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Content-class: urn:content-classes:message
Subject: Re: make -j with j <= 4 seems to only load a single CPU core
Date: Tue, 21 Feb 2006 16:54:53 -0500
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Thread-Topic: make -j with j <= 4 seems to only load a single CPU core
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From: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
To: "Jesper Juhl" <jesper.juhl@gmail.com>
Cc: "Paul Fulghum" <paulkf@microgate.com>,
       "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
       "Sam Ravnborg" <sam@ravnborg.org>, "Andrew Morton" <akpm@osdl.org>
Reply-To: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
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On Tue, 21 Feb 2006, Jesper Juhl wrote:

> On 2/21/06, Paul Fulghum <paulkf@microgate.com> wrote:
>> On Tue, 2006-02-21 at 22:10 +0100, Jesper Juhl wrote:
>>
>>> I should probably mention that the kernel I'm currently running and
>>> observing this behaviour with is 2.6.16-rc4-mm1.
>> ...
>>>> I find this quite strange since anything from 'make -j 2' and up
>>>> should be able to keep both cores resonably busy, but there seems to
>>>> be a huge difference between j <= 4 and j > 4.
>>
>> I've seen the same thing (on Athlon 64x2 64 bit)
>> but was not sure if it was a problem.
>>
>> The break point for me seems to be between -j 2 and -j 3
>> -j 2 = serialized (or the appearance of)
>> -j 3 = both cores mostly busy
>>
>> I'm pretty sure with an earlier 2.6 kernel source (but same environment)
>> I did not see this. I'll start back tracking to earlier kernels
>> to see if I can identify when this started.
>>
>
> I know positively that I've seen this with previous 2.6.16-<something>
> kernels, but not sure which ones exactely. I just dismissed it as
> something that would probably be fixed soon and then today when I
> build a few test kernels I noticed it again and thought "ohh, so it
> didn't get fixed, better report it".
>
> I don't recall seeing it with 2.6.15 and earlier kernels, but I'm not
> at all sure - especially since I only got this Athlon X2 box recently
> and the first kernels I ever ran on it were 2.6.15-<something>.
>
> --
> Jesper Juhl <jesper.juhl@gmail.com>
> Don't top-post  http://www.catb.org/~esr/jargon/html/T/top-post.html
> Plain text mails only, please      http://www.expita.com/nomime.html

Could it be, simply, that you are now I/O bound with nothing
for these CPUs to do in user-space, being busy handling the
kernel I/O?

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.49 BogoMips).
Warning : 98.36% of all statistics are fiction.
_


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