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Other Compiler Assisted Techniques

The multi-call mechanism can be extended further to include code other than the system calls, error checking, and loops in the multi-call. Specifically, we can extend the basic code-motion transformations to identify a clusterable region, possibly containing arbitrary code, that can then be added to the body of a multi-call. Optimization techniques like dead-code elimination, loop invariant elimination, redundancy elimination, and constant propagation can then be applied to optimize the program. For example, the data transformation code in programs such as compression or multimedia encoding/decoding can be included in the multi-call.

Another avenue of optimization is to replace general purpose code in the kernel by compiler-generated case-specific code in user-space. Examples of such general code are the register saves and restores executed by the kernel before and after each system call. Since the kernel does not know which registers are actually used by the application process, it must save and restore all of them. This can be quite expensive on processors with a large number of registers. However, the compiler has this information, and it can therefore generate specialized user-space code for saving and restoring registers. Simulations using this strategy for a 3 parameter read system call on the Intel StrongARM processor show up to 20% reduction in the number of cycles required to enter the kernel. Other such examples include the general permission checking performed by each system call. Note that both of the above extensions require use of a trusted compiler for safety reasons.

The profiling and compiler-based optimization can also be used to enable controlled information sharing between address spaces. Traditionally, components in different address spaces optimize their internal behavior not knowing what type of interactions will be received from other address spaces. For example, the operating system conserves battery power by switching hardware devices such as the CPU, display, hard disk, and wireless cards into power-saving modes based on a period of inactivity. These policies are generally based on statistical models of application behavior that attempt to predict future (in)activity based on patterns of past activity. Because of their stochastic nature, they can be quite inaccurate for individual applications, and result in significant performance overheads [21,22,23]. However, by carefully exposing some of the components internal state to other address spaces using a translucent boundary API, each address space can optimize its behavior to better match the requirements of other system components, and hence aim for a global optimum. The profiling and compiler techniques can be used to collect and generate the information at the application's translucent boundary API to the kernel. The same approach can be used for compiler assisted scheduling, where an adaptive scheduler can fine-tune the scheduling policy based on the processes running in the system and their requirements. The compiler could place "yield" points within the body of the program to indicate schedulable regions and changes in requirements. Conversely, if the kernel exposes changes in the state of an existing resource, e.g., a reduction in CPU speed to conserve power, the application process may be able to adapt its internal algorithms [9] to degrade the service gracefully while still satisfying user requirements.

Finally, note that the same principles can be applied to any address space crossing, including distributed programs where the ``boundary crossing'' cost involving network communication may have a delay of tens or hundreds of milliseconds. In particular, clustering multiple remote procedure calls (or remote method invocations in distributed object systems such as CORBA and Java RMI) can lead to significant savings [25]. Furthermore, more general code movement techniques such as moving client code to the server or server code to the client when appropriate can also be used [24]. Note that the object migration techniques used in systems such as Emerald [10] have the same goal, but without the systematic support provided by our profiling and compiler techniques.

next up previous
Next: Concluding Remarks Up: Cassyopia: Compiler Assisted System Previous: Experimental results.
Mohan Rajagopalan 2003-06-16