Prefetch Thread

In this subsection we describe the basic operations in the prefetch thread and its interaction with the computation thread and the prefetch library.

Figure 2: Computation thread (left) and prefetch thread (right). The main function without the lines marked by ## is the original application code. This code is modified to add a prefetch thread represented by the prefetch function. The lines in the prefetch function that are not marked with $$ are extracted from the original main function. Other additional functions are added for the two threads to communicate with each other. Notice that the computation part of the main function does not appear in the prefetch function

Figure 2 shows how AASFP's translator converts an example application into a computation and a prefetch thread. The programming interface provided by the prefetch library includes the following four calls:

1. create_prefetch_thread (prefetch_function): This function allows an application to fork a prefetch thread, and to execute the prefetch_function. The prefetch_function is passed as the input argument, as shown in line 1 of Figure 2.

2. prefetch_XXX(): These are a set of functions that the prefetch thread can use to specify prefetch calls. The prefetch calls replace the original I/O calls and use almost the same syntax. For example, for the I/O call read(stream, ptr, size), AASFP provides the prefetch call prefetch_read(stream, size). The parameter ptr for the data is not needed, since the prefetch call only generates the target data's starting address but never actually performs the real I/O. Lines P4 and P5 in Figure 2 represent the prefetch calls corresponding to the I/O calls at lines C5 and C6 of the main function respectively.

3. inform_open(file_pointer), inform_close(file_pointer): These two functions are used by the prefetch thread to inform the prefetch library that some file is opened or closed. Such notification is necessary so that the prefetch library can maintain a file table consisting of {file pointer, current offset} for those files accessed by the prefetch thread. Lines P2 and P6 in Figure 2 represent the inform calls corresponding to the open and close system calls in the computation thread.

4. synchronize(synchronization_point, type): This function synchronizes the two threads. The argument type can be signal or wait. Synchronization is discussed in detail in the next subsection.