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From: Keith Owens <kaos@ocs.com.au>
To: Erik Slagter <erik@slagter.name>
cc: linux-kernel@vger.kernel.org
Subject: Re: md mirror / ext3 / dual core performance strange phenomenon? 
In-reply-to: Your message of "Mon, 14 Aug 2006 15:12:14 +0200."
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Date: Mon, 14 Aug 2006 23:22:02 +1000
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Erik Slagter (on Mon, 14 Aug 2006 15:12:14 +0200) wrote:
>Now, what puzzles me, is that compiling the kernel (2.6.17.7) using
>either "make -j1 ..." or "make -j2 ..." or "make -j3 ..." makes the
>building take about 6.5 minutes, which is really dissatisfying for this
>cpu/harddisks combination. Also, top shows that most of the time both
>core are between 10-40% idle.
>
>BUT... starting from -j4 (and upwards) the compile time suddenly goes to
>3.5 minutes!

Nothing to do with the disks, it is a design flaw in the kernel build
system.  If you want a useful parallel make using -j<n>, set <n> to 3,
4 or 5 higher than the real number of parallel jobs that you want.  The
exact value to add depends on which kernel tree you are building.  See
http://marc.theaimsgroup.com/?l=linux-kernel&m=115553906404695&w=2

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