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Distributed Shared Memory
1.
2. Basic Concepts Of DSM:
A DSM system provides a logical abstraction
of shared memory which is built using a set
of interconnected nodes having physically
distributed memories.
4. Advantages of DSM
Simple abstraction
Improved portability of distributed
application programs
Provides better performance in some
applications
Large memory space at no extra cost
Better than message passing systems
5. Hardware architectures
On chip memory
Bus based multiprocessor
Ring based multiprocessor
Switched multiprocessor
6. On chip memory
In this CPU portion of the chip has a address and data lines that directly
connect to the memory portion.
Such chips are used in cars appliances and even toys
7. Bus based multiprocessor
All CPUs connected to one bus (backplane)
Memory and peripherals are accessed via shared bus. System looks the
same from any processor.
11. NUMA Multiprocessor
Non-uniform memory access (NUMA) is a
computer memory design used multiprocessing,
where the memory access time depends on the
memory location relative to the processor.
Under NUMA, a processor can access its own
local memory faster than non-local memory.
The benefits of NUMA are limited to particular
workloads, notably on servers where the data is
often associated strongly with certain tasks or
users.
12. UMA Multiprocessor
Uniform memory access (UMA) is a shared memory
architecture used in parallel computers. All theprocessors
in the UMA model share the physical memoryuniformly.
In a UMA architecture, access time to a memory location
is independent of which processor makes the request or
which memory chip contains the transferred data.
Uniform memory access computer architectures are often
contrasted with non-uniform memory access (NUMA)
architectures.
In the UMA architecture, each processor may use a
private cache. Peripherals are also shared in some
fashion.
The UMA model is suitable for general purpose and time
sharing applications by multiple users. It can be used to
speed up the execution of a single large program in time-
critical applications.
13.
14. DSM design issues
Granularity of sharing
Structure of data
Consistency models
Coherence protocols
16. Thrashing:
False sharing
Techniques to reduce
thrashing:
Application controlled lock
Pin the block to a node for
specific time
Customize algorithm to
shared data usage pattern
20. Sequential consistency
All processors in the system observe the same
ordering of reads and writes which are issued in
sequence by the individual processors
21. Causal consistency
Weakening of sequential consistency for
better concurrency
Causally related operation is the one which
has influenced the other operation
22. PRAM consistency
Pipelined Random Access Memory consistency
Write operations performed by different processes may
be seen by different processes in different orders
All write operations of a single process are pipelined
Simple, easy to implement and has good performance.
23. Processor consistency
Adheres to the PRAM consistency
Constraint on memory coherence
Order in which the memory operations
are seen by two processors need not be
identical, but the order of writes issued
by each processor must be preserved
24. Weak consistency
Use a special variable called the
synchronization variable.
Very difficult to show and keep track of the changes at
time to time.
25. Properties of the weak
consistency model:
Access to synchronization variables is
sequentially consistent.
Only when all previous writes are
completed everywhere, access to
synchronizations variable is allowed.
Until all previous accesses to
synchronization variables are performed,
no read write data access operations will
be allowed.
26. Entry consistency
• Use acquire and release at the start and
end of each critical section, respectively.
• Each ordinary shared variable is
associated with some synchronization
variable such as a lock or barrier.
• Entry consistency (EC) is similar to LRC
but more relaxed; shared data is explicitly
associated with synchronization primitives
and is made consistent when such an
operation is performed
27. Scope consistency
A scope is a limited view of memory with
respect to which memory references are
performed