I/O Scheduling results in poor responsiveness

[Les Mikesell]

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
    lesmikesell at gmail.com