high CPU utilization of RAID or JBOD

Hi,

I am confused about the numbers regarding CPU utilization I got
lately on a RAID or JBOD.

With linux 2.4.18 kernel, Intel motherboard, 4 IBM DTLA 7200rpm
disk connected to system through Promise Ultra100 card.
I used vmstat to get CPU utilization. It is quite high.

One testing program open a raw device and sequentially read 64k chunk from
it. I got 40% CPU utilization by sequential read one ide disk at 64K size.
The throughput is about 37 MB/sec.
100% when accessing four disk as JBOD, i.e., four testing program
accessing four  disk separately.
40% when accessing these 4 drives as one RAID-5 disk array.


It is just too high. Similar high numbers of CPU utilziation can be seen
from others' tests like http://www.nobell.org/~gjm/linux/ide-raid/

What does the kernel do then? DMA is on. Is it memcpy? call back funtions?
I don't think it is memcpy, as I simulate the amount of bytes to be
memcpyed in use-level, it takes less than 5% CPU.

Thanks,
Lan