Re: Linux 2.4.17-pre5

From: James Cleverdon (
Date: Mon Dec 10 2001 - 18:12:38 EST

On Monday 10 December 2001 12:28 am, Alan Cox wrote:
> > Agreed, but does the current x86 code does map them like this or not?
> > If it does, I'm curious as to why they saw a problem which this fixed.
> The current x86 code maps the logical cpus as with the physical ones. In
> other words its how they come off the mainboard. Which for HT seems to
> be with each HT as (n, n+1)

Yes. Intel has defined the LSB of the physical APIC ID to be the
"hyperthreading" bit. Even numbered IDs are real CPUs; odd IDs are the
virtual CPUs. (Or, as wli calls them, Schwarzenegger and Di Vito. ;^)

This may complicate Rusty's zen scheduler scheme. It certainly has made life
complicated for the BIOS folks. They had to sort all the real CPUs to the
front of the ACPI table, lest those folks so benighted as to run the crippled
version of Win2K (which only on-lines 8 CPUs) only get four real CPUs out of

Anyway, with Intel's new numbering scheme you only get two real CPUs per
logical cluster of 4, which is kind of a pain....

> > understand what is happening. I posted my findings, and I'd really
> > like to get some feedback from others doing the same thing.

[ Snip! ]

> Alan

James Cleverdon, IBM xSeries Platform (NUMA), Beaverton   |

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