linux-kernel - Do we need to call calibrate_delay() for all cores/hyperthreads on a socket?

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Message-ID: <20101203065212.GQ6858@sgi.com>
Date:	Fri, 3 Dec 2010 00:52:12 -0600
From:	Robin Holt <holt@....com>
To:	linux-kernel@...r.kernel.org
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Adrian Bunk <bunk@...nel.org>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jack Steiner <steiner@....com>
Subject: Do we need to call calibrate_delay() for all cores/hyperthreads on
 a socket?

I do not know all the different combinations of sockets, cores, and
hyperthreads out there, but it seems like all cores and their hyperthreads
on a socket should compute the same value for their calculate_delay()
function.

When booting a 4096 cpu system without specifying lpj on the command line,
we spend approximately 0.1 seconds per core/hyperthread calculating the
lpj value for that cpu.

If we were to, on the other hand, only calculate the delay value for the
first core on a socket, we would reduce the time spent booting a 4096 cpu
(256 sockets, 8 cores per socket hyperthreaded) down from nearly seven
minutes to approx 25 seconds.  This seems like a very safe optimization,
but I repeat that I do not know all the different potential combinations
of socket, core, hyperthread out that.  Please note these are just rough
approximations taken from memory.  I am doing a couple of test boots
now without and with lpj= specified on the command line to get a more
accurate approximation.

Thanks,
Robin Holt
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