Multiprocessors
• ELEC 6200: Computer Architecture and Design
• Instructor : Agrawal
• Name: Nam
Why we have to use multiprocessors?
• Definition: Multiprocessors are parallel
processors with a single shared address.
• Multiprocessors have the highest performance
, it is higher than the fastest uni-processor.
• Multiprocessors have a lot of more effective
applications than a uni-processor: search
engines, web servers, databases….
• Multitasking within an application
Instruction and data stream of
Multiprocessors.
• SIMD (single instruction, multiple data): every
unit will be executing the same instruction, each
execution unit has its own address registers, and
so each unit can have different data addresses.
• MIMD ( multiple instructions, multiple data):
Machines using MIMD have a number of
processors that function asynchronously and
independently. At any time, different processors
may be executing different instructions on
different pieces of data
• MISD…
How multiprocessors work?
• How do parallel processors share data?
• How do parallel processors coordinate?
• How many processors?
1. Multiprocessors communicate through
shared variables in memory , all processors
can access any memory location via loads
and stores
2. As processors operating in parallel, they
normally share data. Only one processor at a
time can acquire the lock and other processors
interested in shared data have to wait until the
original processor unlocks the variable so called
Lock approach.
Or processors can communicate by sending and
receiving message.
• Parallel processing program: to refer to a single
program that runs on multiple processors
simultaneously
• it is difficult to write parallel processing
programs, the programmer must know a good
deal about the hardware.
Multiprocessors connected by a single
bus
Processors
Processors
Processors
Cashes
Cashes
Cashes
Single Bus
Memory
I/O
• Each microprocessor is much smaller than a
multichip processor, so more processors can be
placed on a bus.
• Caches can lower bus traffic.
• Mechanisms were invented to keep caches and
memory consistent for multiprocessors
Traffic per processor and the bus bandwidth
determine the useful number of processors in
such a multiprocessor.
Single bus designs are attractive but limited: high
bandwidth, low latency, and long length,
Limit to the bandwidth of a single memory
module as well----> limited number of processors
Multiprocessors connected by a
network
Processors
Processors
Processors
Cashes
Cashes
Cashes
Memory
Memory
Memory
Network
• In machines without a single global address
space, communication is explicit ;the
programmer or the compiler must send
messages to ship data to another node and
must receive messages to accept data from
another node.
• Send and receive also have the advantage of
making it easier for the programmer to
optimize communication: it’s simpler to
overlap computation with communication by
using explicit sends and receives rather than
with implicit loads and stores.
New tendency
• An alternative to multiple microprocessors
sharing an interconnect is bringing the processors
inside the chip. In such designs, the processors
typically share some of the caches and the
external memory interface.
• Advantage: amortize the instruction accesses, the
latencies associated with chip-to-chip
communication disappear ,Shared data structures
are also much less of a problem
• Challenge lies in software. What architecture
makes software easier.
Thank You