Date: | Tue, 15 Nov 2011 11:48:52 -0500 |
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From: | "abhisekpan@xxxxxxxxx" <abhisekpan@xxxxxxxxx> |
Subject: | [Gems-users] L2 misses in MESI_CMP directory protocol |
Hi, I am simulating a CMP (single chip with a shared last level cache) using the MESI_CMP_Directory protocol. I was looking at how the cache misses are counted in the MESI_CMP_directory-L2cache.sm files. These are the conditions when an L2 miss is registered. Current State Events NP GETS, GETX IS, ISS GETS SS GETS, GETX, UPGRADE M GETs, GETX, MT GETS, GETX The relevant state descriptions are as follows: stable states: NP, desc="Not present in either cache"; SS, desc="L2 cache entry Shared, also present in one or more L1s"; M, desc="L2 cache entry Modified, not present in any L1s"; MT, desc="L2 cache entry Modified in a local L1, assume L2 copy stale"; transient states: ISS, desc="L2 idle, got single L1_GETS, issued memory fetch, have not seen response yet"; IS, desc="L2 idle, got L1_GET_INSTR or multiple L1_GETS, issued memory fetch, have not seen response yet"; I am confused why misses are registered for cases when the L2 block is in M, MT, or SS states. M means the L2 has the value and owns it already, MT means some L1 cache has exclusive ownership, which will supply the value. There is no off-chip access. The only time when there is an off-chip memory access (a_issueFetchToMemory) is when the L2 is in NP state. So I thought these are the only cases where we should count an L2 miss. I am struggling to explain all these other cases where L2 miss is registered. Any ideas / explanation regarding this would be very helpful! Thanks a lot! -- Abhisek Live Long and Prosper |
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