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==================================
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Performance sensitive environments
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==================================
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Overview
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========
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This document describes suggested feature options for optimizing Duktape
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performance for performance sensitive environments.
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Compiler optimization level
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===========================
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Size optimization using ``-Os`` is a good default when performance is
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not critical. However, it's not ideal when performance matters for
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several reasons:
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* Although ``-Os`` optimized code performs reasonably well, even
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``-O2`` will yield significantly better results.
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* Code performance with ``-Os`` can vary a great deal even when source
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code changes are innocent. It's not uncommon for some performance
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test result to change +/- 10-30% with unrelated changes. Presumably
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this is caused by changes in code alignment etc.
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Because of this, ``-Os`` is definitely a bad idea for measuring
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performance.
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* Overall suggestion is to use ``-O2`` and try ``-O3`` if the end result
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is better. Note that ``-O3`` is not always better because the code is
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larger and may not fit in caches as well as with ``-O2``.
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Suggested feature options
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=========================
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* On some platforms ``setjmp/longjmp`` store the signal mask and may be
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much slower than alternative like ``_setjmp/_longjmp`` or
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``sigsetjmp/siglongjmp``. Use the long control transfer options to use
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an alternative:
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- ``DUK_OPT_UNDERSCORE_SETJMP``
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- ``DUK_OPT_SIGSETJMP``
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- On some platforms (e.g. OSX/iPhone) Duktape will automatically use
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a faster alternative.
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* Consider enabling "fastints":
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- ``DUK_OPT_FASTINT``
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Fastints are often useful on platforms with soft floats, but they can also
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speed up execution on some hard float platforms (even on x64). The benefit
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(or penalty) depends on the kind of Ecmascript code executed, e.g. code
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heavy on integer loops benefits.
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* If you don't need debugging support or execution timeout support, ensure
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the following are **not enabled**:
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- ``DUK_OPT_INTERRUPT_COUNTER``
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- ``DUK_OPT_DEBUGGER_SUPPORT``
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Especially interrupt counter option will have a measurable performance
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impact because it includes code executed for every bytecode instruction
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dispatch.
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