Date: Sun, 4 Feb 2007 14:52:08 -0800 (PST)
From: Davide Libenzi <davidel@xmailserver.org>
To: Jakub Jelinek <jakub@redhat.com>
cc: Bill Davidsen <davidsen@tmr.com>, Arjan van de Ven <arjan@infradead.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [ANN] Userspace M-on-N threading model implementation. Alpha
 release.
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On Sun, 4 Feb 2007, Jakub Jelinek wrote:

> On Sun, Feb 04, 2007 at 03:12:32PM -0500, Bill Davidsen wrote:
> > Arjan van de Ven wrote:
> > >>Because user threading can avoid context switches, there will always be 
> > >>cases where it will outperform o/s threads for hardware reasons.
> > >
> > >actually.. switching from one "real" thread to another in Linux is not
> > >an actual context switch in the hardware sense... at least this part of
> > >your argument seems to be incorrect ;)
> > >
> > How does that work? Switching between kernel threads requires going into 
> > the kernel, user level thread switches are all done in user mode.
> > 
> > Do you have some way to change o/s threads w/o going into the kernel?
> 
> But going into kernel is not very expensive on Linux.
> 
> On the other side, the overhead you need to add for every single syscall
> that might block for the M:N threads and the associated complications
> which make it far harder to conform to POSIX IMHO far outweight the costs
> of going into the kernel for a context switch.

Agreed, definitely. A libpcl (using swapcontext(3)) cobench is about 50 
times faster than an context switch measured by lmbench (although I have 
serious doubts about about the ability of lat_ctx to measure it - but 
that's another story) on an Opteron 254. One may say "Wow! Really?!?".
The point is, who cares. We are talking about differences between 
super-fast (~2us) and ultra-fast (~0.04us).
The time (and code) that you'll have to drop in the syscall path to handle 
M:N is very likely to make you lose way more of what you gain by avoiding 
an OS context switch (a "soft" context switch you still have to do it).
Either use N:N (requires locking, but spread over multiple CPUs) or 1:N 
(I/O driven state machines or coroutines - no locking, once-CPU bound).


- Davide


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