Re: [ANNOUNCE] Native POSIX Thread Library 0.1

From: Ingo Molnar (
Date: Mon Sep 23 2002 - 15:36:28 EST

On Mon, 23 Sep 2002, Peter Waechtler wrote:

> Getting into kernel is not the same as a context switch. Return EAGAIN
> or EWOULDBLOCK is definetly _not_ causing a context switch.

this is a common misunderstanding. When switching from thread to thread in
the 1:1 model, most of the cost comes from entering/exiting the kernel. So
*once* we are in the kernel the cheapest way is not to piggyback to
userspace to do some userspace context-switch - but to do it right in the

in the kernel we can do much higher quality scheduling decisions than in
userspace. SMP affinity, various statistics are right available in
kernel-space - userspace does not have any of that. Not to talk about


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