Re: multi thread app and often variable accessing

[wallace owen l-3com com]

On Fri, 2007-06-08 at 19:41 -0600, Jim George wrote:
> > Begin Curiosity:
> > > Also, the hardware must have something like the Pentium CMPXCHG8B [...]
> > Is it not
> > sufficient to be able to write an int in one single bus access? ie
> > have a 32-bit wide data bus
> > Or is that exactly the point? :D
> No, in most cases, if you want to atomically increment or decrement a
> value (for example, a semaphore or ref count) without locking, you
> have to read the value, increment it, then write it only if it's not
> changed. The write and check to see if it's not changed must be
> atomic, this is where CMPXCHG comes in.
>
> > > MOV instructions with a bus-lock prefix, which is slow.
> > OK, I guess it'd be faster (and leaner) than using a mutex though
> > But that can't be done in pure C, right?
> Maybe, but I don't see how any end-user program (such as one with a
> GTK interface) would benefit from the small speed increase. Put such
> stuff in another program which merely reports its progress to the gtk
> GUI program through a socket or pipe.

Another effect of using the CMPXCHG is that it forces the memory manager
to read the value from memory and not cache, and write through the cache
to physical memory on write.  This is important in the case where you've
got more than one core running threads on your code: Each core has it's
own cache.  If the instruction didn't force a sync with physical memory,
your code would break.


  // Wally

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Wallace Owen L-3Com com
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