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Subjectkswapd using a lot of CPU
Dear kernel/VM gurus,

What the f**k is going on here (RedHat 7.1 kernel-2.4.9-12smp, glibc-2.2.4-19)
?

10:58pm up 6 days, 12:06, 1 user, load average: 3.33, 3.31, 3.20
146 processes: 142 sleeping, 4 running, 0 zombie, 0 stopped
CPU0 states: 36.2% user, 63.3% system, 35.4% nice, 0.0% idle
CPU1 states: 84.1% user, 15.1% system, 84.2% nice, 0.0% idle
Mem: 512244K av, 202012K used, 310232K free, 1072K shrd, 13480K buff
Swap: 2048248K av, 0K used, 2048248K free 58708K cached

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
5 root 20 0 0 0 0 RW 78.4 0.0 97:45 kswapd
3342 magma 20 2 71200 43M 15196 R N 60.2 8.7 54:20 MAGMAfill
3341 magma 18 2 71516 44M 16044 R N 60.0 8.8 54:37 MAGMAfill


# vmstat 2
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
2 0 1 0 309100 13480 58932 2 3 10 15 31 32 18 1 4
2 0 1 0 309076 13480 58940 0 0 0 28 180 161 60 40 0
2 0 1 0 309020 13480 58944 0 0 0 46 215 191 60 40 0
2 0 1 0 309080 13480 58948 0 0 0 20 152 143 61 39 0
2 0 1 0 309088 13480 58948 0 0 0 6 131 141 59 41 0
2 0 1 0 309064 13480 58952 0 0 0 12 109 129 59 41 0
2 0 1 0 309028 13480 58956 0 0 0 56 167 153 57 43 0
2 0 1 0 308912 13480 58960 0 0 0 32 211 201 60 39 0
2 0 1 0 309072 13480 58960 0 0 0 0 115 118 62 38 0
2 0 1 0 309072 13480 58960 0 0 0 12 134 142 60 40 0
2 0 1 0 309076 13480 58960 0 0 0 0 104 122 59 41 0
2 0 1 0 309072 13480 58960 0 0 0 0 130 127 61 39 0

kswapd is using plenty of CPU (this is, as you can see, a dual processor box,
more exactly 2xPIII 1GHz), but there's no swapping going on ?!?. And even more
strange is that there is actually nothing that is swapped out (2GB free).

If I try the following :

# cat /proc/swaps
Filename Type Size Used Priority
/dev/hda7 partition 2048248 0 -1

# /sbin/swapoff /dev/hda7

Still kswapd is using a lot of CPU :

11:07pm up 6 days, 12:15, 1 user, load average: 3.33, 3.33, 3.24
146 processes: 141 sleeping, 5 running, 0 zombie, 0 stopped
CPU0 states: 40.0% user, 58.0% system, 40.0% nice, 0.1% idle
CPU1 states: 69.1% user, 30.0% system, 68.0% nice, 0.0% idle
Mem: 512244K av, 324476K used, 187768K free, 1072K shrd, 13608K buff
Swap: 0K av, 0K used, 0K free 151296K cached

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
5 root 20 0 0 0 0 RW 80.0 0.0 105:19 kswapd
3342 magma 20 2 65036 37M 15368 R N 62.3 7.5 60:10 MAGMAfill
3341 magma 18 2 104M 78M 16096 R N 60.5 15.7 60:12 MAGMAfill



When I do :

# /sbin/swapon /dev/hda7

Nothing changes for kswapd, but the swap device gets a lower priority :

# cat /proc/swaps
Filename Type Size Used Priority
/dev/hda7 partition 2048248 0 -2


If I repetedly do 'swapoff/swapon' the Priority decreases each time.


If I try a test application I've written which just mallocs a given amount of
memory (I tried 450MBytes in this case), memsets it and then frees it, I'm able
to force something to swap out, but after the application has exited kswapd is
still using a lot of CPU.


The only thing that seems to solve the issue is to kill the two user processes
(MPI processes) that is also using CPU.

Any feedback is highly appreciated,

Thanks,
--
Steffen Persvold | Scalable Linux Systems | Try out the world's best
mailto:sp@scali.no | http://www.scali.com | performing MPI implementation:
Tel: (+47) 2262 8950 | Olaf Helsets vei 6 | - ScaMPI 1.12.2 -
Fax: (+47) 2262 8951 | N0621 Oslo, NORWAY | >300MBytes/s and <4uS latency
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