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

[Kevin Moore <kmoore@xxxxxxxxxxx>]

Enrique,
	The TBE is allocated (and immediately deallocated) in that 
case because the actions, setWbitIfTrans and addLogEntry 
check the state of the Trans flag in the TBE.  They fail if 
there's no TBE allocated.  We could have created separate 
actions that ignore this check, but allocating the TBE 
seemed cleaner.  You're right that if we took into account 
multiple outstanding memory requests and limited TBEs this 
decision might result in unnecessary stalls.
--Kevin

Enrique Vallejo Gutiérrez wrote:
> 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
>
>
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>
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