PNTM demonstrates a new concurrent programming model, providing explicit concurrency among cooperative transactions with correctness. It integrates a special Petri net and transactional memory, and improves the performance of transactions by decreasing the rate of conflicts. The GUI part of PNTM environment is based on PNK [1], the compiler is a modified GJC [2], and the runtime is based on DSTM2 [3]. Editor The IDE provides a simple GUI with a code editor and a net editor. The editor for Petri net system is modified from PNK. All elements can have extra fields and be edited visually. The extra fields such as resources in places and code in transitions all correspond special variables and functions in the code, as in Table 1.
After checking the correctness at the level of Petri nets, the editor will append a piece of code for building Petri Net simulator at runtime. Hence the simulator is created and initialized at runtime. The runtime provide 5 APIs as below: W. Wu, Y. Zhang, S. Wang, Y. Dong: Petri Nets and Transactional Memory
2. AddPlace(place, resource) adds place. 3. AddArc(source, target, inscription) adds arc. 4. Start() starts simulation. 5. Join() terminates simulation.
The runtime is a Petri nets VM using DSTM2. The first 3 API can build up a simulator of a Petri net and the last 2 API control the simulator. The simulator allocate a DSTM2 Thread for every transition in the net. The Threads are all waiting for notification. Only notified Thread can consume resources, call function and produce new resources. A global lock is used to protect all resources in order to make manipulation on resources atomic and prevent deadlock. Some optimizations accelerate the check-and-consume process. Hence the overhead is relatively low. When the transition consumes resources and is ready to fire, corresponding petrinet function is called using reflection. If all global variables protected by STM successfully commit, the transition will produce new resources. Otherwise the transition will revert all state and return consumed resources.
Compilation At the early stage of compilation, the compiler will recognize and mark new keyword petrinet, resource, global. The global variables need to transform to instances of pre-defined interfaces in order to meet requirement of DSTM2’s APIs. For example, equivalent code to transformed “ global int a;” is shown in Table 2. AInt refers to “ Atomic Integer”.
After AST is built, the compiler will check the semantic correctness. The functions with petrinet modifier and Petri nets themselves should meet constraints below: 1. petrinet function must have function body. 2. petrinet function should have no parameter. 3. Only resource, global and local variables can be used in a petrinet function. 4. Only petrinet function can be called in transition. 5. Resources in incoming arcs of a transition must be a superset of all resource variable used in corresponding petrinet function.
PNSE’11 – Petri Nets and Software Engineering
In addition, all references of global variables and all left-values consisted of global variables must be transformed to proper getter and setter in order to meet requirement of DSTM2’s API. Equivalent code to getter and setter is shown in Table 3. transform type original code equivalent code initializer global int a = 0; ...;a.setValue(0); reference x = a + 1; x = a.getValue() + 1; left-value a = x + 1; a.setValue(x + 1); self-operation a++; a.setValue(a.getValue() + 1);
Future Work
We are making efforts to some basic performance evaluation.