I/O Scheduling results in poor responsiveness

Les Mikesell lesmikesell at gmail.com
Wed Mar 5 18:42:45 UTC 2008

Nathan Grennan wrote:
>> 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.
>  Are you saying you think that the controller isn't using DMA? You think 
> the controller or driver is just poorly written?

I'm not sure how to tell - but disk activity shouldn't take a lot of CPU 
other than tying it up in iowait if it doesn't have anything else to do.

>  I will try a SCSI system, but the closest I have is a CentOS 4.6 
> machine, which using a kernel based on 2.6.9. It is also a server, so I 
> can't run firefox, thunderbird, xchat, or pidgin on it.

There are 2 things that could be going wrong - one is that the driver is 
keeping the cpu too busy to do anything else, and the other is that the 
system might need to page in some needed code or flush a work buffer 
before doing anything else (neither of which seems likely in your vi 
insert character example) and the disk heads are far away and busy with 
the writing.  The latter case could be helped by putting the OS on a 
different drive from your data with large writes.

    Les Mikesell
     lesmikesell at gmail.com

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