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Storage And CPU Load: DMA?

post #1 of 15
Thread Starter 
This thread, plus my own observations, put a (n00b) question in my head.

I've currently copying about 500GB of data from an external eSATA drive to my main RAID array (6x 2TB F4EG, Linux MD RAID level 5). I'm running KDE, so it's the kio_file process doing the copying.

In top, I'm seeing an average 20% CPU load for md0_raid5 and average 25% load for kio_file, on an i3 530 @2.93GHz with HT on. I can understand the RAID 5 process loading the CPU to a certain extent, but why kio_file?

Why is this? Isn't DMA supposed avoid the CPU from being involved in copying data between devices?
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post #2 of 15
Thread Starter 
Anyone?
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post #3 of 15
Are you using onboard RAID or a dedicated card? if onboard, then the CPU is used for all processing.

Or drivers might not support DMA on your system perhaps?
    
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post #4 of 15
Thread Starter 
Well, I'm using MD RAID, so I can understand the md0_raid5 kernel process taking up CPU time. It's the kio_file process I'm not sure about; I don't understand why copying data should involve the CPU, at least not to such a degree.
Edited by parityboy - 8/15/11 at 12:42pm
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post #5 of 15
Well, if it's software RAID, then the parity calculations are likely hitting the CPU.
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post #6 of 15
Thread Starter 
@mbreita

I know that, and I said so. It's the userland file copying process kio_file (which is part of KDE) which is taking up CPU time.

As a further test, I copied a file to a software RAID 1 array. The in-kernel process md1_raid1 took about 2% CPU time, but again the kio_file process took up ~25%.

I copied data to the RAID 1 again, this time using cp, and the result was the same. cp took up around 25% CPU time, with no parity processing of any kind.

Why would a process simply copying files from disk to disk take up that much CPU time?
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post #7 of 15
What file system are you using? ext2 ext3?
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post #8 of 15
Thread Starter 
I'm using ext4 for both the RAID 5 array and the RAID 1 array. I'm not doing compression or encryption or anything like that.
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post #9 of 15
I guess it could be processing permissions. In the Windows world, copying an item causes it to take up the permissions of the folder it is being copied to. Where as moving an item would cause the permissions to stay intact (that is why it seems quicker top cut and paste).
Edited by trueg50 - 8/17/11 at 3:27am
    
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post #10 of 15
Thread Starter 
But wouldn't it only have to process the permissions once per file? Once it has permission, the file copy should just continue, surely? It can't possibly have to get permission for every block of the file?
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