The Numba Runtime (NRT) provides the language runtime to the nopython mode Python subset. NRT is a standalone C library with a Python binding. This allows NPM runtime feature to be used without the GIL. Currently, the only language feature implemented in NRT is memory management.
NRT implements memory management for NPM code. It uses atomic reference count
for threadsafe, deterministic memory management. NRT maintains a separate
MemInfo structure for storing information about each allocation.
For NRT to cooperate with CPython, the NRT python binding provides adaptors for
converting python objects that export a memory region. When such an
object is used as an argument to a NPM function, a new MemInfo is created
and it acquires a reference to the Python object. When a NPM value is returned
to the Python interpreter, the associated MemInfo (if any) is checked. If
the MemInfo references a Python object, the underlying Python object is
released and returned instead. Otherwise, the MemInfo is wrapped in a
Python object and returned. Additional process maybe required depending on
the type.
The current implementation supports Numpy array and any buffer-exporting types.
NRT reference counting requires the compiler to emit incref/decref operations according to the usage. When the reference count drops to zero, the compiler must call the destructor routine in NRT.
The compiler is allowed to emit incref/decref operations naively. It relies on an optimization pass that to remove the redundant reference count operations.
The optimization pass runs on block level to avoid control flow analysis. It depends on LLVM function optimization pass to simplify the control flow, stack-to-register, and simplify instructions. It works by matching and removing incref and decref pairs within each block.
Since the refcount optimization pass requires LLVM function optimization pass, the pass works on the LLVM IR as text. The optimized IR is then materialized again as a new LLVM in-memory bitcode object.
To debug reference leaks in NRT MemInfo, each MemInfo python object has a
.refcount attribute for inspection. To get the MemInfo from a ndarray
allocated by NRT, use the .base attribute.
To debug memory leaks in NRT, the numba.runtime.rtsys defines
.get_allocation_stats(). It returns a namedtuple containing the
number of allocation and deallocation since the start of the program.
Checking that the allocation and deallocation counters are matching is the
simplest way to know if the NRT is leaking.
The start of numba/runtime/nrt.h has these lines:
/* Debugging facilities - enabled at compile-time */
/* #undef NDEBUG */
#if 0
# define NRT_Debug(X) X
#else
# define NRT_Debug(X) if (0) { X; }
#endif
Undefining NDEBUG (uncomment the #undef NDEBUG line) enables the assertion
check in NRT.
Enabling the NRT_Debug (replace #if 0 with #if 1) turns on
debug print inside NRT.
During the compilation of a pair of mutually recursive functions, one of the
functions will contain unresolved symbol references since the compiler handles
one function at a time. The memory for the unresolved symbols is allocated and
initialized to the address of the unresolved symbol abort function
(nrt_unresolved_abort) just before the machine code is
generated by LLVM. These symbols are tracked and resolved as new functions are
compiled. If a bug prevents the resolution of these symbols,
the abort function will be called, raising a RuntimeError exception.
The unresolved symbol abort function is defined in the NRT with a zero-argument signature. The caller is safe to call it with arbitrary number of arguments. Therefore, it is safe to be used inplace of the intended callee.
The plan for NRT is to make a standalone shared library that can be linked to Numba compiled code, including use within the Python interpreter and without the Python interpreter. To make that work, we will be doing some refactoring: