Further testing of the "Reduce system disruption due to kswapd" discovered
a few problems. First, as pages were not being swapped, the file LRU was
being scanned faster and clean file pages were being reclaimed resulting
in some cases in larger amounts of read IO to re-read data from disk.
Second, more pages were being written from kswapd context which can
adversly affect IO performance. Lastly, it was observed that PageDirty
pages are not necessarily dirty on all filesystems (buffers can be clean
while PageDirty is set and ->writepage generates no IO) and not all
filesystems set PageWriteback when the page is being written (e.g. ext3).
This disconnect confuses the reclaim stalling logic. This follow-up series
is aimed at these problems.
The tests were based on three kernels
vanilla: kernel 3.9 as that is what the current mmotm uses as a baseline
mmotm-20130522 is mmotm as of 22nd May with "Reduce system disruption due to
kswapd" applied on top as per what should be in Andrew's tree
right now
lessdisrupt-v5r4 is this follow-up series on top of the mmotm kernel
The first test used memcached+memcachetest while some background IO
was in progress as implemented by the parallel IO tests implement in
MM Tests. memcachetest benchmarks how many operations/second memcached
can service. It starts with no background IO on a freshly created ext4
filesystem and then re-runs the test with larger amounts of IO in the
background to roughly simulate a large copy in progress. The expectation
is that the IO should have little or no impact on memcachetest which is
running entirely in memory.
3.9.0 3.9.0 3.9.0
vanilla mm1-mmotm-20130522 mm1-lessdisrupt-v5r4
Ops memcachetest-0M 23117.00 ( 0.00%) 23088.00 ( -0.13%) 22815.00 ( -1.31%)
Ops memcachetest-715M 23774.00 ( 0.00%) 23504.00 ( -1.14%) 23342.00 ( -1.82%)
Ops memcachetest-2385M 4208.00 ( 0.00%) 23740.00 (464.16%) 24138.00 (473.62%)
Ops memcachetest-4055M 4104.00 ( 0.00%) 24800.00 (504.29%) 24930.00 (507.46%)
Ops io-duration-0M 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%)
Ops io-duration-715M 12.00 ( 0.00%) 7.00 ( 41.67%) 7.00 ( 41.67%)
Ops io-duration-2385M 116.00 ( 0.00%) 21.00 ( 81.90%) 21.00 ( 81.90%)
Ops io-duration-4055M 160.00 ( 0.00%) 37.00 ( 76.88%) 36.00 ( 77.50%)
Ops swaptotal-0M 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%)
Ops swaptotal-715M 140138.00 ( 0.00%) 18.00 ( 99.99%) 18.00 ( 99.99%)
Ops swaptotal-2385M 385682.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%)
Ops swaptotal-4055M 418029.00 ( 0.00%) 0.00 ( 0.00%) 2.00 (100.00%)
Ops swapin-0M 0.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%)
Ops swapin-715M 144.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%)
Ops swapin-2385M 134227.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%)
Ops swapin-4055M 125618.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%)
Ops minorfaults-0M 1536429.00 ( 0.00%) 1533759.00 ( 0.17%) 1537248.00 ( -0.05%)
Ops minorfaults-715M 1786996.00 ( 0.00%) 1606613.00 ( 10.09%) 1610854.00 ( 9.86%)
Ops minorfaults-2385M 1757952.00 ( 0.00%) 1608201.00 ( 8.52%) 1614772.00 ( 8.14%)
Ops minorfaults-4055M 1774460.00 ( 0.00%) 1620493.00 ( 8.68%) 1625930.00 ( 8.37%)
Ops majorfaults-0M 1.00 ( 0.00%) 0.00 ( 0.00%) 0.00 ( 0.00%)
Ops majorfaults-715M 184.00 ( 0.00%) 159.00 ( 13.59%) 162.00 ( 11.96%)
Ops majorfaults-2385M 24444.00 ( 0.00%) 108.00 ( 99.56%) 151.00 ( 99.38%)
Ops majorfaults-4055M 21357.00 ( 0.00%) 218.00 ( 98.98%) 189.00 ( 99.12%)
memcachetest is the transactions/second reported by memcachetest. In
the vanilla kernel note that performance drops from around
23K/sec to just over 4K/second when there is 2385M of IO going
on in the background. With current mmotm, there is no collapse
in performance and with this follow-up series there is little
change.
swaptotal is the total amount of swap traffic. With mmotm and the follow-up
series, the total amount of swapping is much reduced.
3.9.0 3.9.0 3.9.0
vanillamm1-mmotm-20130522mm1-lessdisrupt-v5r4
Minor Faults 11160152 10592704 10620743
Major Faults 46305 771 788
Swap Ins 260249 0 0
Swap Outs 683860 18 20
Direct pages scanned 0 0 850
Kswapd pages scanned 6046108 18523180 1598979
Kswapd pages reclaimed 1081954 1182759 1093766
Direct pages reclaimed 0 0 800
Kswapd efficiency 17% 6% 68%
Kswapd velocity 5217.560 16027.810 1382.231
Direct efficiency 100% 100% 94%
Direct velocity 0.000 0.000 0.735
Percentage direct scans 0% 0% 0%
Zone normal velocity 5105.086 15217.472 636.579
Zone dma32 velocity 112.473 810.338 746.387
Zone dma velocity 0.000 0.000 0.000
Page writes by reclaim 1929612.00016620834.000 43115.000
Page writes file 1245752 16620816 43095
Page writes anon 683860 18 20
Page reclaim immediate 7484 70 147
Sector Reads 1130320 94964 97244
Sector Writes 13508052 11356812 11469072
Page rescued immediate 0 0 0
Slabs scanned 33536 27648 21120
Direct inode steals 0 0 0
Kswapd inode steals 8641 1495 0
Kswapd skipped wait 0 0 0
THP fault alloc 8 9 39
THP collapse alloc 508 476 378
THP splits 24 0 0
THP fault fallback 0 0 0
THP collapse fail 0 0 0
There are a number of observations to make here
1. Swap outs are almost eliminated. Swap ins are 0 indicating that the
pages swapped were really unused anonymous pages. Related to that,
major faults are much reduced.
2. kswapd efficiency was impacted by the initial series but with these
follow-up patches, the efficiency is now at 66% indicating that far
fewer pages were skipped during scanning due to dirty or writeback
pages.
3. kswapd velocity is reduced indicating that fewer pages are being scanned
with the follow-up series as kswapd now stalls when the tail of the
LRU queue is full of unqueued dirty pages. The stall gives flushers a
chance to catch-up so kswapd can reclaim clean pages when it wakes
4. In light of Zlatko's recent reports about zone scanning imbalances,
mmtests now reports scanning velocity on a per-zone basis. With mainline,
you can see that the scanning activity is dominated by the Normal
zone with over 45 times more scanning in Normal than the DMA32 zone.
With the series currently in mmotm, the ratio is slightly better but it
is still the case that the bulk of scanning is in the highest zone. With
this follow-up series, the ratio of scanning between the Normal and
DMA32 zone is roughly equal.
5. As Dave Chinner observed, the current patches in mmotm increased the
number of pages written from kswapd context which is expected to adversly
impact IO performance. With the follow-up patches, far fewer pages are
written from kswapd context than the mainline kernel
6. With the series in mmotm, fewer inodes were reclaimed by kswapd. With
the follow-up series, there is less slab shrinking activity and no inodes
were reclaimed.
7. Note that "Sectors Read" is drastically reduced implying that the source
data being used for the IO is not being aggressively discarded due to
page reclaim skipping over dirty pages and reclaiming clean pages. Note
that the reducion in reads could also be due to inode data not being
re-read from disk after a slab shrink.
Overall, the system is getting less kicked in the face due to IO.
fs/buffer.c | 34 ++++++++++++++++++++++++++++++++++
fs/ext2/inode.c | 1 +
fs/ext3/inode.c | 3 +++
fs/ext4/inode.c | 2 ++
fs/gfs2/aops.c | 2 ++
fs/ntfs/aops.c | 1 +
fs/ocfs2/aops.c | 1 +
fs/xfs/xfs_aops.c | 1 +
include/linux/buffer_head.h | 3 +++
include/linux/fs.h | 1 +
mm/vmscan.c | 45 ++++++++++++++++++++++++++++++++++++++-------
11 files changed, 87 insertions(+), 7 deletions(-)
--
1.8.1.4
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