Distributed Shared Memory
Presentation by
Deepthi Reddy
What is DSM?
DSM is an abstraction used for sharing data between computers
that do not share physical memory.
Differences between DSM and Message Passing
DSM
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Involved shared memory directly no
marshalling necessary.
Can alter erroneously causing one
another to fail.
Synchronization is achieved through
normal constructs shared memory
programming such as locks and
semaphores.
Process can leave data in an
agreed memory location for the
other to examine when it runs.
Underlying run-time ensures the
transparency.
Message Passing
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Variables have to be marshalled
and unmarshalled.
Protected one another
by
having private address space.
Synchronization is achieved
through
message
passing
primitives such as lock server.
Process communicating must
execute at the same time.
All remote
explicit.
data
access
is
Implementation Approaches
• Hardware
• Paged Virtual Memory
• Middleware
Design and Implementation Issues
Structure of data held in DSM:
• Syncronization model
• DSM consistency model
• Update options
• Granularity
• Thrashing
Structure
DSM is a replication system in which each application process is
presented some abstraction of a collection of objects (but in this
case collection looks more or less like memory). Different
approaches to DSM vary in what they consider to be and
‘object’ and how the objects are addressed.
Byte-Oriented:
Accessed as ordinary virtual memory – a
contiguous array of bytes. Operations allowed on Objects are :
read or LOAD and write or Store.
Example: Mether system, IVY.
Object-oriented: Structured as a collection of language-level
objects with higher-level semantics such as stacks and
dictionaries. Example: Orca
Immutable data: Viewed as collection of immutable data items that
process can read, add and remove form.
Examples: Agora, Linda, Tspaces and JavaSpaces.
Synchronization model
If a and b are 2 variables stored in DSM, the a constraint might be
that a=b always. If 2 or more process execute the following
code:
a:=a+1;
b:=b+1;
then an inconsistency arise.
A Distributed synchronization need to be provided, which includes
familiar constructs such as locks and semaphores.
Consistency model
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The local replica manager is implemented by a combination of
middleware and the kernel. Middleware performs the majority of
DSM processing.
Amortize communication costs by spreading them over multiple
updates.
Memory consistency: Specifies the consistency guarantees
that a DSM makes about the values that process read from
objects
Example:
process 1
process 2
br:=b;
a:=a+1;
ar:=a;
b:=b+1;
if(ar>=br)
print(“OK”);
Consistency cotd..
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Atomic Consistency (Linearizability): A replicated shared object is said
to be linearizable if for any execution there is some interleaving of the
series of operations issued by all the clients that satisfies the
following two criteria:
 The interleaved sequence of operations meets the
specification of a
correct copy of the objects
 The order of operations in the interleaving is consistent with
the real
times at which the operations occurred in the actual
execution.
Sequential Consistency: Linearizability is too strict for most practical
purposes. This could be implemented by using a single server to hold
all the shared data by making all process perform reads or writes by
sending requests to the server.
Coherence: Is an example of weaker form of consistency.
Weak Consistency: Dubois, et al define this
model, called weak consistency, by saying that it
has three properties:
 Accesses to synchronization
variables are sequentially consistent.
 No access to a synchronization
variable is allowed to be performed
until all previous writes have completed
everywhere.
 No data access ( read or write )
is allowed to be performed until all
previous
accesses to synchronization variables have been
performed.
Update Options
Two implementation Choices:
• Write-update : The updates made by a process are made locally and multicast
to all other replica managers possessing a copy of the data item, which
immediately modify the data read by local processes. Processes read the local
copies of data items, without the need for communication. In addition to
allowing multiple readers, several processes may write the same data item at
the same time; this is known as multiple-reader-multiple-writer sharing.
• Write-invalidate: This protocol is commonly implemented in the form of
multiple-reader-single-writer sharing. At any time, a data item may either
be:
 accessed in read-only mode by one or more processes
 read and written by a single process
Granularity
Granularity refers to the size of the shared memory unit. A page
size of that is a multiple of the size provided by the underlying
hardware or the memory management system allows for the
integration of DSM and the memory management systems. By
integrating DSM with the underlying memory management
system, a DSM system can take advantage of the built in
protection mechanism to detect incoherent memory references,
and use built in fault handlers to prevent and recover from
inappropriate references.
Thrashing
• A potential problem with Write-invalidate protocols
• Occur where the DSM run-time spends on inordinate
amount of time invalidating and transferring shared data
compared with the time spent by application process doing
useful work.
Sequential Consistency and IVY
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Consistency model: sequential consistency
Granularity
: virtual memory pages
Data location : distributed directory
Modifications : detected via page faults
Synchronization : not integrated (locks)
The system model: A collection of process shares a segment of DSM
which is mapped to the same range of addresses in each process.
The problem of write update: Used with page-based implementations,
where the page is left with write permissions after an initial page fault
and several writes are allowed to occur before the updated page is
propagated.
The write-invalidate:
 When one or more process
are reading the page, they have read-only
permissions.
 When one process is
updating a page, all other process have no
access
permissions to the page.
 A process with most up-todate version of a page p is designated as its
owner – owner(p).
 The set of processes that have
a copy of a page p is called its copy set
–
copyset(p)
IVY contd..
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Invalidation protocols:
 Centralized manager algorithm: A single server called manager stores the
location of owner(p) for every page p and the set copyset(p) is stored at
owner(p)
 Using Multicast: When a process faults, it multicasts its page request to all
the other process and only the owner of the page replies.
Dynamic distributed manager algorithm:
 divides the overhead of locating pages between those computers
that
access them.
 Every process keeps, for every page p, a hint as to the page’s current owner
– the probable owner of p or probOwner(p).
 Hints are updates and requests are forwarded.
•Thrashing: Mirage extends the coherence protocol of the IVY
system to control thrashing. memory page is transferred to a
processor, the processor is allowed to keep the page for a duration
d. If a processor receives an invalidation message or has to
relinquish the page to some other writer, it checks to see whether
d has expired. If not, the processor informs the control manager
of the amount of time it must wait before the processor can honor
the request. The control manager waits until d expires and then
requests the page again.
Release Consistency and Munin
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Consistency model: release consistency ("eager") - note: multiple COHERENCY
PROTOCOLS
Granularity : cross between variables and virtual memory pages
Data location : distributed directory
Modifications : detected via page faults
Synchronization : integrated (locks, barriers, and condition variables)
Memory Access: Two types –
Competing access: 2 types
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Synchronization access
 acquire
 release
Non-synchronization access
non-competing access
Release Consistency:
 All modifications to
shared data made within a critical
section must be made visible to
other processors that might access
the data prior to completion of the
critical section.
 A shared memory system is release
consistent if it obeys the following rules
1. Before any ordinary read or
write access to shared data may be performed,
all
previous acquires done by the process
must have completed successfully.
2. Before a release operation
may be performed, all previous ordinary reads
and
writes done by the process must have
been performed.
3. Acquire and release
operations must be performed in "processor
consistency" order.
Munin
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Implementation of release consistency:
 Munin sends upadte or invalidation information as soon as lock is released.
 programmers can take annotations that associate a lock with particular data
items.
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Sharing Annotations:
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Read-only
Migratory
Write shared
Producer-consumer
Reduction
Result
Conventional
Other Consistency models
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Casual consistency
Processor consistency
Pipelined RAM
Entry Consistency
Scope consistency
Weak Consistency