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SubjectRe: Abysmal RAID 0 performance on 2.4.0-test10 for IDE?
On Tue, 26 Dec 2000, Felix von Leitner wrote:
> Thus spake Rik van Riel (riel@conectiva.com.br):
> > > One more detail: top says the CPU is 50% system when reading from either
> > > one of the disk or raid devices. That seems awfully high considering
> > > that the Promise controller claims to do UDMA.
> > >
> > > Any comments?
> > Your program reads in data at 30MB/second, on a memory bus
> > that most likely supports something like 60 to 100MB/second.
>
> 100.

So that's 30% for the UDMA controller and maybe
30% for the CPU (if your program reads in all the
data).

> > Part of this memory bandwidth is needed for the UDMA controller
> > to push the data to memory, probably between 30% and 50%.
>
> That would be 30%.

Add to that the overhead of allocating and reclaiming
the memory, doing the RAID mapping, sending commands
to the hard disk, ...

> > Every time the UDMA controller has the memory bus for itself the
> > CPU will busy-wait on memory, which shows up as CPU busy time.
>
> So, you are saying, when I add a gigabit ethernet card, CPU will hit
> 100% at about 30 MB/second? That sounds like a weak architecture ;-)

Hey, there's a reason PCs are so cheap ;)

regards,

Rik
--
Hollywood goes for world dumbination,
Trailer at 11.

http://www.surriel.com/
http://www.conectiva.com/ http://distro.conectiva.com.br/

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