Multi-core processors

1
Multi-core processors

• 12/1/09

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Multiprocessors inside a single chip

• It is now possible to implement multiple
  processors (cores) inside a single chip.
• The number of cores in multicore processors is
  expected to double every two years.
• Manycore processors will become the norm.
• Such processors typically share some of the
  caches and external memory interfaces.
• Eliminates latencies associated with chip-to-chip
  communications.

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Applications

• In some multi-core applications processors are
  running the same code.
• SIMD
• Multithreading
• State of tread must be saved when a processor
  switches to a new thread.
• Requires separate copies of registers.

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Multithreading

• Fine-grained multithreading
• Course-grained multithreading
• Simultaneous multithreading (SMT)

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Fine-grained multithreading

• Switches between threads on each clock.
• Interleaved execution of threads.
• Must be able to switch threads very quickly.
• On each clock
• Stalled threads are skipped
• Primary disadvantage is that execution of any
  particular thread is slowed since it is
  interleaved with other threads.

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Course-grained multithreading

• Invented as an alternative to fine-grain
  multithreading
• Switches threads only on costly stall.
• Such as level 2 cache miss
• Relieves need to switch threads on each clock.
• Less likely to slow execution of an individual
  thread.
• Disadvantage
• Throughput suffers because of inability to switch
  threads on shorter stalls.
• Pipeline must be empted (or frozen) and started
  up when a thread is switched.

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Simultaneous multithreading

• A variation of multithreading
• Uses a multiple-issue, dynamically scheduled
  processor to exploit thread-level parallelism at
  the same time exploiting instruction-level
  parallelism.
• Motivated by the fact that modern multiple-issue
  processors often have more functional unit
  parallelism than can be used by a single thread.
• Register renaming and dynamic scheduling enable
  multiple instructions from independent treads
  without regard to the dependencies among them.
• Dependencies are resolved by the dynamic
  scheduling capability.

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Example Four threads executing independently on
a superscalar processor with no multithreading
support.
9
Example The same four threads executing together
on the a processor for the three types of
multithreading.
Thread A
Thread A
Thread B
Thread A
Thread B
Thread B