PRAM consistency (pipelined
random access memory
Random-access memory (RAM; ) is a form of computer memory that can be read and changed in any order, typically used to store working Data (computing), data and machine code. A Random access, random-access memory device allows data items to b ...
) also known as ''
FIFO consistency
In classical deductive logic, a consistent theory is one that does not lead to a logical contradiction. The lack of contradiction can be defined in either semantic or syntactic terms. The semantic definition states that a theory is consistent ...
''.
All
processes see
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered, ...
writes from one process in the order they were issued from the process.
Writes from different processes may be seen in a different order on different processes. Only the write order needs to be
consistent
In classical deductive logic, a consistent theory is one that does not lead to a logical contradiction. The lack of contradiction can be defined in either semantic or syntactic terms. The semantic definition states that a theory is consistent i ...
, thus the name ''pipelined''.
PRAM consistency is easy to implement. In effect it says that there are no guarantees about the order in which different processes see writes, except that two or more writes from a single source must arrive in order, as though they were in a pipeline.
P1:W(x)1
P2: R(x)1W(x)2
P3: R(x)1R(x)2
P4: R(x)2R(x)1
Time ---->
Fig: A valid sequence of events for PRAM consistency.
The above sequence is not valid for
Causal consistency Causal consistency is one of the major memory consistency models. In concurrent programming, where concurrent processes are accessing a shared memory, a consistency model restricts which accesses are legal. This is useful for defining correct data ...
because W(x)1 and W(x)2 are causal, so different processes must read it in the same sequence.
References
{{Reflist
Consistency models