COMP 206: Computer Architecture and Implementation

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COMP 206Computer Architecture and Implementation

• Montek Singh
• Mon, Nov 14, 2005
• Topic Cache Coherence

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Outline

• Cache Coherence
• Reading HP3 Section 6.3 Appendix I

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Cache Coherence

• Common problem with multiple copies of mutable
  information (in both hardware and software)
• If a datum is copied and the copy is to match
  the original at all times, then all changes to
  the original must cause the copy to be
  immediately updated or invalidated. (Richard L.
  Sites, co-architect of DEC Alpha)

Copy becomes stale
1 2 3 4 A A A C - A B B
Copies diverge hard to recover from
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Example of Cache Coherence

• I/O in uniprocessor with primary unified cache
• MM copy and cache copy of memory block not always
  coherent
• WT cache
• MM copy stale while write update to MM in transit
• WB cache
• MM copy stale while cache copy Dirty
• Inconsistency of no concern if no one
  reads/writes MM copy
• If I/O directed to main memory, need to maintain
  coherence

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Example of Cache Coherence (contd)

• Uniprocessor with a split primary cache
• I-cache contains instruction
• D-cache contains data
• Often contents are disjoint
• If self-modifying code is allowed, then same
  cache block may appear in both caches, and
  consistency must be enforced
• MS-DOS allows self-modifying code
• Strong motivation for unified caches in Intel
  i386 and i486
• Pentium has split primary cache, and supports SMC
  by enforcing coherence between I and D caches
• Coordinating primary and secondary caches in
  uniprocessor
• Shared memory multiprocessors

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Two Snoopy Protocols

• We will discuss two protocols
• A simple three-state protocol
• Section 6.3 Appendix I of HP3
• The MESI protocol
• IEEE standard
• Used by many machines, including Pentium and
  PowerPC 601
• Snooping
• monitor memory bus activity by individual caches
• taking some actions based on this activity
• introduces a fourth category of miss to the 3C
  model coherence misses
• First, we need some notation to discuss the
  protocols

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Notation Write-Through Cache
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Notation Write-Back Cache
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Three-State Write-Invalidate Protocol

• Minor modification of WB cache
• Assumptions
• Single bus and MM
• Two or more CPUs, each with WB cache
• Every cache block in one of three states
  Invalid, Clean, Dirty (called Invalid, Shared,
  Exclusive in Figure 6.10 of HP3)
• MM copies of blocks have no state
• At any moment, a single cache owns bus (is bus
  master)
• Bus master does not obey bus command
• All misses (reads or writes) serviced by
• MM if all cache copies are Clean
• the only Dirty cache copy (which is no longer
  Dirty), and MM copy is written instead of being
  read

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Understanding the Protocol

• Only two global states
• Most up-to-date copy is MM copy, and
• all cache copies are Clean
• Most up-to-date copy is a single unique
• cache copy in state Dirty

• Bus owner Clean
• Another Clean copy exists
• Can read without notifying
• other caches

• Bus owner Dirty
• No other cache copies
• Can read or write without
• notifying other caches

• Bus owner Clean
• No other cache copies
• Can read without notifying
• other caches

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State Diagram of Cache Block (Part 1)
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State Diagram of Cache Block (Part 2)
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Comparison with Single WB Cache

• Similarities
• Read hit invisible on bus
• All misses visible on bus
• Differences
• In single WB cache, all misses are serviced by
  MM in three-state protocol, misses are serviced
  either by MM or by unique cache block holding
  only Dirty copy
• In single WB cache, write hit is invisible on
  bus in three-state protocol, write hit of Clean
  block
• invalidates all other Clean blocks by a Bus Write
  Miss (necessary action)

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Correctness of Three-State Protocol

• Problem State transition of FSM is supposed to
  be atomic, but they are not in this protocol,
  because of the bus
• Example CPU read miss in Dirty state
• CPU access to cache detects a miss
• Request bus
• Acquire bus, and change state of cache block
• Evict dirty block to MM
• Put Bus Read Miss on bus
• Receive requested block from MM or another cache
• Release bus, and read from cache block just
  received
• Bus arbitration may cause gap between steps 2 and
  3
• Whole sequence of operations no longer atomic
• App. I.1 argues that protocol will work correctly
  if steps 3-7 are atomic, i.e., bus is not a
  split-transaction bus

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Adding More Bits to Protocols

• Add third bit, called Shared, to Valid and Dirty
  bits
• Get five states (M, O, E, S, I)
• Developed in context of Futurebus, with
  intention of explaining all snoopy protocols, all
  of which use 3, 4, or 5 states

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MESI Protocol

• Four-state, write-invalidate
• Improved version of three-state protocol
• Clean state split into Exclusive and Shared
  states
• Dirty state equivalent to Modified state
• Several slightly different versions of MESI
  protocol
• Will describe version implemented by Futurebus
• PowerPC 601 MESI protocol does not support
  cache-to-cache transfer of blocks

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State Diag. of MESI Cache Block (Part 1)
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State Diag. of MESI Cache Block (Part 2)
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Comparison with Three-State Protocol

• Similarities
• Read hit invisible on bus
• All misses handled the same way
• Differences
• Big improvement in handling write hits
• Write hit in Exclusive state invisible on bus
• Write hit in Shared state involves no block
  transfer, only a control signal

• Exclusive state
• Can be read or written

• Shared state
• Can be read only

• Modified state
• Can be read and written

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Comments on Write-Invalidate Protocols

• Performance
• Processor can lose cache block through
  invalidation by another processor
• Average memory access time goes up, since writes
  to shared blocks take more time (other copies
  have to be invalidated)
• Implementation
• Bus and CPU want to simultaneously access same
  cache
• Either same block or different blocks, but
  conflict nonetheless
• Three possible solutions
• Use a single tag array, and accept structural
  hazards
• Use two separate tag arrays for bus and CPU,
  which must now be kept coherent at all times
• Use a multiported tag array (both Intel Pentium
  and PowerPC 601 use this solution)