Transactional Memory: Lo bueno y lo posiblemente feo

El gran Greg Pfister tiene un artículo en su blog sobre memoria transaccional, soportada por la próxima familia de chips de Intel, Haswell. Explica qué es y para qué puede servir:

With the usual lock semantics, you could simply have one coarse lock around the whole table: Only one person, read or write, gets in at any time. This works, and is simple, but all access to the table is now serialized, so will cause a problem as you scale to more processors.

Alternatively, you could have a lock per hash bucket, for fine-grained rather than coarse locking. That’s a lot of locks. They take up storage, and maintaining them all correctly gets more complex.

Or you could do either of those – one lock, or many – but also get out your old textbooks and try once again to understand those multiple reader / single writer algorithms and their implications, and, by the way, did you want reader or writer priority? Painful and error-prone.

On the other hand, suppose everybody – readers and writers – simply says “Transaction On!” (I keep wanting to write “Flame On!”) before starting a read or a write; then does a “Commit!” when they exit. This is only as complicated as the single coarse lock (and sounds a lot like an “atomic” keyword on a class, hint hint).