Re: [Caml-list] STM support in OCaml

[Brian Hurt <bhurt@spnz.org>]

On Wed, 8 Mar 2006, William Lovas wrote:

> As the quoted paper said, though, some running transaction can always
> commit -- so not all of the associated costs are still paid.  In
> particular, the cost of potential non-termination due to deadlock or
> livelock is not paid.  It doesn't matter that there's a mutex under the
> hood, since it's used only in such a way as to preserve the property that
> some transaction can always complete.

One comment I will make is that a mutex is expensive, but not *that* 
expensive.  I just wrote a quick program (available if anyone cares) in 
GNU C that measures the cost, in clocks, of locking and unlocking a posix 
mutex.  On my desktop box (AMD Athlon XP 2200+ 1.8GHz), I'm getting a cost 
of like 44 clock cycles.  Which makes it less expensive than an L2 cache 
miss.

At this point correctness is a much bigger concern of mine than absolute 
performance.  A fair bit of conservative or unnecessary locking is 
acceptable, given than I can write a complicated and highly scalable 
application and know that I don't have deadlocks or livelocks.  Not having 
race conditions would also be nice, but I don't think that's possible with 
mutable data.

Brian