[CR] Triple buffer

[CharlesB]

Name: Triple buffer

Submitter: CharlesB

Submitted: 21 Oct 2014

Category: *Uncertified*

LabVIEW Version: 2011

License Type: BSD (Most common)

(initial discussion, with other implementations here)

 

In the need for displaying large images at a high performance, I wanted to use triple buffering in my program. This type of acquisition allows to acquire large data in buffers, and have it used without copying images back and forth between producer and consumer.

 

This way consumer thread doesn't wait if a buffer is ready, and producer works at max speed because it never waits or copy any data.

If the consumer makes the request when a buffer is ready, it is atomically turned into a "lock" state. If a buffer isn't ready, it waits for it, atomically lock it when it is ready.

 

This class allows to have a producer loop running at its own rate, independently from the consumer. It is useful in the case of a fast producer faster than the consumer, where the consumer doesn't need to process all the data (like a display).

 

How to use

Buffers are provided at initialization, through refnums. They can be DVRs, or IMAQ refnums, or any pointer to some memory area.

Once initialized, consumer gets the refnums with "get latest or wait". The refnum given is locked and guaranteed to stay uncorrupted from the producer loop. If new data has been produced between two consumer calls, the call doesn't wait for new data, and returns the latest one. If not, it waits for the next data.

At each producer iteration, producer starts with a "reserve data", which returns the refnum in which to fill. Once data is ready, it calls "reserved data is ready". These two calls never wait, so producer is always running at a fastest pace.

Implementation details

A condition variable is shared between producer and consumer. This variable is a cluster holding indexes "locked", "grabbing", and "ready". The condition variable has a mechanism that allows to acquire mutex access to the cluster, and atomically release it and wait. When the variable is signaled by the producer, the mutex is re-acquired by the consumer. This guarantees that the consumer that the variable isn't accessed by producer between end of consumer wait and lock by consumer.

Reference for CV implementation: "Implementing Condition Variables with Semaphores ", Andrew D. Birrell, Microsoft Research

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