Re: [gem5-dev] Functional Memory Accesses in Ruby

[Nilay Vaish]

On Sat, 28 May 2011, Nilay Vaish wrote:


Hi Brad

I am trying to complete the patch on functional accesses in Ruby. I came 
across this problem while testing the patch for higher number of processors. 
I am working with MESI CMP directory protocol right now. So I will describe 
the problem in its context.


Assume we are trying to functionally write some thing to block A. It is in 
state S_I in the L2 cache. When a block moves to state S_I from state SS, 
then the cache block in the cache is deallocated. Therefore, when viewed from 
the CacheMemory's perpespective, since the cache does not have block A, 
therefore, the L2 cache is of no consequence for this access. But the 
controller has a TBE for this block. And this TBE will have this block with 
AccessPermission:Busy. Also, there are L1 caches in the system that hold this 
block in S state.


Now, as per the current condition for write functional accesses,

if((num_busy == 0  num_ro  0) || num_rw == 1)

this access would go ahead as num_busy would evaluate to 0 and num_ro would 
evaluate to some value greater than 0. But clearly we do not want this access 
to be performed since that state S_I is a busy state and no other cache holds 
the block in a read-write state.


It seems to me that the controller should supply the function for deciding 
the access permissions, since it is possible that one the TBE holds the 
block.


--
Nilay



Brad, I went over the discussion that we had this morning. I think the 
getState() function cannot be used for extracting access permissions. This 
is because the getState() function needs pouinters to transaction buffer 
and cache entries, apart from the address.


I think we should let the Controller provide a function for getting the 
access permissions. This function would be a virtual function declared in 
the AbstractController class, but would not be pure. The 
AbstractController implementation of the function would always return 
busy, so that functional accesses are not enabled at all for protocols 
that do not provide such a function.


--
Nilay
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[gem5-dev] Functional Memory Accesses in Ruby

[Nilay Vaish]

Hi Brad

I am trying to complete the patch on functional accesses in Ruby. I came 
across this problem while testing the patch for higher number of 
processors. I am working with MESI CMP directory protocol right now. So I 
will describe the problem in its context.


Assume we are trying to functionally write some thing to block A. It is in 
state S_I in the L2 cache. When a block moves to state S_I from state SS, 
then the cache block in the cache is deallocated. Therefore, when viewed 
from the CacheMemory's perpespective, since the cache does not have block 
A, therefore, the L2 cache is of no consequence for this access. But the 
controller has a TBE for this block. And this TBE will have this block 
with AccessPermission:Busy. Also, there are L1 caches in the system that 
hold this block in S state.


Now, as per the current condition for write functional accesses,

 if((num_busy == 0  num_ro  0) || num_rw == 1)

this access would go ahead as num_busy would evaluate to 0 and num_ro 
would evaluate to some value greater than 0. But clearly we do not want 
this access to be performed since that state S_I is a busy state and no 
other cache holds the block in a read-write state.


It seems to me that the controller should supply the function for deciding 
the access permissions, since it is possible that one the TBE holds the 
block.


--
Nilay
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