linux-kernel - Re: getting processor numbers

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Date:	Tue, 03 Apr 2007 10:45:35 -0700
From:	Ulrich Drepper <drepper@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: getting processor numbers

Andi Kleen wrote:
>>> Topology is dependent on the number of CPUs.
>> Not all of it.
> 
> What is not?

Memory banks can exist without a CPU present.  The places where you can
plug in memory don't change and so the memory hierarchy can be described.

> Hmm, e.g. in OpenMP you would have another thread that just reads /proc/cpuinfo
> in a loop and starts new threads on new CPUs?
> 
> That sounds ...... "expensive" 

That's the cost of doing business.

There is an inexpensive solution: finally make the vdso concept a bit
more flexible.  You could add a vdso call to get the processor count.
The vdso code itself can use a data page mapped in from the kernel.
This page (read-only at userlevel) would contain global information such
as processor count and topology.

But we're getting IMO off topic here.  That's a separate and far more
complicated issue.

Here we now have the concrete issue that determining the CPU count is
terribly expensive and there is a simple proposal to make it faster by
keeping /sys/devices/system/cpu/ free from anything but cpu* directories.

-- 
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖

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