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Invalidation vs. Update
- Two main classes of protocols:
- Dictates what action should be taken on a store
- Invalidation-based protocols invalidate sharers when a store miss appears
- Update-based protocols update the sharer caches with new value on a store
- Advantage of update-based protocols: sharers continue to hit in the cache while in invalidation-based protocols sharers will miss next time they try to access the line
- Advantage of invalidation-based protocols: only store misses go on bus and subsequent stores to the same line are cache hits
- When is update-based protocol good?
- What sharing pattern? (large-scale producer/consumer)
- Otherwise it would just waste bus bandwidth doing useless updates
- When is invalidation-protocol good?
- Sequence of multiple writes to a cache line
- Saves intermediate write transactions
- Overhead of initiating small updates
- Invalidation-based protocols are much more popular
- Some systems support both or maybe some hybrid based on dynamic sharing pattern of a cache line
Sharing patterns
- Producer-consumer (initially flag, done are zero)
T0: while (!exit) {x=y; flag=1; while (done != k); flag=0; done=0;}
T1 to Tk: while (!exit) {while (!flag); use x; done++; while (flag);}
- Exit condition not shown
- What if T1 to Tk do not have the outer loop?
- Migratory (initially flag is zero)
T0: x = f0(x); flag++;
T1 to Tk: while (flag != pid); x = f1(x); flag++;
Migratory hand-off
- Needs a memory writeback on every hand-off
- r0, w0, r1, w1, r2, w2, r3, w3, r4, w4, …
- How to avoid these unnecessary writebacks?
- Saves memory bandwidth
- Solution: add an owner state (different from M) in caches
- Only owner can write a line back on eviction
- Ownership shifts along the migratory chain
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