linux-kernel - Re: performance differences: "maxcpus=1" vs. "echo 0

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Date:	Sat, 29 Mar 2008 23:16:50 +0100 (CET)
From:	Michael Meyer <mike65134@...oo.de>
To:	Pavel Machek <pavel@....cz>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online"


--- Pavel Machek <pavel@....cz> schrieb:

> Hi!
> 
> > > > what is the difference between booting a dual core
> > > > machine with "maxcpus=1" or by deactivating the second
> > > > core at run time with "echo 0 >
> > > > /sys/devices/system/cpu/cpu1/online"?
> > > 
> > > maxcpus=1 : core stays powered off
> > > 0 > online : core enters halt.
> > > 
> > > > I observed a funny behaviour of apache ant: although
> > > > it uses javac which is single threaded, a compile run
> > > > with "maxcpus=1" is actually faster than a compile run
> > > > with both cores activated. But with the second core
> > > > deactivated using "echo 0 >
> > > > /sys/devices/system/cpu/cpu1/online" it is even slower
> > > > than with both cores.
> > > 
> > > Thermal fun? Check cooling.
> > 
> > I have an extremely well cooled case and a Thermaltake Typhoon
> sitting
> > on the E6600 (Intel TDP 65 Watt). At a room temperature of 20°C
> both
> > cores idle at 21°C and after one hour of two instances of prime95
> > running they are both at 28-33°C. As the E6600 is specified until
> 60°C
> > (I believe), this should prevent any thermal throttling?
> 
> Ok, and your chip is too old to have "Enhanced Dynamic Acceleration
> Technology" (Intel, can't you use some reasonable names?!)... both
> would explain effects you see, and it is neither...

It is too old to have that. I bought it at the beginning of 2007 and it
is from the first line of Core 2 Duo processors (released in summer
2006). I think "Enhanced Dynamic Acceleration Technology" is only
available recently.

> You could try implementing deep sleep (C4) state for cpu hotplug....

I have to confess that I sneaked unto LKML illegitimately, as I am not
a kernel hacker.

But thank you all very much for all the hints!


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