CSC/ECE 506 Fall 2007/wiki3 2 aY3w

SCI. The IEEE Scalable Coherent Interface is a superset of the SSCI protocol we have been considering in class. A lot has been written about it, but it is still difficult to comprehend. Using SSCI as a starting point, explain why additional states are necessary, and give (or cite) examples that demonstrate how they work. Ideally, this would still be an overview of the working of the protocol, referencing more detailed documentation on the Web.

A Brief Overview of the SSCI Protocol

The Simple Scalable Coherence Interface (SSCI) protocol is a simplified version of the SCI protocol. While the SCI protocol has 29 stable states and many pending states, the SSCI protocol has only 3 states. Consequently, many race conditions are possible. Here we give a brief overview of the SSCI protocol, and elaborate on the possible race conditions in the final section.

As previously states, in the SSCI protocol the memory directory is in one of three states. They are:

• UNOWNED - the block is unowned by any cache.
• SHARED - the block is owned by multiple caches. Main memory is up to date.
• EXCLUSIVE/MODIFIED - the (possibly dirty) block is owned by a single cache. Main memory is possibly out of date.

A Brief Overview of the SCI Protocol

Memory Directory States

Similar to the SSCI protocol, in SCI the memory directory is one of three states:

• HOME - no remote cache contains a copy of the block.
• FRESH - one of more remote caches may have a read-only copy of the block. Main memory is up to date.
• GONE - the cache at the head of the sharing list contains an exclusive (possibly dirty) copy of the block.

Cache States

Additionally, each cache has a state, either stable or pending. Stable states have two parts:

1. Where the cache entry is located in the sharing list for that block
• ONLY - singleton list
• HEAD - head of sharing list of more than one node
• MID - not head or tail of sharing list of more than one node
• TAIL - tail of sharing list of more than one node
2. State of the cached block
• DIRTY - modified and writable
• CLEAN - unmodified and writable, same contents as memory
• FRESH - data is readable and writable, but memory must be informed before memory is written
• COPY - unmodified and readable
• many others

These two parts together describe the cache state, e.g. FRESH-ONLY or MID-VALID.

Why Additional States are Needed

The principle difference between SCI and SSCI is that SCI provides mechanisms for resolving race conditions and preserving serialization.

Links

http://www.scizzl.com/WhatIsSCI.html
http://www.lrr.in.tum.de/~gerndt/home/Teaching/scalable_shared_memory_systems/Kapitel8.pdf