Thread-local arenas

Currently, all threads use the same arena for imaging. This can
result in a lot of contention when there are enough workers and
the mutex is constantly being checked.

This commit instead introduces lockless thread-local arenas for
environments that support it.

setup.py

@@ -8,18 +8,21 @@
 # ------------------------------
 from __future__ import annotations
 
+import distutils.ccompiler
 import os
 import re
 import shutil
 import struct
 import subprocess
 import sys
+import tempfile
 import warnings
 from collections.abc import Iterator
 from typing import Any
 
 from setuptools import Extension, setup
 from setuptools.command.build_ext import build_ext
+from setuptools.errors import CompileError
 
 
 def get_version() -> str:
@@ -292,6 +295,47 @@ def _pkg_config(name: str) -> tuple[list[str], list[str]] | None:
     return None
 
 
+def _try_compile(compiler: distutils.ccompiler.CCompiler, code: str) -> bool:
+    try:
+        with tempfile.TemporaryDirectory() as d:
+            fn = os.path.join(d, "test.c")
+            with open(fn, "w") as f:
+                f.write(code)
+            compiler.compile([fn], output_dir=d, extra_preargs=["-Werror"])
+        return True
+    except CompileError:
+        return False
+
+
+def _try_compile_attr(compiler: distutils.ccompiler.CCompiler, attr: str) -> bool:
+    code = f"""
+        #pragma GCC diagnostic error "-Wattributes"
+        #pragma clang diagnostic error "-Wattributes"
+
+        int {attr} foo;
+        int main() {{
+            return 0;
+        }}
+    """
+
+    return _try_compile(compiler, code)
+
+
+def _try_compile_tls_define_macros(
+    compiler: distutils.ccompiler.CCompiler,
+) -> list[tuple[str, str | None]]:
+    if _try_compile_attr(compiler, "thread_local"):  # C23
+        return [("HAVE_THREAD_LOCAL", None)]
+    elif _try_compile_attr(compiler, "_Thread_local"):  # C11/C17
+        return [("HAVE__THREAD_LOCAL", None)]
+    elif _try_compile_attr(compiler, "__thread"):  # GCC/clang
+        return [("HAVE___THREAD", None)]
+    elif _try_compile_attr(compiler, "__declspec(thread)"):  # MSVC
+        return [("HAVE___DECLSPEC_THREAD_", None)]
+    else:
+        return []
+
+
 class pil_build_ext(build_ext):
     class ext_feature:
         features = [
@@ -426,13 +470,14 @@ class pil_build_ext(build_ext):
     def _update_extension(
         self,
         name: str,
-        libraries: list[str] | list[str | bool | None],
+        libraries: list[str] | list[str | bool | None] | None = None,
         define_macros: list[tuple[str, str | None]] | None = None,
         sources: list[str] | None = None,
     ) -> None:
         for extension in self.extensions:
             if extension.name == name:
-                extension.libraries += libraries
+                if libraries is not None:
+                    extension.libraries += libraries
                 if define_macros is not None:
                     extension.define_macros += define_macros
                 if sources is not None:
@@ -890,7 +935,10 @@ class pil_build_ext(build_ext):
 
         defs.append(("PILLOW_VERSION", f'"{PILLOW_VERSION}"'))
 
-        self._update_extension("PIL._imaging", libs, defs)
+        tls_define_macros = _try_compile_tls_define_macros(self.compiler)
+        self._update_extension("PIL._imaging", libs, defs + tls_define_macros)
+        self._update_extension("PIL._imagingmath", define_macros=tls_define_macros)
+        self._update_extension("PIL._imagingmorph", define_macros=tls_define_macros)
 
         #
         # additional libraries
@@ -913,7 +961,9 @@ class pil_build_ext(build_ext):
                         libs.append(feature.get("fribidi"))
                     else:  # building FriBiDi shim from src/thirdparty
                         srcs.append("src/thirdparty/fribidi-shim/fribidi.c")
-            self._update_extension("PIL._imagingft", libs, defs, srcs)
+            self._update_extension(
+                "PIL._imagingft", libs, defs + tls_define_macros, srcs
+            )
 
         else:
             self._remove_extension("PIL._imagingft")
@@ -922,19 +972,19 @@ class pil_build_ext(build_ext):
             libs = [feature.get("lcms")]
             if sys.platform == "win32":
                 libs.extend(["user32", "gdi32"])
-            self._update_extension("PIL._imagingcms", libs)
+            self._update_extension("PIL._imagingcms", libs, tls_define_macros)
         else:
             self._remove_extension("PIL._imagingcms")
 
         webp = feature.get("webp")
         if isinstance(webp, str):
             libs = [webp, webp + "mux", webp + "demux"]
-            self._update_extension("PIL._webp", libs)
+            self._update_extension("PIL._webp", libs, tls_define_macros)
         else:
             self._remove_extension("PIL._webp")
 
         tk_libs = ["psapi"] if sys.platform in ("win32", "cygwin") else []
-        self._update_extension("PIL._imagingtk", tk_libs)
+        self._update_extension("PIL._imagingtk", tk_libs, tls_define_macros)
 
         build_ext.build_extensions(self)
 

src/_imaging.c

@@ -3938,7 +3938,7 @@ _get_stats(PyObject *self, PyObject *args) {
         return NULL;
     }
 
-    MUTEX_LOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
     ImagingMemoryArena arena = &ImagingDefaultArena;
 
     v = PyLong_FromLong(arena->stats_new_count);
@@ -3965,7 +3965,7 @@ _get_stats(PyObject *self, PyObject *args) {
     PyDict_SetItemString(d, "blocks_cached", v ? v : Py_None);
     Py_XDECREF(v);
 
-    MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
     return d;
 }
 
@@ -3975,14 +3975,14 @@ _reset_stats(PyObject *self, PyObject *args) {
         return NULL;
     }
 
-    MUTEX_LOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
     ImagingMemoryArena arena = &ImagingDefaultArena;
     arena->stats_new_count = 0;
     arena->stats_allocated_blocks = 0;
     arena->stats_reused_blocks = 0;
     arena->stats_reallocated_blocks = 0;
     arena->stats_freed_blocks = 0;
-    MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
 
     Py_INCREF(Py_None);
     return Py_None;
@@ -3994,9 +3994,9 @@ _get_alignment(PyObject *self, PyObject *args) {
         return NULL;
     }
 
-    MUTEX_LOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
     int alignment = ImagingDefaultArena.alignment;
-    MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
     return PyLong_FromLong(alignment);
 }
 
@@ -4006,9 +4006,9 @@ _get_block_size(PyObject *self, PyObject *args) {
         return NULL;
     }
 
-    MUTEX_LOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
     int block_size = ImagingDefaultArena.block_size;
-    MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
     return PyLong_FromLong(block_size);
 }
 
@@ -4018,9 +4018,9 @@ _get_blocks_max(PyObject *self, PyObject *args) {
         return NULL;
     }
 
-    MUTEX_LOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
     int blocks_max = ImagingDefaultArena.blocks_max;
-    MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
     return PyLong_FromLong(blocks_max);
 }
 
@@ -4041,9 +4041,9 @@ _set_alignment(PyObject *self, PyObject *args) {
         return NULL;
     }
 
-    MUTEX_LOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
     ImagingDefaultArena.alignment = alignment;
-    MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
 
     Py_INCREF(Py_None);
     return Py_None;
@@ -4066,9 +4066,9 @@ _set_block_size(PyObject *self, PyObject *args) {
         return NULL;
     }
 
-    MUTEX_LOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
     ImagingDefaultArena.block_size = block_size;
-    MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
 
     Py_INCREF(Py_None);
     return Py_None;
@@ -4092,9 +4092,9 @@ _set_blocks_max(PyObject *self, PyObject *args) {
         return NULL;
     }
 
-    MUTEX_LOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
     int status = ImagingMemorySetBlocksMax(&ImagingDefaultArena, blocks_max);
-    MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
     if (!status) {
         return ImagingError_MemoryError();
     }
@@ -4111,9 +4111,9 @@ _clear_cache(PyObject *self, PyObject *args) {
         return NULL;
     }
 
-    MUTEX_LOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
     ImagingMemoryClearCache(&ImagingDefaultArena, i);
-    MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
 
     Py_INCREF(Py_None);
     return Py_None;

src/libImaging/Imaging.h

@@ -149,6 +149,32 @@ struct ImagingPaletteInstance {
     int keep_cache; /* This palette will be reused; keep cache */
 };
 
+#define IMAGING_ARENA_LOCK(m)
+#define IMAGING_ARENA_UNLOCK(m)
+
+#if defined(__cplusplus)
+#define IMAGING_ARENA_TLS thread_local
+#elif defined(HAVE_THREAD_LOCAL)
+#define IMAGING_ARENA_TLS thread_local
+#elif defined(HAVE__THREAD_LOCAL)
+#define IMAGING_ARENA_TLS _Thread_local
+#elif defined(HAVE___THREAD)
+#define IMAGING_ARENA_TLS __thread
+#elif defined(HAVE___DECLSPEC_THREAD_)
+#define IMAGING_ARENA_TLS __declspec(thread)
+#elif defined(Py_GIL_DISABLED)
+#define IMAGING_ARENA_TLS
+#define IMAGING_ARENA_LOCKING
+
+#undef IMAGING_ARENA_LOCK
+#undef IMAGING_ARENA_UNLOCK
+
+#define IMAGING_ARENA_LOCK(m) PyMutex_Lock(m)
+#define IMAGING_ARENA_UNLOCK(m) PyMutex_Unlock(m)
+#else
+#define IMAGING_ARENA_TLS
+#endif
+
 typedef struct ImagingMemoryArena {
     int alignment;     /* Alignment in memory of each line of an image */
     int block_size;    /* Preferred block size, bytes */
@@ -161,7 +187,8 @@ typedef struct ImagingMemoryArena {
     int stats_reallocated_blocks; /* Number of blocks which were actually reallocated
                                      after retrieving */
     int stats_freed_blocks;       /* Number of freed blocks */
-#ifdef Py_GIL_DISABLED
+
+#ifdef IMAGING_ARENA_LOCKING
     PyMutex mutex;
 #endif
 } *ImagingMemoryArena;
@@ -169,7 +196,7 @@ typedef struct ImagingMemoryArena {
 /* Objects */
 /* ------- */
 
-extern struct ImagingMemoryArena ImagingDefaultArena;
+extern IMAGING_ARENA_TLS struct ImagingMemoryArena ImagingDefaultArena;
 extern int
 ImagingMemorySetBlocksMax(ImagingMemoryArena arena, int blocks_max);
 extern void

src/libImaging/Storage.c

@@ -218,9 +218,9 @@ ImagingNewPrologueSubtype(const char *mode, int xsize, int ysize, int size) {
             break;
     }
 
-    MUTEX_LOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
     ImagingDefaultArena.stats_new_count += 1;
-    MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
 
     return im;
 }
@@ -259,7 +259,7 @@ ImagingDelete(Imaging im) {
 
 #define IMAGING_PAGE_SIZE (4096)
 
-struct ImagingMemoryArena ImagingDefaultArena = {
+IMAGING_ARENA_TLS struct ImagingMemoryArena ImagingDefaultArena = {
     1,                 // alignment
     16 * 1024 * 1024,  // block_size
     0,                 // blocks_max
@@ -270,7 +270,7 @@ struct ImagingMemoryArena ImagingDefaultArena = {
     0,
     0,
     0,  // Stats
-#ifdef Py_GIL_DISABLED
+#ifdef IMAGING_ARENA_LOCKING
     {0},
 #endif
 };
@@ -369,12 +369,12 @@ ImagingDestroyArray(Imaging im) {
     int y = 0;
 
     if (im->blocks) {
-        MUTEX_LOCK(&ImagingDefaultArena.mutex);
+        IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
         while (im->blocks[y].ptr) {
             memory_return_block(&ImagingDefaultArena, im->blocks[y]);
             y += 1;
         }
-        MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+        IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
         free(im->blocks);
     }
 }
@@ -504,11 +504,11 @@ ImagingNewInternal(const char *mode, int xsize, int ysize, int dirty) {
         return NULL;
     }
 
-    MUTEX_LOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
     Imaging tmp = ImagingAllocateArray(
         im, &ImagingDefaultArena, dirty, ImagingDefaultArena.block_size
     );
-    MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
     if (tmp) {
         return im;
     }
@@ -516,9 +516,9 @@ ImagingNewInternal(const char *mode, int xsize, int ysize, int dirty) {
     ImagingError_Clear();
 
     // Try to allocate the image once more with smallest possible block size
-    MUTEX_LOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_LOCK(&ImagingDefaultArena.mutex);
     tmp = ImagingAllocateArray(im, &ImagingDefaultArena, dirty, IMAGING_PAGE_SIZE);
-    MUTEX_UNLOCK(&ImagingDefaultArena.mutex);
+    IMAGING_ARENA_UNLOCK(&ImagingDefaultArena.mutex);
     if (tmp) {
         return im;
     }