From: Robert Hancock <hancockr@shaw.ca>
To: Ulrich Windl <ulrich.windl@rz.uni-regensburg.de>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Q: (2.6.16 & ext3) bad SMP load balancing when writing to ext3 on slow device
Date: Sat, 06 Sep 2008 12:15:31 -0600 [thread overview]
Message-ID: <48C2C8C3.8080706@shaw.ca> (raw)
In-Reply-To: <fa.iG3Mum/QrNI8vr+DNw6zItBIFJM@ifi.uio.no>
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
next parent reply other threads:[~2008-09-06 18:15 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <fa.iG3Mum/QrNI8vr+DNw6zItBIFJM@ifi.uio.no>
2008-09-06 18:15 ` Robert Hancock [this message]
2008-09-08 7:44 ` Q: (2.6.16 & ext3) bad SMP load balancing when writing to ext3 on slow device Ulrich Windl
2008-09-08 14:36 ` Robert Hancock
2008-09-05 7:37 Ulrich Windl
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