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Date: Tue, 26 Dec 2000 14:35:39 -0200 (BRDT)
From: Rik van Riel <riel@conectiva.com.br>
To: Felix von Leitner <leitner@convergence.de>
cc: linux-kernel@vger.kernel.org
Subject: Re: Abysmal RAID 0 performance on 2.4.0-test10 for IDE?
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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
--
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	Trailer at 11.

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