[cpp-threads] Alternatives to SC

[Paul E. McKenney]

On Mon, Jan 15, 2007 at 07:57:52AM +0100, Alexander Terekhov wrote:
> On 1/14/07, Raul Silvera <rauls at ca.ibm.com> wrote:
> [...]
> >Furthermore, as several other people have mentioned already, SC requires
> >reads to wait for any writes observed from other threads to become globally
> >visible. This means you need a StoreLoad barrier ...
> 
> P1: x = 1;
> P2: if (x == 1) y = 2;
> P3: if (y == 2) assert(x == 1);
> 
> PowerPC Book II discusses that example and says that it requires
> 
> P1: x = 1;
> P2: if (x == 1) sync(), y = 2;
> P3: if (y == 2) sync(), assert(x == 1);
> 
> ('Cumulative ordering' property of 'sync' instruction.)
> 
> I somehow doubt that it will outperform a sync()-less version doing
> load of x on P3 via stwcx-validated lwarx. ;-)

Hard to say which would be faster.  Raul's version has the advantage of
permitting the caches to retain read sharing.  Your version has the
advantage of getting rid of an expensive sync() instruction.  If there
are lots of P3 executions and very few P1 and P2 executions, I would
guess that Raul's approach wins.

Of course, even better would be avoiding -both- the sync and the
lwarx/stwcx.  This can actually be done in some specialized but
very commonly occurring cases, and this is what RCU is all about.

						Thanx, Paul