.90     90.25
512M-4X-xfs          Elapsed Time mmap-strm                261.73    255.38
512M-4X-xfs          Kswapd efficiency fsmark                 49%       50%
512M-4X-xfs          Kswapd efficiency simple-wb              54%       56%
512M-4X-xfs          Kswapd efficiency mmap-strm              37%       36%
512M-16X-xfs         Files/s  mean                 60.89 ( 0.00%)     65.22
( 6.64%)
512M-16X-xfs         Elapsed Time fsmark                    67.47     58.25
512M-16X-xfs         Elapsed Time simple-wb                103.22     90.89
512M-16X-xfs         Elapsed Time mmap-strm                237.09    198.82
512M-16X-xfs         Kswapd efficiency fsmark                 45%       46%
512M-16X-xfs         Kswapd efficiency simple-wb              53%       55%
512M-16X-xfs         Kswapd efficiency mmap-strm              33%       33%

Up until 512-4X, the FSmark improvements were statistically
significant. For the 4X and 16X tests the results were within standard
deviations but just barely. The time to completion for all tests is
improved which is an important result. In general, kswapd efficiency
is not affected by skipping dirty pages.

1024M1P-xfs          Files/s  mean                 39.09 ( 0.00%)     41.15
( 5.01%)
1024M1P-xfs          Elapsed Time fsmark                    84.14     80.41
1024M1P-xfs          Elapsed Time simple-wb                210.77    184.78
1024M1P-xfs          Elapsed Time mmap-strm                162.00    160.34
1024M1P-xfs          Kswapd efficiency fsmark                 69%       75%
1024M1P-xfs          Kswapd efficiency simple-wb              71%       77%
1024M1P-xfs          Kswapd efficiency mmap-strm              43%       44%
1024M-xfs            Files/s  mean                 35.45 ( 0.00%)     37.00
( 4.19%)
1024M-xfs            Elapsed Time fsmark                    94.59     91.00
1024M-xfs            Elapsed Time simple-wb                229.84    195.08
1024M-xfs            Elapsed Time mmap-strm                405.38    440.29
1024M-xfs            Kswapd efficiency fsmark                 79%       71%
1024M-xfs            Kswapd efficiency simple-wb              74%       74%
1024M-xfs            Kswapd efficiency mmap-strm              39%       42%
1024M-4X-xfs         Files/s  mean                 32.63 ( 0.00%)     35.05
( 6.90%)
1024M-4X-xfs         Elapsed Time fsmark                   103.33     97.74
1024M-4X-xfs         Elapsed Time simple-wb                204.48    178.57
1024M-4X-xfs         Elapsed Time mmap-strm                528.38    511.88
1024M-4X-xfs         Kswapd efficiency fsmark                 81%       70%
1024M-4X-xfs         Kswapd efficiency simple-wb              73%       72%
1024M-4X-xfs         Kswapd efficiency mmap-strm              39%       38%
1024M-16X-xfs        Files/s  mean                 42.65 ( 0.00%)     42.97
( 0.74%)
1024M-16X-xfs        Elapsed Time fsmark                   103.11     99.11
1024M-16X-xfs        Elapsed Time simple-wb                200.83    178.24
1024M-16X-xfs        Elapsed Time mmap-strm                397.35    459.82
1024M-16X-xfs        Kswapd efficiency fsmark                 84%       69%
1024M-16X-xfs        Kswapd efficiency simple-wb              74%       73%
1024M-16X-xfs        Kswapd efficiency mmap-strm              39%       40%

All FSMark tests up to 16X had statistically significant
improvements. For the most part, tests are completing faster with
the exception of the streaming writes to a mixture of anonymous and
file-backed mappings which were slower in two cases

In the cases where the mmap-strm tests were slower, there was more
swapping due to dirty pages being skipped. The number of additional
pages swapped is almost identical to the fewer number of pages written
from reclaim. In other words, roughly the same number of pages were
reclaimed but swapping was slower. As the test is a bit unrealistic
and stresses memory heavily, the small shift is acceptable.

4608M1P-xfs          Files/s  mean                 29.75 ( 0.00%)     30.96
( 3.91%)
4608M1P-xfs          Elapsed Time fsmark                   512.01    492.15
4608M1P-xfs          Elapsed Time simple-wb                618.18    566.24
4608M1P-xfs          Elapsed Time mmap-strm                488.05    465.07
4608M1P-xfs          Kswapd efficiency fsmark                 93%       86%
4608M1P-xfs          Kswapd efficiency simple-wb              88%       84%
4608M1P-xfs          Kswapd efficiency mmap-strm              46%       45%
4608M-xfs            Files/s  mean                 27.60 ( 0.00%)     28.85
( 4.33%)
4608M-xfs            Elapsed Time fsmark                   555.96    532.34
4608M-xfs            Elapsed Time simple-wb                659.72    571.85
4608M-xfs            Elapsed Time mmap-strm               1082.57   1146.38
4608M-xfs            Kswapd efficiency fsmark                 89%       91%
4608M-xfs            Kswapd efficiency simple-wb              88%       82%
4608M-xfs            Kswapd efficiency mmap-strm              48%       46%
4608M-4X-xfs         Files/s  mean                 26.00 ( 0.00%)     27.47
( 5.35%)
4608M-4X-xfs         Elapsed Time fsmark                   592.91    564.00
4608M-4X-xfs         Elapsed Time simple-wb                616.65    575.07
4608M-4X-xfs         Elapsed Time mmap-strm               1773.02   1631.53
4608M-4X-xfs         Kswapd efficiency fsmark                 90%       94%
4608M-4X-xfs         Kswapd efficiency simple-wb              87%       82%
4608M-4X-xfs         Kswapd efficiency mmap-strm              43%       43%
4608M-16X-xfs        Files/s  mean                 26.07 ( 0.00%)     26.42
( 1.32%)
4608M-16X-xfs        Elapsed Time fsmark                   602.69    585.78
4608M-16X-xfs        Elapsed Time simple-wb                606.60    573.81
4608M-16X-xfs        Elapsed Time mmap-strm               1549.75   1441.86
4608M-16X-xfs        Kswapd efficiency fsmark                 98%       98%
4608M-16X-xfs        Kswapd efficiency simple-wb              88%       82%
4608M-16X-xfs        Kswapd efficiency mmap-strm              44%       42%

Unlike the other tests, the fsmark results are not statistically
significant but the min and max times are both improved and for the
most part, tests completed faster.

There are other indications that this is an improvement as well. For
example, in the vast majority of cases, there were fewer pages scanned
by direct reclaim implying in many cases that stalls due to direct
reclaim are reduced. KSwapd is scanning more due to skipping dirty
pages which is unfortunate but the CPU usage is still acceptable

In an earlier set of tests, I used blktrace and in almost all cases
throughput throughout the entire test was higher. However, I ended
up discarding those results as recording blktrace data was too heavy
for my liking.

On a laptop, I plugged in a USB stick and ran a similar tests of tests
using it as backing storage. A desktop environment was running and for
the entire duration of the tests, firefox and gnome terminal were
launching and exiting to vaguely simulate a user.

1024M-xfs            Files/s  mean               0.41 ( 0.00%)        0.44
( 6.82%)
1024M-xfs            Elapsed Time fsmark               2053.52   1641.03
1024M-xfs            Elapsed Time simple-wb            1229.53    768.05
1024M-xfs            Elapsed Time mmap-strm            4126.44   4597.03
1024M-xfs            Kswapd efficiency fsmark              84%       85%
1024M-xfs            Kswapd efficiency simple-wb           92%       81%
1024M-xfs            Kswapd efficiency mmap-strm           60%       51%
1024M-xfs            Avg wait ms fsmark                5404.53     4473.87
1024M-xfs            Avg wait ms simple-wb             2541.35     1453.54
1024M-xfs            Avg wait ms mmap-strm             3400.25     3852.53

The mmap-strm results were hurt because firefox launching had
a tendency to push the test out of memory. On the postive side,
firefox launched marginally faster with the patches applied.  Time to
completion for many tests was faster but more importantly - the "Avg
wait" time as measured by iostat was far lower implying the system
would be more responsive. It was also the case that "Avg wait ms"
on the root filesystem was lower. I tested it manually and while the
system felt slightly more responsive while copying data to a USB stick,
it was marginal enough that it could be my imagination.

For the most part, this series has a positive impact. Is there anything
else that should be done before I send this to Andrew requested it
be merged?

 fs/ext4/inode.c             |    6 +++-
 fs/xfs/linux-2.6/xfs_aops.c |    7 ++--
 include/linux/mmzone.h      |    1 +
 mm/vmscan.c                 |   67
++++++++++++++++++++++++++++++------------
 mm/vmstat.c                 |    1 +
 5 files changed, 58 insertions(+), 24 deletions(-)

-- 
1.7.3.4

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