Re: [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores)

Hubertus Franke
2002-03-11T14:13:49Z

On Friday 08 March 2002 09:12 pm, Linus Torvalds wrote:
> On Fri, 8 Mar 2002, Hubertus Franke wrote:
> > > The point being that the difference between a "decl" and a "lock ;
> > > decl" is about 1:12 or so in performance.
> >
> > I am no expert in architecture, but if its done through the cache
> > coherency mechanism, the overhead shouldn't be 12:1. You simply mark the
> > cache line as part of you instruction to avoid a cache line transfer. How
> > can that be 12 times slower. .. Ready to be educated....
>
> A lock in a SMP system also needs to synchronize the instruction stream,
> and not let stores move "out" from the locked region.
>
> On a UP system, this all happens automatically (well, getting it to happen
> right is obviously one of the big issues in an out-of-order CPU core, but
> it's a very fundamental part of the core, so it's "free" in the sense that
> if it isn't done, the CPU simply doesn't work).
>
> On SMP, it's a memory barrier. This is why a "lock ; decl" is more
> expensive than a "decl" - it's the implied memory ordering constraints (on
> other architectures they are explicit). On an intel CPU, this basically
> means that the pipeline is drained, so a locked instruction takes roughly
> 12 cycles on a PPro core (AMD's K7 core seems to be rather more graceful
> about this one). I haven't timed a P4 lately, I think it's worse.
>
> Other architectures do the memory ordering explicitly, and some are
> better, some are worse. But it always costs you _something_.
>
> Linus

Sure, not contending that. Right now I think our focus should be to get the
right functionality out and address people's concerns.
Improvements, as you suggested, are orthogonal and can always be put
in later.

--
-- Hubertus Franke (frankeh@watson.ibm.com)