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SubjectQ: (2.6.16 & ext3) bad SMP load balancing when writing to ext3 on slow device
Hi,

while copying large remote files for an USB memory stick formatted with ext3 using
scp, I noticed a stall in wrie speed. Looking at the system with top I saw:
top - 09:25:25 up 55 days, 23:49, 2 users, load average: 11.09, 7.41, 4.43
Tasks: 128 total, 1 running, 127 sleeping, 0 stopped, 0 zombie
Cpu0 : 7.6%us, 0.3%sy, 0.0%ni, 0.0%id, 90.4%wa, 0.3%hi, 1.3%si, 0.0%st
Cpu1 : 0.0%us, 0.0%sy, 0.0%ni, 0.0%id,100.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu2 : 0.0%us, 0.0%sy, 0.0%ni, 0.0%id,100.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu3 : 0.0%us, 1.7%sy, 0.0%ni, 0.0%id, 98.3%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 1028044k total, 1017956k used, 10088k free, 34784k buffers
Swap: 2097140k total, 616k used, 2096524k free, 733100k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
11284 root 18 0 29168 1960 1504 D 2 0.2 0:11.81 scp
137 root 15 0 0 0 0 D 0 0.0 14:16.59 pdflush
10865 root 15 0 0 0 0 D 0 0.0 0:00.50 kjournald
11355 root 15 0 0 0 0 D 0 0.0 0:00.09 pdflush
11396 root 15 0 0 0 0 D 0 0.0 0:00.12 pdflush
11397 root 15 0 0 0 0 D 0 0.0 0:00.06 pdflush
12007 root 15 0 0 0 0 D 0 0.0 0:00.02 pdflush
12070 root 16 0 23976 2376 1744 R 0 0.2 0:00.28 top
12294 root 15 0 0 0 0 D 0 0.0 0:00.00 pdflush
12295 root 15 0 0 0 0 D 0 0.0 0:00.02 pdflush
12296 root 15 0 0 0 0 D 0 0.0 0:00.02 pdflush
27490 root 10 -5 0 0 0 D 0 0.0 0:02.93 usb-storage

First, it's impressive that a singly copy job can raise the load to above 10, and
the next thing is that writing to a slow device can make 4 CPUs (actually two with
hyperthreading) busy. The pdflush daemons are expected to bring dirty blocks onto
the device, I guess. Does it make any sense to make four CPUs busy with doing so?

Here's another snapshot showing the assigned CPU also:

top - 09:32:18 up 55 days, 23:56, 2 users, load average: 10.63, 9.99, 6.78
Tasks: 127 total, 1 running, 126 sleeping, 0 stopped, 0 zombie
Cpu0 : 0.0%us, 0.0%sy, 0.0%ni, 0.0%id,100.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu1 : 0.0%us, 0.0%sy, 0.0%ni, 0.0%id,100.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu2 : 0.0%us, 0.0%sy, 0.0%ni, 1.7%id, 98.3%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu3 : 0.0%us, 0.0%sy, 0.0%ni, 0.0%id,100.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 1028044k total, 1017896k used, 10148k free, 18044k buffers
Swap: 2097140k total, 616k used, 2096524k free, 741616k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ P COMMAND
137 root 15 0 0 0 0 D 0 0.0 14:16.71 1 pdflush
4299 root 17 0 5860 752 596 D 0 0.1 9:36.19 1 syslogd
10865 root 15 0 0 0 0 D 0 0.0 0:00.62 1 kjournald
11284 root 18 0 29168 1960 1504 D 0 0.2 0:14.76 3 scp
11355 root 15 0 0 0 0 D 0 0.0 0:00.19 0 pdflush
11396 root 15 0 0 0 0 D 0 0.0 0:00.24 1 pdflush
11397 root 15 0 0 0 0 D 0 0.0 0:00.22 1 pdflush
12294 root 15 0 0 0 0 D 0 0.0 0:00.11 1 pdflush
12295 root 15 0 0 0 0 D 0 0.0 0:00.14 1 pdflush
12296 root 15 0 0 0 0 D 0 0.0 0:00.13 1 pdflush
12591 root 16 0 23976 2376 1744 R 0 0.2 0:00.07 3 top
27490 root 10 -5 0 0 0 D 0 0.0 0:03.13 3 usb-storage

At times like shown, the scp seems to come to a complete halt. (Previously I had
been using VFAT filesystem on the stick, and copy went much smoother, but the
filesystem was full, so I tried another filesystem.)

Would anybody bee so kind to explain why the system looks like that? I'm not
subscribed, so please honor the CC:.

Regards,
Ulrich Windl



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