TM methods and method groups

libitm supports several ways of synchronizing transactions with each other. These TM methods (or TM algorithms) are implemented in the form of subclasses of abi_dispatch, which provide methods for transactional loads and stores as well as callbacks for rollback and commit. All methods that are compatible with each other (i.e., that let concurrently running transactions still synchronize correctly even if different methods are used) belong to the same TM method group. Pointers to TM methods can be obtained using the factory methods prefixed with dispatch_ in libitm_i.h. There are two special methods, dispatch_serial and dispatch_serialirr, that are compatible with all methods because they run transactions completely in serial mode.

TM method life cycle

The state of TM methods does not change after construction, but they do alter the state of transactions that use this method. However, because per-transaction data gets used by several methods, gtm_thread is responsible for setting an initial state that is useful for all methods. After that, methods are responsible for resetting/clearing this state on each rollback or commit (of outermost transactions), so that the transaction executed next is not affected by the previous transaction.

There is also global state associated with each method group, which is initialized and shut down (method_group::init() and fini()) when switching between method groups (see retry.cc).

Selecting the default method

The default method that libitm uses for freshly started transactions (but not necessarily for restarted transactions) can be set via an environment variable (ITM_DEFAULT_METHOD), whose value should be equal to the name of one of the factory methods returning abi_dispatch subclasses but without the “dispatch_” prefix (e.g., “serialirr” instead of GTM::dispatch_serialirr()).

Note that this environment variable is only a hint for libitm and might not be supported in the future.