Linux-kernel mailing list archive 1998-49,: Re: Linux threads -- as seen in NT Magazine

# Re: Linux threads -- as seen in NT Magazine

Scott Doty (scott@sonic.net)
Wed, 9 Dec 1998 07:36:44 -0800

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On Tue, Dec 08, 1998 at 03:53:40PM -0500, Paul Barton-Davis wrote:
> Scott Doty quoted:
> >The December issue of _Windows NT Magazine_ features an article:
> >"NT vs. Unix." The article includes a sidebar "Linux and
> >the Enterprise," which begins:
> >
> > Whereas Windows NT and all leading commercial operating systems
> > (OSs) implement kernel-mode threads, Linux does not.
>
> Well, lets get a few things clear.
>
> Linux does implement kernel threads via the clone(2) interface.

That's the important part.

> Linux does *NOT* implement kernel threads in the sense that its
> scheduler equates a "task" (a traditional Unix process) with a
> "thread".

Sure it does. (A task is not necessarily a "traditional Unix process".)

To avoid confusion, I recommend the following terminology:

task -- a "context of execution". (That is, one task structure being
scheduled.)
process -- a task that has been clone()d with COPYVM.
thread -- a task that has been clone()d without COPYVM.

Each kernel thread has it's own task struct -- therefore, they are
tasks. The scheduler only deals with tasks, which I think we can
appreciate: that's pretty elegant. A full-blown process adds its own
page tables (but not necessarily its own pages -- they are shared
with the parent until written to, which is another stroke of elegance).

> There is no support in any current Linus-approved kernels
> for the notion of a process ("task") that is implemented as a set of
> kernel threads.

I think I can understand why you'd want cooperating threads to be scheduled
as a group. (It seems like you could take advantage of a processor's cache,
as well as speed up a context shift.)

> [Scheduling model and multiprocessor hardware]
> [Dynamically loadable schedulers]
> I have had no enthusiastic feedback from anyone on the list, and right
> now, I'm busy with another fairly ambitious Linux project, so its on the
> backburner for now, but once 2.2 comes out, and stuff like this might get
> more attention, I'll focus on it again.

Dynamically-loadable schedulers? That sounds like a snazzy project!
(Enthusiasm! Enthusiasm! :)

However, I do find some code in 2.0.36 that appears to support
tasks "spiking" themselves on a particular processor:

#ifdef __SMP__
[...]
#ifdef PAST_2_0
/* This process is locked to a processor group */
if (p->processor_mask && !(p->processor_mask & (1<<this_cpu))
return -1000;
#endif

linux/kernel/sched.c, in goodness() -- there's also some other goodness in
goodness(), so take a look. :)

I'm starting to quote "#ifdef __SMP__", so I suspect we should
continue this discussion on the linux-smp list. However, I'll point
out that it seems the benefits of dynamically-loaded schedulers could
be gained with a single, highly-configurable scheduler, and that
could preserve the elegance of the current implementation.

But that is beside the point: Linux has kernel threads, and they have
been supported in commercial distributions since May, 1998 (at least --
probably earlier.)

I've written the author of that awful article. That author is an
acquantance of a good friend of mine -- the same friend who brought the
article to my attention. I'll report Dr. Russinovich's response (if
any).

Thanks to folks who gave me much-needed feedback!

-Scott

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