262 lines
11 KiB
Plaintext
262 lines
11 KiB
Plaintext
This file describes some special Python build types enabled via compile-time
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preprocessor defines.
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IMPORTANT: if you want to build a debug-enabled Python, it is recommended that
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you use ``./configure --with-pydebug``, rather than the options listed here.
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However, if you wish to define some of these options individually, it is best
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to define them in the EXTRA_CFLAGS make variable;
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``make EXTRA_CFLAGS="-DPy_REF_DEBUG"``.
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Py_REF_DEBUG
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------------
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Turn on aggregate reference counting. This arranges that extern _Py_RefTotal
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hold a count of all references, the sum of ob_refcnt across all objects. In a
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debug-mode build, this is where the "8288" comes from in
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>>> 23
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23
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[8288 refs]
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>>>
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Note that if this count increases when you're not storing away new objects,
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there's probably a leak. Remember, though, that in interactive mode the special
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name "_" holds a reference to the last result displayed!
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Py_REF_DEBUG also checks after every decref to verify that the refcount hasn't
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gone negative, and causes an immediate fatal error if it has.
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Special gimmicks:
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sys.gettotalrefcount()
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Return current total of all refcounts.
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Py_TRACE_REFS
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-------------
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Turn on heavy reference debugging. This is major surgery. Every PyObject grows
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two more pointers, to maintain a doubly-linked list of all live heap-allocated
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objects. Most built-in type objects are not in this list, as they're statically
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allocated. Starting in Python 2.3, if COUNT_ALLOCS (see below) is also defined,
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a static type object T does appear in this list if at least one object of type T
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has been created.
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Note that because the fundamental PyObject layout changes, Python modules
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compiled with Py_TRACE_REFS are incompatible with modules compiled without it.
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Py_TRACE_REFS implies Py_REF_DEBUG.
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Special gimmicks:
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sys.getobjects(max[, type])
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Return list of the (no more than) max most-recently allocated objects, most
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recently allocated first in the list, least-recently allocated last in the
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list. max=0 means no limit on list length. If an optional type object is
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passed, the list is also restricted to objects of that type. The return
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list itself, and some temp objects created just to call sys.getobjects(),
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are excluded from the return list. Note that the list returned is just
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another object, though, so may appear in the return list the next time you
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call getobjects(); note that every object in the list is kept alive too,
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simply by virtue of being in the list.
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envvar PYTHONDUMPREFS
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If this envvar exists, Py_Finalize() arranges to print a list of all
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still-live heap objects. This is printed twice, in different formats,
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before and after Py_Finalize has cleaned up everything it can clean up. The
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first output block produces the repr() of each object so is more
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informative; however, a lot of stuff destined to die is still alive then.
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The second output block is much harder to work with (repr() can't be invoked
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anymore -- the interpreter has been torn down too far), but doesn't list any
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objects that will die. The tool script combinerefs.py can be run over this
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to combine the info from both output blocks. The second output block, and
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combinerefs.py, were new in Python 2.3b1.
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PYMALLOC_DEBUG
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--------------
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When pymalloc is enabled (WITH_PYMALLOC is defined), calls to the PyObject_
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memory routines are handled by Python's own small-object allocator, while calls
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to the PyMem_ memory routines are directed to the system malloc/ realloc/free.
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If PYMALLOC_DEBUG is also defined, calls to both PyObject_ and PyMem_ memory
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routines are directed to a special debugging mode of Python's small-object
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allocator.
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This mode fills dynamically allocated memory blocks with special, recognizable
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bit patterns, and adds debugging info on each end of dynamically allocated
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memory blocks. The special bit patterns are:
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#define CLEANBYTE 0xCB /* clean (newly allocated) memory */
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#define DEADBYTE 0xDB /* dead (newly freed) memory */
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#define FORBIDDENBYTE 0xFB /* forbidden -- untouchable bytes */
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Strings of these bytes are unlikely to be valid addresses, floats, or 7-bit
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ASCII strings.
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Let S = sizeof(size_t). 2*S bytes are added at each end of each block of N bytes
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requested. The memory layout is like so, where p represents the address
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returned by a malloc-like or realloc-like function (p[i:j] means the slice of
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bytes from *(p+i) inclusive up to *(p+j) exclusive; note that the treatment of
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negative indices differs from a Python slice):
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p[-2*S:-S]
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Number of bytes originally asked for. This is a size_t, big-endian (easier
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to read in a memory dump).
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p[-S]
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API ID. See PEP 445. This is a character, but seems undocumented.
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p[-S+1:0]
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Copies of FORBIDDENBYTE. Used to catch under- writes and reads.
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p[0:N]
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The requested memory, filled with copies of CLEANBYTE, used to catch
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reference to uninitialized memory. When a realloc-like function is called
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requesting a larger memory block, the new excess bytes are also filled with
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CLEANBYTE. When a free-like function is called, these are overwritten with
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DEADBYTE, to catch reference to freed memory. When a realloc- like function
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is called requesting a smaller memory block, the excess old bytes are also
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filled with DEADBYTE.
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p[N:N+S]
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Copies of FORBIDDENBYTE. Used to catch over- writes and reads.
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p[N+S:N+2*S]
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A serial number, incremented by 1 on each call to a malloc-like or
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realloc-like function. Big-endian size_t. If "bad memory" is detected
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later, the serial number gives an excellent way to set a breakpoint on the
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next run, to capture the instant at which this block was passed out. The
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static function bumpserialno() in obmalloc.c is the only place the serial
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number is incremented, and exists so you can set such a breakpoint easily.
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A realloc-like or free-like function first checks that the FORBIDDENBYTEs at
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each end are intact. If they've been altered, diagnostic output is written to
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stderr, and the program is aborted via Py_FatalError(). The other main failure
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mode is provoking a memory error when a program reads up one of the special bit
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patterns and tries to use it as an address. If you get in a debugger then and
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look at the object, you're likely to see that it's entirely filled with 0xDB
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(meaning freed memory is getting used) or 0xCB (meaning uninitialized memory is
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getting used).
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Note that PYMALLOC_DEBUG requires WITH_PYMALLOC.
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Special gimmicks:
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envvar PYTHONMALLOCSTATS
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If this envvar exists, a report of pymalloc summary statistics is printed to
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stderr whenever a new arena is allocated, and also by Py_Finalize().
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Changed in 2.5: The number of extra bytes allocated is 4*sizeof(size_t).
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Before it was 16 on all boxes, reflecting that Python couldn't make use of
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allocations >= 2**32 bytes even on 64-bit boxes before 2.5.
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Py_DEBUG
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--------
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This is what is generally meant by "a debug build" of Python.
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Py_DEBUG implies LLTRACE, Py_REF_DEBUG, Py_TRACE_REFS, and PYMALLOC_DEBUG (if
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WITH_PYMALLOC is enabled). In addition, C assert()s are enabled (via the C way:
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by not defining NDEBUG), and some routines do additional sanity checks inside
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"#ifdef Py_DEBUG" blocks.
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COUNT_ALLOCS
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------------
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Each type object grows three new members:
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/* Number of times an object of this type was allocated. */
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int tp_allocs;
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/* Number of times an object of this type was deallocated. */
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int tp_frees;
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/* Highwater mark: the maximum value of tp_allocs - tp_frees so
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* far; or, IOW, the largest number of objects of this type alive at
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* the same time.
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*/
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int tp_maxalloc;
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Allocation and deallocation code keeps these counts up to date. Py_Finalize()
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displays a summary of the info returned by sys.getcounts() (see below), along
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with assorted other special allocation counts (like the number of tuple
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allocations satisfied by a tuple free-list, the number of 1-character strings
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allocated, etc).
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Before Python 2.2, type objects were immortal, and the COUNT_ALLOCS
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implementation relies on that. As of Python 2.2, heap-allocated type/ class
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objects can go away. COUNT_ALLOCS can blow up in 2.2 and 2.2.1 because of this;
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this was fixed in 2.2.2. Use of COUNT_ALLOCS makes all heap-allocated type
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objects immortal, except for those for which no object of that type is ever
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allocated.
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Starting with Python 2.3, If Py_TRACE_REFS is also defined, COUNT_ALLOCS
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arranges to ensure that the type object for each allocated object appears in the
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doubly-linked list of all objects maintained by Py_TRACE_REFS.
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Special gimmicks:
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sys.getcounts()
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Return a list of 4-tuples, one entry for each type object for which at least
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one object of that type was allocated. Each tuple is of the form:
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(tp_name, tp_allocs, tp_frees, tp_maxalloc)
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Each distinct type object gets a distinct entry in this list, even if two or
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more type objects have the same tp_name (in which case there's no way to
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distinguish them by looking at this list). The list is ordered by time of
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first object allocation: the type object for which the first allocation of
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an object of that type occurred most recently is at the front of the list.
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LLTRACE
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-------
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Compile in support for Low Level TRACE-ing of the main interpreter loop.
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When this preprocessor symbol is defined, before PyEval_EvalFrame (eval_frame in
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2.3 and 2.2, eval_code2 before that) executes a frame's code it checks the
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frame's global namespace for a variable "__lltrace__". If such a variable is
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found, mounds of information about what the interpreter is doing are sprayed to
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stdout, such as every opcode and opcode argument and values pushed onto and
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popped off the value stack.
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Not useful very often, but very useful when needed.
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CALL_PROFILE
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------------
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Count the number of function calls executed.
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When this symbol is defined, the ceval mainloop and helper functions count the
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number of function calls made. It keeps detailed statistics about what kind of
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object was called and whether the call hit any of the special fast paths in the
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code.
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WITH_TSC
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--------
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Super-lowlevel profiling of the interpreter. When enabled, the sys module grows
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a new function:
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settscdump(bool)
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If true, tell the Python interpreter to dump VM measurements to stderr. If
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false, turn off dump. The measurements are based on the processor's
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time-stamp counter.
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This build option requires a small amount of platform specific code. Currently
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this code is present for linux/x86 and any PowerPC platform that uses GCC
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(i.e. OS X and linux/ppc).
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On the PowerPC the rate at which the time base register is incremented is not
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defined by the architecture specification, so you'll need to find the manual for
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your specific processor. For the 750CX, 750CXe and 750FX (all sold as the G3)
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we find:
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The time base counter is clocked at a frequency that is one-fourth that of
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the bus clock.
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This build is enabled by the --with-tsc flag to configure.
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