Brian Hurt wrote:
> 
> ---------- Forwarded message ----------
> Date: Tue, 21 Mar 2006 16:32:51 -0600 (CST)
> From: Brian Hurt <bhurt@spnz.org>
> To: Robert Roessler <roessler@rftp.com>
> Subject: Re: [Caml-list] Severe loss of performance due to new signal 
> handling
> 
> On Tue, 21 Mar 2006, Robert Roessler wrote:
> 
>> Well, I *thought* there was a marked absence of "bit-level 
>> parallelism" in the signal-handling... ;)
>>
>> So the "expense" of individual atomic operations is not really what is 
>> at the heart of this performance problem...
> 
> Hmm.  Maybe not.  I'm measuring a 4 clock cycle cost for a xchgl, both 
> with and without a lock on my Athlon XP 1.8GHz.  See attached code. 
> Naturally, this is a uniprocessor machine and the memory location is in 
> L1 cache (or will be soon), and no contention, so this is definately 
> best case.  4 clocks is about rights for a read and a write to L1 cache 
> (each L1 cache access taking 2 clocks).

And after adjusting the inline assembly syntax for vc7.1, I get

Minimum time for a rdtsc instruction (in clocks): 38
Minimum time for a read_and_clear() + rdtsc (in clocks): 75

This is on a P-III S (Tualatin) @ 1.4GHz on Windows XP SP2.

Robert Roessler
roessler@rftp.com
http://www.rftp.com