1 Multi-core processors 12/1/09

2 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.

3 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.

4 Multithreading Fine-grained multithreading Course-grained multithreading Simultaneous multithreading (SMT)

5 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.

6 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.

7 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.

8 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 B Thread A Thread B