An Adaptive Cache Coherence Protocol Optimized for Migratory Sharing

CS258 Paper SummaryComputer ScienceJaein Jeong

Migratory Sharing

Pi has exclusive copy of X

Location X

Pi

Pj

Pk

Dirty

Dirty

Invalid

Invalid

Pi

Migratory Sharing

Pj sends read request to X

Location X

Pi

Pj

Pk

Dirty Rd

Dirty

Invalid

Invalid

Pi

Migratory Sharing

D.C. sends flush to the exclusive sharer Pi

Location X

Pi

Pj

Pk

Dirty

RdDirty

Invalid

Invalid

Pi

Migratory Sharing

Pi sends data to Pj and main memoryState changes to shared, with sharer Pi and Pj

Location X

Pi

Pj

Pk

Shared

WBShared

Shared

Invalid

Pi, Pj

WB

Migratory Sharing

Pj sends read exclusive request to X

Location X

Pi

Pj

Pk

Shared RdX

Shared

Shared

Invalid

Pi, Pj

Migratory Sharing

D.C. invalidates Pi

Location X

Pi

Pj

Pk

Shared

Inv.Shared

Shared

Invalid

Pi, Pj

Migratory Sharing

Pi is invalidatedState for memory and Pj changes to dirtywith sharer Pj

Location X

Pi

Pj

Pk

Dirty

Invalid

Dirty

Invalid

Pj

Transaction Cost under W.I.

Two separate requests from Pj: Read and Read Exclusive requests

One flush and one invalidation to PiIdea for improvement One request from Pj After Pi sends data, its state becomes invalid

Migratory Sharing (modified)

Pj sends read request (migrating read) to X

Location X

Pi

Pj

Pk

Dirty Rd

Dirty

Invalid

Invalid

Pi

Migratory Sharing (modified)

D.C. sends read to the exclusive sharer Pi

Location X

Pi

Pj

Pk

Dirty

RdDirty

Invalid

Invalid

Pi

Migratory Sharing (modified)

Pi sends data to Pj and main memoryand invalidates its copy

Location X

Pi

Pj

Pk

Dirty

WBInvalid

Invalid

Invalid

Pi

WB

Migratory Sharing (modified)

D.C. sends ACK to PjState changes with sharer Pj

Location X

Pi

Pj

Pk

Dirty ACK

Invalid

Dirty

Invalid

Pj

Extensions to support migratory sharing

Invalid

Dirty Shared

Migrating

Switches b/w W.I. And migratory protocolExtensions are needed: LW (last writer), N (num sharer) 2 states for home, 1 state for local

Detection of migratory sharing

Condition for migratory sharingRxq_i && N == 2 && LW != i

Location X

Pi

Pj

Pk

Dirty Rd

Dirty

Invalid

Invalid

Pi

Detection of non-migratory sharing

Condition for W-IRr && local state == migrating

<States of Processor Pi>

Location X

Pi

Pj

Pk

Dirty

Rd_j Migrating

Invalid

Invalid

Pi

Experimental Results(Execution Time)

Effective for read-and-modifiy accessesReduces number of writesNo negative effect for non-migratory apps

Experimental Results(Network Traffic)

Same trend with the execution time

Experimental Results(Various memory consistency models)

Adaptive protocol more effective for stricter memory consistency models

Experimental Results(Smaller cache size)

Adaptive protocol more effective when capacity and conflict misses are reduced with larger cache

Discussion

Is the adaptive protocol worthwhile? The effectiveness varies depending

on applications (1% to 54%). Benefits on migratory sharing

Added complexity Increased number of states More complex transition