Counted loops

Counted loops

The simplest form of iteration in coNCePTuaL repeats a <simple_stmt> a given number of times. In its most basic form the syntax is simply “ FOR <expr> REPETITIONS <simple_stmt>”. As could be expected, the <expr> term (see Arithmetic expressions) specifies the number of repetitions to perform. Hence, the following <simple_stmt> outputs the phrase “I will not talk in class” 100 times:

FOR 100 REPETITIONS ALL TASKS OUTPUT "I will not talk in class."

FOR… REPETITIONS can optionally specify a number of “warmup” repetitions to perform in addition to the base number of repetitions. The syntax is “ FOR <expr> REPETITIONS PLUS <expr> WARMUP REPETITIONS <simple_stmt>”. During warmup repetitions, the OUTPUTS statement (see Writing to standard output), the LOGS statement (see Writing to a log file), and the COMPUTES AGGREGATES statement (see Computing aggregates) are all suppressed (i.e., they have no effect) and none of the special variables predeclared by coNCePTuaL (see Predeclared variables) are updated. Many benchmarks synchronize all tasks after performing a set of warmup repetitions. This behavior can be expressed conveniently as part of a coNCePTuaL FOR loop by appending the AND A SYNCHRONIZATION clause:

FOR 1000 REPETITIONS PLUS 3 WARMUP REPETITIONS AND A SYNCHRONIZATION
TASK 0 MULTICASTS A 1 MEGABYTE MESSAGE TO ALL OTHER TASKS

coNCePTuaL also provides a separate SYNCHRONIZES statement. This is described in Synchronizing.

The importance of performing warmup repetitions is that many communication layers give atypically poor performance on the first few transmissions. This may be because the messages miss in the cache; because the communication layer needs to establish connections between pairs of communicating tasks; or, because the operating system needs to “register” message buffers with the network interface. Regardless of the reason, specifying warmup repetitions helps make performance measurements less variable.