7.3 Memory efficiency

As described in Generated code, the c_generic backend (and therefore all derived backends) generates programs that run by executing a sequence of events in an event list. While this form of program execution makes it possible to hoist a significant amount of computation out of the timing loop, it does imply that a program’s memory requirements are proportional to the number of statements that the program executes.

coNCePTuaL’s memory usage can be reduced by taking advantage of repeat counts within statements that support such a construct. The language’s <send_stmt> (see Sending), <receive_stmt> (see Receiving), and <touch_stmt> (see Touching memory) are all examples of statements that accept repeat counts. For other statements and for groups of statements that repeat, the FOR… REPETITIONS statement produces a single EV_REPEAT event followed by a single instance of the events in the loop body. This technique is valid because coNCePTuaL knows a priori that every iteration is identical to every other iteration. In contrast, the more general FOR EACH statement can induce different behavior each iteration based on the value of the loop variable so programs must conservatively instantiate the events in the loop body for every iteration. Consider the following examples:

Least efficient:

‘FOR EACH i IN {1, ..., 1000} TASK 0 TOUCHES A 1 WORD MEMORY REGION’
(1000 EV_TOUCH events on task 0)

More efficient:

‘FOR 1000 REPETITIONS TASK 0 TOUCHES A 1 WORD MEMORY REGION’
(an EV_REPEAT event and an EV_TOUCH event on task 0)

Most efficient:

‘TASK 0 TOUCHES A 1 WORD MEMORY REGION 1000 TIMES’
(one EV_TOUCH event on task 0)

Least efficient:

‘FOR EACH i IN {1, ..., 1000} TASK 0 SENDS A 32 KILOBYTE MESSAGE TO TASK 1’
(1000 EV_SEND events on task 0 and 1000 EV_RECV events on task 1)

More efficient:

‘FOR 1000 REPETITIONS TASK 0 SENDS A 32 KILOBYTE MESSAGE TO TASK 1’
(an EV_REPEAT event and an EV_SEND event on task 0 plus an EV_REPEAT event and an EV_RECV event on task 1)

Most efficient:

‘TASK 0 SENDS 1000 32 KILOBYTE MESSAGES TO TASK 1’
(currently the same as the above although a future release of coNCePTuaL may reduce this to a single EV_SEND event on task 0 and a single EV_RECV event on task 1)