[Gems-users] Temporary reservation of TBE in transactional stores?

[Enrique Vallejo Gutiérrez <enrique@xxxxxxxxxxxxx>]

Dear list,

 

            I have a question regarding the released LogTM SLICC protocols. In both MOESI and MESI LogTM protocols, when a store is received during a transaction (event: Store_XACT) and the block is in any “ready-to-be-modified” state (such as M or MM in MOESI, or M or E in MESI) the W bit of the cache line must be set. However, instead of directly setting this W bit, the protocol performs the following series of actions (among others):

 

 · i – allocate a TBE

· xt – set the transactional flag (TBE[address].Trans) in the TBE

· xiw – set the W bit in the cache line IF the TBE is marked TRANS

· s – deallocate TBE

 

            It is not obvious to me why the reservation of the TBE is necessary, given that no message is sent to the network and no reply is awaited. In fact, given that the transaction is atomic, the only “visible” effect might be that the transaction wouldn’t occur if there wasn’t a free TBE. I don’t know if this is a simulation artifact, for example for profiling purposes (the reservation of the TBE has some append_transition_comment calls), or there is some real reason to not directly set the bit (or maybe only historical reason). The final detail is that the bit R IS directly set when a load_XACT event happens.

 

            Someone can help? Thanks in advance,

 

Enrique Vallejo Gutiérrez

University of Cantabria, Spain