Optimization ideas

Once the new C API will succeed to hide implementation details, it becomes possible to experiment radical changes in CPython to implement new optimizations.

See Experimental runtime.

Remove debug checks

See Regular runtime: since a debug runtime will be provided by default, many sanity checks can be removed from the release build.

Change the garbage collector and remove reference counting: unlikely

CPython 3.7 garbage collector (GC) uses “stop-the-world” which is a big issue for realtime applications like games and can be major issue more generally for performance critical applications. There is a desire to use a GC which doesn’t need to “stop the world”. PyPy succeeded to use an incremental GC.

There are discussing to use a tracing garbage collector for CPython, but this idea remains highly hypothetical since it very likely require deep changes in the C API, which is out of the scope of the new C API project. The main risk is to break too many C extensions which would make this idea unusable in practice.

It may be possible to emulate reference counting for the C API. Py_INCREF() and Py_DECREF() would be re-implemented using an hash table: object => reference counter.

Larry Hastings consider to use a tracing garbage collector for Gilectomy.

See also Reference counting.

Remove the GIL: unlikely

Removing the Global Interpreter Lock (GIL) from CPython, or at least being able to use one GIL per Python interpreter (when using multiple interpreters per process) is an old dream. It means replacing a single big lock with many smaller locks, maybe one lock per Python object.

Jython has not GIL.

Reference counting remains a good and convenient API for C extension. Maybe this design can be kept for the public C API for external C extensions, but CPython internals can be modified to avoid reference counting, like using a tracing garbage collector for example. Once the C API stops leaking implementation details, many new options become possible.

Gilectomy project is CPython 3.6 fork which tries to remove the GIL. In 2017, the project did not succeed yet to scale linearly performances with the number of threads. It seems like reference counting is a performance killer for multi-threading.

By the way, using atomic operations to access (increase in Py_INCREF(), decrease and test in Py_DECREF()) the reference count has been proposed, but experiment showed a slowdown of 20% on single threaded micro-benchmarks.

Tagged pointers: doable

See Wikipedia: Tagged pointer.

Tagged pointers are used by MicroPython to reduce the memory footprint.

Using tagged pointers is a common optimization technique to reduce the boxing/unboxing cost and to reduce the memory consumption.

Currently, it’s not possible to implement such optimization, since most of the C API rely on real pointer values for direct access to Python objects.

Note: ARM64 was recently extended its address space to 48 bits, causing issue in LuaJIT: 47 bit address space restriction on ARM64.

Copy-on-Write (CoW): doable

Copy-on-Write (CoW). Instagram is using prefork with Django but has memory usage issues caused by reference counting. Accessing a Python object modifies its reference counter and so copies the page which was created a COW in the forked child process. Python 3.7 added gc.freeze() workaround.

  • Replace Py_ssize_t ob_refcnt; (integer) with Py_ssize_t *ob_refcnt; (pointer to an integer).
  • Same change for the GC header?
  • Store all reference counters in a separated memory block (or maybe multiple memory blocks)

Expected advantage: smaller memory footprint when using fork() on UNIX which is implemented with Copy-On-Write on physical memory pages.

See also Dismissing Python Garbage Collection at Instagram.

Transactional Memory: highly experimental

PyPy experimented Software Transactional Memory (STM) but the project has been abandoned, PyPy STM.

Specialized list for small integers

If C extensions don’t access structure members anymore, it becomes possible to modify the memory layout.

For example, it’s possible to design a specialized implementation of PyListObject for small integers:

typedef struct {
    PyVarObject ob_base;
    int use_small_int;
    PyObject **pyobject_array;
    int32_t *small_int_array;   // <-- new compact C array for integers
    Py_ssize_t allocated;
} PyListObject;

PyObject* PyList_GET_ITEM(PyObject *op, Py_ssize_t index)
{
    PyListObject *list = (PyListObject *)op;
    if (list->use_small_int) {
        int32_t item = list->small_int_array[index];
        /* create a new object at each call */
        return PyLong_FromLong(item);
    }
    else {
        return list->pyobject_array[index];
    }
}

Each call to PyList_GET_ITEM() of this example creates a new temporary object which leads the memory leak (reference leak). This is one concrete example of issue with borrowed references.

List specialized for numbers is just a example easy to understand to show that it becomes possible to modify PyObject structures. The main benefit of the memory footprint, but the overall on performances is unknown at this point.

O(1) conversion of bytearray to bytes

TODO: find a better method name :-)

Problem: memory copy, memory usage

When a function produces a bytes string but the output length is enough, using a temporary bytearray object is recommended to use the efficient bytearray += bytes pattern (bytearray overallocates its internal buffer and so reduce the number of reallocations). Problem: if the result type must be bytes, the bytearray must be converted to bytes… and this operation currently requires to copy the memory. For example, _pyio.FileIO.readall() copies the full content of a binary file and doubles the memory usage.

In Python 3.7, a bytes object always use a single memory block: content follows the object header, whereas a bytearray uses two memory blocks. It’s not possible to transfer data from bytearray to bytes to implement a O(1) conversion.

Solution: support bytes using two memory blocks

If the bytes type is modified to also support storing data in a second memory block, it becomes possible to implement O(1) conversion of bytearray to bytes. The bytearray would pass its memory block to the bytes object and then “loose its content” (becomes an empty buffer).

And more!

Insert your new cool idea here!