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Quasistatic Parameters and qinfluence Statements

A quasistatic parameter (hereafter referred to as qint) is an constant integer used by the program in such a way that the initialization value of the constant integer may be selected from the any of the N values specified at the definition of the parameter, without changing the functional behavior of the program; it is similar to the notion of dynamically-valued constants discussed in [].

A qinfluence statement is used to indicate to the clever compiler what parts of the program should have their performance associated with a given qint parameter.

Again, I present examples to demonstrate the intended semantics of the construct.

Matrix Multiplication

The code in Figure illustrates that when matrices to be multiplied are too large to work on at once, they can be divided up into sub-matrices and multiplied, and then the results composed to get the final answer; the possible values for blocking size are 1024, 2048, and 4096 in this case. Figure is semantically equivalent to Figure , and demonstrates the use of the `` :'' notation for specifying ranges of values. Exactly which blocking size results in the best performance depends heavily on both static machine characteristics like cache size/associativity, and dynamic characteristics like cache competition introduced by other processes under multitasking operating systems.

The effect of using a qint parameter can be achieved by writing N pieces of alternative code (in each of which the quasistatic parameter symbol is substituted with a literal from the parameter's set of possible values) chained together in a qif statement; however, the net effect achieved is desired commonly enough to merit special treatment. Furthermore, encoding the alternatives using a qint parameter instead of N quasistatic variables in long qif statements potentially allows a clever compiler to more efficiently search the version space by guessing that to a first approximation, performance of a particular version of a program can be estimated by interpolation on the differences in qint parameter values between it and program versions which have already been tried. E.g., if the program versions and have already been tried, then a clever compiler might guess that to a first approximation, performance of the version of the program falls somewhere between the performance of those two versions.

Figure: qint example: matrix multiplication.

Figure: qint example (alternative syntax).

As another example, in Figure , the programmer has access to remote computation services but is not certain whether their performance is good compared to just performing computation locally --- the servers are likely to be shared with other users, for example, so if other users are often making heavy demands, the servers will be relatively unresponsive. Using qint parameters allows dynamically balancing between client and server processing.

The access scope of a qint parameter should be the same as that of a C integer variable definition appearing in the same textual position in the source code file. By default, if there are no qinfluence statements in the program which reference a qint, for profiling feedback purposes, the entire part of the program where the qint is lexically visible is assumed to be influenced by which particular value which the clever compiler assigns to the qint. Note that this default can be simultaneously both too broad and too narrow in scope. The value of a qint parameter may not affect program performance for all the code within its lexical scope, and including more code than necessary will tend to swamp out any performance effects of changing the value of the qint parameter; conversely, mutations allow the qint value to influence code performance outside its lexical scope.

Where the programmer believes this default would be substantially inaccurate, the qinfluence construct allows him or her to indicate explicitly what parts of the code he or she believes will have their performance influenced. Figure demonstrates usage of the qinfluence construct; the programmer has provided more information to the compiler than in Figure . Note that no qinfluence(USE_REMOTE_COMPUTATION_SERVER_LIMIT) statement is necessary in this example since the entire lexical scope of that qint parameter is the correct scope for the clever compiler to be looking at profiling feedback.

Figure: qint example ( qinfluence syntax).

Next: Justification Up: Quasistatic Constructs Previous: Quasistatic Variables and

Reinventing Computing, MIT AI Lab. Author: pshuang@ai.mit.edu (Ping Huang)