feature: support pymalloc for subinterpreters. each subinterpreter has pymalloc_state

[JunyiXie]

BPO	43313
Nosy	@nascheme, @vstinner, @methane, @JunyiXie
PRs	bpo-43313: support pymalloc for subinterpreters. each subinterpreter ha… #24857

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-02-24.09:43:29.704>
labels = ['expert-subinterpreters', '3.10']
title = 'feature: support pymalloc for subinterpreters. each subinterpreter has pymalloc_state'
updated_at = <Date 2021-03-15.14:39:50.325>
user = 'https://github.com/JunyiXie'

bugs.python.org fields:

activity = <Date 2021-03-15.14:39:50.325>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Subinterpreters']
creation = <Date 2021-02-24.09:43:29.704>
creator = 'JunyiXie'
dependencies = []
files = []
hgrepos = []
issue_num = 43313
keywords = ['patch']
message_count = 10.0
messages = ['387614', '388668', '388670', '388671', '388734', '388735', '388736', '388737', '388743', '388745']
nosy_count = 4.0
nosy_names = ['nascheme', 'vstinner', 'methane', 'JunyiXie']
pr_nums = ['24857']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue43313'
versions = ['Python 3.10']

[JunyiXie]

ericsnowcurrently/multi-core-python#73

JunyiXie@8209548
changes:
move pymalloc state in obmalloc.h
_is add pymalloc_state
pymalloc_allocxx api use subinterpreter pymalloc_state

[JunyiXie]

Made two changes:

1. support pymalloc for subinterpreters. each subinterpreter has pymalloc_state

2. _copy_raw_string api alloc memory use PyMem_RawFree and PyMem_RawMalloc.

I extend _xxsubinterpretermodule.c to support call any function in sub interpreter.
when i need return result from sub interpreter call.

1. i need create item->name in shared item. will use pymem_xxx api to manage memory. when with_pymalloc macro defined, it will create memory and bound to interpreter(iterp1) pymalloc state.

2. after switch interpreter state, now in iterp2 state, get return value from shareditem, and i need free shared item. but item->name memory managed by interp1 pymalloc state. if i want to free them, i need switch to interpreter state 1. it's complicated. to implementation it, we need save interpid in shared item.

so i think, in _sharednsitem_init _copy_raw_string, need malloc by PyMem_RawAPI. easy to management.

static int
_sharednsitem_init(struct _sharednsitem *item, PyObject *key, PyObject *value)
{
    item->name = _copy_raw_string(key);

_sharedns *result_shread = _sharedns_new(1);


#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
    // Switch to interpreter.
    PyThreadState *new_tstate = PyInterpreterState_ThreadHead(interp);
    PyThreadState *save1 = PyEval_SaveThread();

    (void)PyThreadState_Swap(new_tstate);
#else
    // Switch to interpreter.
    PyThreadState *save_tstate = NULL;
    if (interp != PyInterpreterState_Get()) {
        // XXX Using the "head" thread isn't strictly correct.
        PyThreadState *tstate = PyInterpreterState_ThreadHead(interp);
        // XXX Possible GILState issues?
        save_tstate = PyThreadState_Swap(tstate);
    }
#endif
    
    PyObject *module = PyImport_ImportModule(PyUnicode_AsUTF8(module_name));
    PyObject *function = PyObject_GetAttr(module, function_name);
    
    result = PyObject_Call(function, args, kwargs);

    if (result == NULL) {
        // exception handler
        ...
    }

    if (result && _sharednsitem_init(&result_shread->items[0], PyUnicode_FromString("result"), result) != 0) {
        PyErr_Format(RunFailedError, "interp_call_function result convert to shared failed");
        return NULL;;
    }
#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
    // Switch back.
    PyEval_RestoreThread(save1);
#else
    // Switch back.
    if (save_tstate != NULL) {
        PyThreadState_Swap(save_tstate);
    }
#endif
    // ...

    if (result) {
        result = _PyCrossInterpreterData_NewObject(&result_shread->items[0].data);
        _sharedns_free(result_shread);
    }

[JunyiXie]

github pr

[JunyiXie]

#24857

[JunyiXie]

There is a problem:
if we bound pymalloc state with a interpreter.
malloc pointer in interpreterA and free pointer is usual.

it's cause a problem.
when we use PyObject_Free,

1. we look up address in pymalloc pool.
2. if not find, current code will call PyMem_RawFree(p) to free. it will cause crash.(address is pymalloc_alloc from another interpreter)

I think it has two way to slove this problem:

1. free/alloc memory in one interpreter. Frequent switch interpreter affects performance
2. when free memory address, find this address in all interpreter pymalloc pool. and free it.(but it need add lock to pymalloc)

[JunyiXie]

malloc pointer in interpreterA and free pointer is usual.

malloc pointer in interpreterA and free pointer in interpreterB is usual.

[JunyiXie]

by the way,
There is no operation to destroy the memory pool in the cpython code. Repeated creation of the pymalloc pool will cause memory leaks.

[JunyiXie]

2. when free memory address, find this address in all interpreter pymalloc pool. and free it.(but it need add lock to pymalloc)

when finalize_interp_delete, we need keep interpreter pymalloc pool in linked list.It will be used when search memory in pymalloc pools.

[vstinner]

I'm not sure that it's needed to have a "per interpreter" allocator. The needed feature is to be able to call PyMem_Malloc() in parallel in different threads. If I understood correctly, the glibc malloc has a per-thread fast allocator (no locking) and then falls back to a slow allocator (locking) if the fast allocator failed. Maybe pymalloc could have per-thread memory arenas.

When I implemented the PEP-587, I spend a significant amount of time to avoid using pymalloc before Py_Initialize() is called: only use PyMem_RawMalloc() before Py_Initialize().

But I'm not 100% sure that pymalloc is not used before Py_Initialize() nor *after* Py_Finalize(). For example, we should check if a daemon thread can call PyMem_Malloc() after Py_Finalize(), even if they are supposed to exit as soon as they try to acquire the GIL, even the GIL must be held to use pymalloc (to use PyMem_Malloc and PyObject_Malloc):
https://docs.python.org/dev/c-api/memory.html#memory-interface

See also bpo-37448:
"Add radix tree implementation for obmalloc address_in_range()"
#14474

[vstinner]

The current workaround is to disable pymalloc when Python is built with EXPERIMENTAL_ISOLATED_SUBINTERPRETERS:

_PyPreConfig_InitCompatConfig(PyPreConfig *config):

#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
    /* bpo-40512: pymalloc is not compatible with subinterpreters,
       force usage of libc malloc() which is thread-safe. */
#ifdef Py_DEBUG
    config->allocator = PYMEM_ALLOCATOR_MALLOC_DEBUG;
#else
    config->allocator = PYMEM_ALLOCATOR_MALLOC;
#endif
#else
    ...
#endif