Re: I/O Scheduling results in poor responsiveness

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Nathan Grennan wrote:
Why is the command below all that is needed to bring the system to it's knees? Why doesn't the io scheduler, CFQ, which is supposed to be all about fairness starve other processes? Example, if I open a new file in vim, and hold down "i" while this is running it will pause the display of new "i"s for seconds, sometimes until the dd write is completely finished. Another example is applications like firefox, thunderbird, xchat, and pidgin will stop refreshing for 10+ seconds.
 dd if=/dev/zero of=test-file bs=2M count=2048

I understand the main difference between using oflag=direct or not relates to if the io scheduler is used, and if the file is cached or not. I can see this clearly by watching cached rise without oflag=direct, stay the same with it, and go way down when I delete the file after running dd without oflag=direct.
 The system in question is running Fedora 8. It is an E6600, 4gb memory,
and 2x300gb Seagate sata drives. The drives are setup with md raid 1,
and the filesystem is ext3. But I also see this with plenty of other
systems with more cpu, less cpu, less memory, raid, and no raid.
Can you compare to systems with SCSI drives? I think this is telling
you that your disk controller is eating all the CPU when the controller
and DMA should be doing all the work.
  Les Mikesell
   [email protected]

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