Re: [Gems-users] Ruby Question about MESI_CMP and implications of a Blocking Protocol

["Jiho \"Ray\" Choi" <jray319@xxxxxxxxx>]

I am not 100% confident with my answers.

1) 'Unblock/Exclusive Unblock' messages are to change L1 cache line states (to/from Exclusive states) and they have nothing to do with blocking/non-blocking cache.

2) Ruby models blocking cache as default. I am not sure if you can model non-blocking cache in Ruby, but if you want, you may look into SimicsDriver, SimicsProcessor and Sequencer.

And blocking cache serves memory transactions such as read and store one by one.

Regarding your example, I think it is possible. Think of the scenario when L1 is full and the cache line of Y is selected as a victim. If the cache line of Y is dirty, L1 needs to write it back to L2. Apparently, STORE X doesn't "block" writing back the cache line of Y.

Ray

On Wed, Mar 23, 2011 at 6:54 AM, Malek Musleh <malek.musleh@xxxxxxxxx> wrote:

Hi,

1) I was wondering if the MESI_CMP protocol is currently implemented
as a 'blocking' protocol, similar to how the MOESI_CMP version is.


I see that on this link on the GEMS page that it indicates that the
MOESI_CMP one is blocking, but doesn't indicate anything about the
MESI_CMP version.

http://www.cs.wisc.edu/gems/doc/gems-wiki/moin.cgi/Protocols?action="">

In the MOESI_CMP Version there are coherenceResponseType Messages such
as 'Unblock' / 'Exclusive_Unblock' which seems to enforce the blocking
aspect, and I also see these types in the MESI version, but just
implementing them does not necessarily enforce blocking in all
possible situations.

2) What does the specific meaning of a 'blocking' protocol refer to?
As in, is the following example not possible:

For example, can Core 0 do a STORE X to L1 as L1 does a writeback of
Data: Y to L2?

I suspect no, that in a blocking Cache Coherence Protocol I cannot do
that, but just wanted to confirm.

Malek
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