Re: [ANNOUNCE] Native POSIX Thread Library 0.1

From: Mark Mielke (mark@mark.mielke.cc)
Date: Mon Sep 23 2002 - 17:44:23 EST

On Mon, Sep 23, 2002 at 11:08:53PM +0200, Peter Wächtler wrote:
> With 1:1 on "hitting" a blocking condition the kernel will
> switch to a different beast (yes, a thread gets a bonus for
> using the same MM and the same cpu).

> But on M:N the "user process" makes some more progress in its
> timeslice (does it get even punished for eating up its
> timeslice?) I would think that it tends to cause less context
> switches but tends to do more syscalls :-(

Think of it this way... two threads are blocked on different resources...
The currently executing thread reaches a point where it blocks.

    OS threads:
        1) thread#1 invokes a system call
        2) OS switches tasks to thread#2 and returns from blocking

    user-space threads:
        1) thread#1 invokes a system call
        2) thread#1 returns from system call, EWOULDBLOCK
        3) thread#1 invokes poll(), select(), ioctl() to determine state
        4) thread#1 returns from system call
        5) thread#1 switches stack pointer to be thread#2 upon determination
           that the resource thread#2 was waiting on is ready.

Certainly the above descriptions are not fully accurate, or complete,
and it is possible that the M:N threading would make a fair compromise
between OS thread sand user-space threads, however, if user-space threads
requires all this extra work, and M:N threads requires some extra work,
some less work, and extra book keeping and system calls, why couldn't
OS threads by themselves be more efficient?

mark 

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