linux-kernel - Re: Q: (2.6.16 & ext3) bad SMP load balancing when writing to ext3 on slow device

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Message-ID: <48C2C8C3.8080706@shaw.ca>
Date:	Sat, 06 Sep 2008 12:15:31 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	Ulrich Windl <ulrich.windl@...uni-regensburg.de>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Q: (2.6.16 & ext3) bad SMP load balancing when writing to ext3
 on slow device

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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