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SubjectKswapd in 3.2.0-rc5 is a CPU hog
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Hello,

I'm using 3.2-rc5 on a machine, which atm does almost nothing except
file system operations and network i/o (i.e. file server). And there is
a problem with kswapd.

I'm playing with dd:
dd if=/some/big/file of=/dev/null bs=8M

I.e. I'm filling page cache.

So when the machine is just rebooted, kswapd during this operation is
almost idle, just 5-8 percent according to top.

After ~5 days of uptime (5 days, 2:10), the same operation demands ~70%
for kswapd:

PID USER S %CPU %MEM TIME+ SWAP COMMAND
420 root R 70 0.0 22:09.60 0 kswapd0
17717 nowhere D 27 0.2 0:01.81 10m dd

In fact, kswapd cpu usage on this operation steadily increases over
time.

Also read performance degrades over time. After reboot:
dd if=/some/big/file of=/dev/null bs=8M
1019+1 records in
1019+1 records out
8553494018 bytes (8.6 GB) copied, 16.211 s, 528 MB/s

After ~5 days uptime:
dd if=/some/big/file of=/dev/null bs=8M
1019+1 records in
1019+1 records out
8553494018 bytes (8.6 GB) copied, 29.0507 s, 294 MB/s

Whereas raw disk sequential read performance stays the same:
dd if=/some/big/file of=/dev/null bs=8M iflag=direct
1019+1 records in
1019+1 records out
8553494018 bytes (8.6 GB) copied, 14.7286 s, 581 MB/s

Also after dropping caches, situation somehow improves, but not to the
state of freshly restarted system:
PID USER S %CPU %MEM TIME+ SWAP COMMAND
420 root S 39 0.0 23:31.17 0 kswapd0
19829 nowhere D 24 0.2 0:02.72 7764 dd

perf shows:

31.24% kswapd0 [kernel.kallsyms] [k] _raw_spin_lock
26.19% kswapd0 [kernel.kallsyms] [k] shrink_slab
16.28% kswapd0 [kernel.kallsyms] [k] prune_super
6.55% kswapd0 [kernel.kallsyms] [k] grab_super_passive
5.35% kswapd0 [kernel.kallsyms] [k] down_read_trylock
4.03% kswapd0 [kernel.kallsyms] [k] up_read
2.31% kswapd0 [kernel.kallsyms] [k] put_super
1.81% kswapd0 [kernel.kallsyms] [k] drop_super
0.99% kswapd0 [kernel.kallsyms] [k] __put_super
0.25% kswapd0 [kernel.kallsyms] [k] __isolate_lru_page
0.23% kswapd0 [kernel.kallsyms] [k] free_pcppages_bulk
0.19% kswapd0 [r8169] [k] rtl8169_interrupt
0.15% kswapd0 [kernel.kallsyms] [k] twa_interrupt

P.S.: The message above was written couple of days ago. Now I'm at 10
days uptime, and this is the result as of today
PID USER S %CPU %MEM TIME+ SWAP COMMAND
420 root R 93 0.0 110:48.48 0 kswapd0
30085 nowhere D 42 0.2 0:04.36 10m dd

PPS: Please CC me.



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