Re: Q: (2.6.16 & ext3) bad SMP load balancing when writing to ext3on slow device
From: Robert Hancock
Date: Sat Sep 06 2008 - 14:15:46 EST
Ulrich Windl wrote:
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?
They're not busy. IO wait means they have nothing to do other than wait
for IO to complete. It's a bit surprising that you get so many pdflush
threads started up, however..
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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