Re: [Gems-users] Finding the values of latency

[Philip Garcia <pcgarcia@xxxxxxxx>]

On Nov 28, 2009, at 4:46 PM, sparsh mittal ISU wrote:

> Hello
> Thanks a lot list-users for the previous replies.
> While using the default configurations (no change) for ruby and opal  
> in gems-2.1 what is the value of L1, L2 and memory latency that is  
> used? I tried to see the config files, but could not understand the  
> right value, amongst the variety of values of latencies.
> I would appreciate any help.
It really depends on what protocol you use for what the latencies you  
get.  I use MOESI_CMP_directory_m for my research.  In other protocols  
the latencies are often completely different.  I had a conversation  
with Derek recently about the latencies involved in different  
scenarios, and I am pretty sure this is what we came up with (please,  
correct me if I'm wrong).

L1 hit:  Sequencer to Controller latency

L1 Miss, L2 hit:  	1.  seq 2 controller
				2.  L1 Req latency
				3.  Network
				4.  L2 response latency
				5.  Network

L1 Miss/L2 Miss/other L1 hit:
1.  seq 2 controller
				2.  L1 Req latency
				3.  Network
				4.  L2 response latency
				5.  Network
				6.  L1 Req latency
				7.  Network
				8.  L1 req latency
				9.  Network

I think on an L2 miss ->main memory it goes

L1 Miss, L2 hit:  	1.  seq 2 controller
				2.  L1 Req latency
				3.  Network
				4.  L2 response latency
				5.  Network
				6.  directory latency
				7.  memory latency (set by a whole bunch of parameters in the _m  
protocols, check the default files for details).
	--not sure if there is other on chip latencies here i'm skipping  
though, as we didn't discuss this case much.
Phil