Spring 2006 1 EE 437 Lillevik 437s06-l21 University of Portland School of Engineering Advanced Computer Architecture Lecture 21 MSP shared cached MSI protocol

Spring 2006

Lillevik 437s06-l21 1University of Portland School of Engineering

EE 437

Advanced ComputerArchitecture

Lecture 21

MSP shared cachedMSI protocol

MESI protocol

Spring 2006


EE 437

Symmetric multiprocessor (SMP)

Memory

...CPU

Memory Controller

CPU

I/O Hub/Bridge

Key Board

Mouse

Monitor

BIOS

EtherNet

PowerSupply

CoolingFan

One address space, uniform access time

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A simpler model

Each processor has a local cache, one main memory

P1 transactions Pn transactions

Bus transactions

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Coherency through snooping

Controllers monitor bus to manage local cache

Single shared memory

Local controller

Local controller

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Coherency requirements

1. Memory operations occur in the order they were issued

2. All reads return the most current value

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Cache coherency solution

• Monitor bus to see when things change

• Must maintain the “state” of each cache line– Modifed (as in write-back)– Others

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Two solutions

• Both write-back invalidation: snooped write (another writer) invalidates local copy

• MSI protocol– Three states– Simpler, uses bus a bit more

• MESI protocol: most popular– Four states– Slightly more complex, uses bus less

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WB invalidation protocol

• Cache states– Modified: dirty, memory inconsistent, local cache has

only valid copy, only one CPU in this state

– Shared: clean, one or more copies, memory consistent

– Invalid: local data is not current, stale

• State transitions– Determined by local controller– States may vary across caches

MSI

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Processor transactions

• Read (PrRd): read instruction

• Write (PrWr): write instruction

• Misses: if modified, must write back data to memory

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Bus transactions

• Read (BusRd): memory read

• Write (BusWr): memory write

• Read Exclusive (BusRdX)– Used to request an exclusive copy of data– Generated by a PrWr if data Invalid or Shared– Data returned may be ignored

• Flush: Modified cache performs a WB, resolves inconsistency

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Processor hits and misses

Action Processor BusRead hit PrRd none

Write hit PrWr none

Read miss PrRd BusRd

Write miss PrWr BusRdX

Invalidates other local copies

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Bus snooping

Bus ActionBusRd Another processor wants to read line

BusRdX Another processor wants to write line

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MSI protocol

Bus/snoop generatedProcessor generated

Controller

Observes/Actionwrite miss

read miss

write hitread hit

read hit

Another reader

Another writer

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Find the coherency?

Action P1 State P2 State P3 State BusData source

P1 read u S I I BusRd mem

P3 read uS I S BusRd

C1 or mem

P3 writes uI I M

BusRdX/Flush

P3

P1 read u S I S BusRd C3

P2 read u S S S BusRd C3

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MSI performance

• Consider single read-write– Read results in Shared state, BusRd– Write results in Modified state, BusRdX

• Unfortunate Result– Two bus actions– Second, BusRdX, not necessary if line is not

shared with other processors– Suggest a new state, exclusive-clean

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WB invalidation protocol

• Improved performance vs. MSI

• States– Modified: dirty, memory inconsistent, local cache has

only valid copy

– Exclusive (clean): local cache owns, but not written, one CPU in this state

– Shared: clean, one or more copies, memory consistent

– Invalid: data is not current, stale

MESI

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Processor transactions

• Read (PrRd): read instruction

• Write (PrWr): write instruction

• Misses: if modified, must write back data to memory

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Bus transactions

• Read (BusRd): memory read

• Write (BusWr): memory write

• Read Exclusive (BusRdX)• Flush: only one need provide WB data

• Shared (S): new signal– Determines if data already shared– Used with BusRd(S) or BusRd(S#)

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MESI protocol

Controller

Observes/Action


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MESI protocol

Controller

Observes/Action


write hitread hit

read miss

one cache does WB

Another writerAnother readerno bus cycle

write miss

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Find the coherency?


P1 read u E I I BusRd(S#) Mem

P3 read uS I S BusRd(S)

C1 or mem

P3 writes uI I M BusRdX/Flush

C3 or mem

P1 read u S I S BusRd(S)

P2 read u S S S

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Spring 2006


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Find the coherency?


P1 read u S I I BusRd Memory

P3 read u S I S BusRd Memory

P3 writes u I I M BusRdX P3

P1 read uS I S

BusRd/Flush

P3 cache

P2 read u S S S BusRd Memory

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Find the coherency?

Action P1 State P2 StateP3 State

BusData source

P1 read u E I I BusRd(S#) Memory

P3 read u S I S BusRd(S)/Flush P1 cache

P3 writes u I I M BusRdX/Flush’ P3

P1 read u S I S BusRd(S)/Flush P3 cache

P2 read uS S S BusRd(S)/Flush’

P1 or P3 cache

Documents

Spring 2006 1 EE 437 Lillevik 437s06-l21 University of Portland School of Engineering Advanced Computer Architecture Lecture 21 MSP shared cached MSI protocol