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| -rw-r--r-- | docs/reference/speed_python.rst | 27 |
1 files changed, 20 insertions, 7 deletions
diff --git a/docs/reference/speed_python.rst b/docs/reference/speed_python.rst index 86884a1577..7bc4868054 100644 --- a/docs/reference/speed_python.rst +++ b/docs/reference/speed_python.rst @@ -85,18 +85,31 @@ elements in contiguous memory locations. Once again to avoid memory allocation i code these should be pre-allocated and passed as arguments or as bound objects. When passing slices of objects such as ``bytearray`` instances, Python creates -a copy which involves allocation. This can be avoided using a ``memoryview`` -object: +a copy which involves allocation of the size proportional to the size of slice. +This can be alleviated using a ``memoryview`` object. ``memoryview`` itself +is allocated on heap, but is a small, fixed-size object, regardless of the size +of slice it points too. .. code:: python - ba = bytearray(100) - func(ba[3:10]) # a copy is passed - mv = memoryview(ba) - func(mv[3:10]) # a pointer to memory is passed + ba = bytearray(10000) # big array + func(ba[30:2000]) # a copy is passed, ~2K new allocation + mv = memoryview(ba) # small object is allocated + func(mv[30:2000]) # a pointer to memory is passed A ``memoryview`` can only be applied to objects supporting the buffer protocol - this -includes arrays but not lists. +includes arrays but not lists. Small caveat is that while memoryview object is live, +it also keeps alive the original buffer object. So, memoryviews isn't universal +panacea. For instance, in the example above, if you are done with 10K buffer and +just need those bytes 30:2000 from it, it may be better to make a slice, and let +the 10K buffer go (be ready for garbage collection), instead of making a +long-living memoryview and keeping 10K blocked for GC. + +Nonetheless, ``memoryview`` is indispensable for advanced preallocated buffer +management. ``.readinto()`` method discussed above puts data at the beginning +of buffer and fills in entire buffer. What if you need to put data in the +middle of existing buffer? Just create a memoryview into the needed section +of buffer and pass it to ``.readinto()``. Identifying the slowest section of code --------------------------------------- |