From: Brian Hurt <brian.hurt@qlogic.com>
To: Will Benton <willb@cs.wisc.edu>
Cc: Ocaml Mailing List <caml-list@inria.fr>
Subject: Re: [Caml-list] Re: feature priorities (multithreading)
Date: Wed, 19 Feb 2003 13:26:47 -0600 (CST) [thread overview]
Message-ID: <Pine.LNX.4.33.0302191307560.2037-100000@eagle.ancor.com> (raw)
In-Reply-To: <20030219121756.C2587@tux15.cs.wisc.edu>
On Wed, 19 Feb 2003, Will Benton wrote:
> On Wed, Feb 19, 2003 at 11:45:52AM -0600, Brian Hurt wrote:
>
> > A better approach is what's called the M-by-N approach. You have M kernel
> > level threads (so you can take advantage of M parallel CPUs) each
> > executing threads from a pool of N user space threads. This gives you the
> > best of both worlds.
>
> Not necessarily -- most m:n threads packages have non-preemptive user
> thread schedulers, which means that a compute-intensive user thread that
> never enters the kernel and doesn't yield the processor can monopolize
> a kernel thread. That might be a pathological example (since if
> you're trying to run >N compute-intensive threads on N processors,
> you're in trouble), but someone might argue that an application that
> context-switches enough to *require* m:n instead of 1:1 is
> pathological in a different way.
Co-operative scheduling is easier- but does run into the problem you point
out, with the possibility of deadlock.
>
> If you need preemptability, then the best-of-both-worlds approach is
> to find a system on which kernel thread context switches are cheap.
> :-) (Making KT switches cheap seems to be the direction that the
> industry is going -- Sun, for example, has dropped their m:n threads
> package for solaris.)
>
Part of the of the question is "what do you mean by 'cheap'"? The last
time I had firm numbers of how much a true task switch cost was the i386-
and that was 300-500 clock cycles. Which on a 33MHz machine was a
signifigant cost. 300-500 clocks on a 3GHz machine is signifigantly
cheaper- in fact, 300 clocks is about the cost of an unpredictable cache
miss on the P4. Also, large portions of the cost of a task switch is
related to swapping out the memory map- cache and TLB misses and flushing.
If you're swapping between different processes with the same memory space
(i.e. you don't have to flush TLB or cache), then I could see the cost
being close to the cost of just saving/restoring the registers- i.e. about
30-60 clock cycles.
I could easily see Sun saying that it wasn't worth it to do true M:N in
the face of this. The savings just aren't signifigant enough to make it
worthwhile- *if* your operating systems can handle 1,000s of threads
(until recently, Linux couldn't). Solaris definately can.
Brian
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next prev parent reply other threads:[~2003-02-19 19:16 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-02-10 18:52 [Caml-list] Request: matrix_init function in Array Brian Hurt
2003-02-10 23:22 ` Pierre Weis
2003-02-11 2:37 ` Chris Hecker
2003-02-13 8:33 ` Pierre Weis
2003-02-13 16:50 ` Chris Hecker
2003-02-13 17:13 ` feature priorities (was Re: [Caml-list] Request: matrix_init function in Array) Ed L Cashin
2003-02-14 17:52 ` brogoff
2003-02-14 20:22 ` rich
2003-02-16 23:07 ` Alessandro Baretta
[not found] ` <Pine.LNX.4.53L.0302170500360.32142@ontil.ihep.su>
2003-02-17 22:27 ` Alessandro Baretta
2003-02-19 9:18 ` [Caml-list] Re: feature priorities (multithreading) James Leifer
2003-02-19 16:46 ` cashin
2003-02-19 17:14 ` Ranjan Bagchi
2003-02-19 17:45 ` Brian Hurt
2003-02-19 18:17 ` Will Benton
2003-02-19 19:26 ` Brian Hurt [this message]
2003-02-19 17:25 ` Brian Hurt
2003-02-19 17:26 ` Noel Welsh
2003-02-20 8:00 ` Michel Schinz
2003-02-20 16:26 ` Brian Hurt
2003-02-13 17:38 ` [Caml-list] Request: matrix_init function in Array Brian Hurt
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