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

[James Wang <jameswang99@xxxxxxxxx>]

Hi Enrique :
    The main reason for the complication of what you
observed is to deal with certain situations like two
legged transactional write, like from I to IM, the
cache line's W bit could not be set until the cache
line is obtained by p0. So, to member to set the w bit
of the cache line, the TBE's w bit is set. The same
procedure is also applied in other situations for
consistency reasons, I surmise. 


--- Enrique Vallejo Gutiérrez <enrique@xxxxxxxxxxxxx>
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
> 
> > _______________________________________________
> Gems-users mailing list
> Gems-users@xxxxxxxxxxx
>
https://lists.cs.wisc.edu/mailman/listinfo/gems-users
> Use Google to search the GEMS Users mailing list by
> adding
> "site:https://lists.cs.wisc.edu/archive/gems-users/";
> to your search.
> 
> 


Regards
James