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Time in SST

Time in SST is represented by a combination of two values: a time base and a count. The time base represents a quantum of time and is encapsulated in the TimeConverter object. The count is the number of these quanta stored as an unsigned 64-bit integer (typedef'd as SimTime t). For example, if the time base is 2ps and the count is 1000, the represented time is 2ns (2ps * 1000). This tuple is for both absolute and relative time. Absolute time is the current simulated time, whereas relative time represents delays, usually based on the current absolute time. There are also two views of time, core and local.

Core time​

Core time is the time as viewed by the SST-Core and absolute core time is the core time that has passed since the beginning of the simulation. We simply refer to absolute core time as current time. The core time base is the minimum time interval that is representable in the simulator and defaults to 1ps. It is stored as a UnitAlgebra object in SST-Core and can be retrieved in components and subcomponents through the call getCoreTimeBase(). The current time is tracked with a SimTime_t in the core and can be found through the call getCurrentSimCycle(). Each Simulation object (which manages all the elements of the simulation for a given partition) keeps track of its own current time. SST uses a conservative lookahead mechanism to synchronize across Simulation objects to ensure that events never occur out of order, even when crossing partitions. The current time is determined by the delivery time of the activity at the head of the core event queue (called the TimeVortex) that is currently being delivered/executed.

Local time​

Local time is an element’s local view of time. The concept of local time allows an element (component or subcomponent) to be written without having to specifically know what absolute timescales it is operating at. This allows an element to deal, for example, with clock cycles instead of absolute time. An element need not care what the actual clock frequency is, just that things occur in a certain number of clock cycles. The mechanisms used for local time ensure that the timings relative to all other elements are consistent.

The local time base is captured using a TimeConverter object. The TimeConverter stores a factor that represents the number of core time base intervals in the time base represented by the Time- Converter. For example, a TimeConverter that represents 1ns would have a factor of 1000 if the core time base was 1ps (1ns / 1ps). The TimeConverter object has two main functions: convertToCoreTime and convertFromCoreTime. Elements can also query the TimeConverter’s factor using getFactor().

The TimeConverter object​

The TimeConverter object is used to manage the conversion of time between global time and the various local views of time.

The convertToCoreTime() function converts the count from the local view of time to the count in the global view. For example, if the local count is 250 and the local timebase is 1ns, the function would return 250,000 if the core time base was 1 ps (250ns * 1000 = 250,000ps). This function is used in Links when adding extra latency (specified in the local view of time) on an event send.

Similarly, the convertFromCoreTime function converts a count from the global view of time to a count in the local view. This function are used when getting the elapsed simulation time in the local view. For example, if a Component has a clock frequency of 2GHz, the local time base would be 500ps (period of a 2GHz clock). If getCurrentSimTime() was called after 2ms of simulation, the call would return 4,000 (2ms / 500ps).

Default time bases​

Components, SubComponents and Links have the concept of a default time base. ComponentExtensions share the default time base with their parent Component. This is the local view of time used by the various member functions when a specific TimeConverter object is not supplied. There are various ways to set the default time base. The most direct is to simply call setDefaultTimeBase() on the object. There are also other implicit ways that the default time base can be set, though a call to setDefaultTimeBase() will always override the implicit methods. A call to registerClock() will, by default, set the element’s default time base to be the clock period of the requested clock. It will also set the default time bases of all the Links in the element to be the same (both links that have already been configured and those that have not). In the case where a default time base has already been explicitly or implicitly set, the time base will not be changed to be the clock period. In general, implicitly setting a default will not overwrite an existing default (implicitly or explicitly set) and explicitly setting a default will unconditionally overwrite any existing default.