Today we continue the discussion on the performance of the Shasta Distributed Shared Memory Protocol. We were discussing protocol optimizations with prefetch and home placement directives. The shared data in the system undergoes state transitions only between three states - invalid, shared and exclusive. By requiring that each load and store is a shared miss check on the data being referenced, Shasta maintains cache coherency. Shared data is split based on address space ranges into block and line size. The operations supported are read, read-exclusive and exclusive/upgrade and a state table is maintained for each line. Each processor maintains a directory and an owner for each line. With these data structures and the tight transitions, coherency is guaranteed via communication.
We now look at detecting migratory sharing patterns. Migratory sharing occurs when data is read and modified by different processors, leading to the migration of the data from one processor to another, By keeping extra information at each directory entry, the protocol detects whether the data in each line exhibits migratory pattern. When a threshold number of times migratory sharing is observed, the is designated for migratory conversion and automatically converted to a read-exclusive request at the directory. This conversion avoids a load miss followed by a store miss to the same line that is typical for migratory shared data. The protocol provides a mechanism to revert a line from migratory conversion. The reply data for a converted read request is cached with a special caching state called exclusive-migratory. The owner processor treats the line as exclusive and subsequently on a store changes the line to the ordinary exclusive state. Breaks in the migratory behavior are easy to detect. If an incoming request from another processor arrives before the owner processor writes to the line while the line is still is exclusive migratory, then a break is observed and a message is sent to the home directory to nullify or revert the migratory conversion for that line.
#codingexercise
Double GetAlternateEvenNumberRangeCount()(Double [] A)
{
if (A == null) return 0;
Return A.AlternateEvenNumberRangeCount();
}
#codingexercise
Double GetAlternateEvenNumberRangeMin()(Double [] A)
{
if (A == null) return 0;
Return A.AlternateEvenNumberRangeMin();
}
The switching to and from migratory conversion state can be stopped for a given line if the line reverts a threshold number of times. This removes needless conversions and avoids continuous switching.
We now look at detecting migratory sharing patterns. Migratory sharing occurs when data is read and modified by different processors, leading to the migration of the data from one processor to another, By keeping extra information at each directory entry, the protocol detects whether the data in each line exhibits migratory pattern. When a threshold number of times migratory sharing is observed, the is designated for migratory conversion and automatically converted to a read-exclusive request at the directory. This conversion avoids a load miss followed by a store miss to the same line that is typical for migratory shared data. The protocol provides a mechanism to revert a line from migratory conversion. The reply data for a converted read request is cached with a special caching state called exclusive-migratory. The owner processor treats the line as exclusive and subsequently on a store changes the line to the ordinary exclusive state. Breaks in the migratory behavior are easy to detect. If an incoming request from another processor arrives before the owner processor writes to the line while the line is still is exclusive migratory, then a break is observed and a message is sent to the home directory to nullify or revert the migratory conversion for that line.
#codingexercise
Double GetAlternateEvenNumberRangeCount()(Double [] A)
{
if (A == null) return 0;
Return A.AlternateEvenNumberRangeCount();
}
#codingexercise
Double GetAlternateEvenNumberRangeMin()(Double [] A)
{
if (A == null) return 0;
Return A.AlternateEvenNumberRangeMin();
}
The switching to and from migratory conversion state can be stopped for a given line if the line reverts a threshold number of times. This removes needless conversions and avoids continuous switching.
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