Draft hybrid pool+malloc allocator

examples/alloc-hybrid/README.rst

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+=====================
+Hybrid pool allocator
+=====================
+
+Example allocator that tries to satisfy memory allocations for small sizes
+from a set of fixed pools, but always falls back to malloc/realloc/free if
+a larger size is requested or the pools have been exhausted.
+
+This may be useful to reduce memory churn when the platform allocator does
+not handle allocations for a lot of small memory areas efficiently.

examples/alloc-hybrid/duk_alloc_hybrid.c

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+/*
+ *  Example memory allocator with pool allocation for small sizes and
+ *  fallback into malloc/realloc/free for larger sizes or when the pools
+ *  are exhausted.
+ *
+ *  Useful to reduce memory churn or work around a platform allocator
+ *  that doesn't handle a lot of small allocations efficiently.
+ */
+
+#include "duktape.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdint.h>
+
+typedef struct {
+	size_t size;
+	int count;
+} pool_size_spec;
+
+static pool_size_spec pool_sizes[] = {
+	{ 32, 1024 },
+	{ 48, 2048 },
+	{ 64, 2048 },
+	{ 128, 2048 },
+	{ 256, 512 },
+	{ 1024, 64 },
+	{ 2048, 32 }
+};
+
+#define  NUM_POOLS  (sizeof(pool_sizes) / sizeof(pool_size_spec))
+
+/* This must fit into the smallest pool entry. */
+struct pool_free_entry;
+typedef struct pool_free_entry pool_free_entry;
+struct pool_free_entry {
+	pool_free_entry *next;
+};
+
+typedef struct {
+	pool_free_entry *free;
+	char *alloc_start;
+	char *alloc_end;
+	size_t size;
+	int count;
+} pool_header;
+
+typedef struct {
+	pool_header headers[NUM_POOLS];
+	size_t pool_max_size;
+	char *alloc_start;
+	char *alloc_end;
+} pool_state;
+
+#define ADDR_IN_STATE_ALLOC(st,p) \
+	((char *) (p) >= (st)->alloc_start && (char *) (p) < (st)->alloc_end)
+#define ADDR_IN_HEADER_ALLOC(hdr,p) \
+	((char *) (p) >= (hdr)->alloc_start && (char *) (p) < (hdr)->alloc_end)
+
+static void dump_pool_state(pool_state *st) {
+	pool_free_entry *free;
+	int free_len;
+	int i;
+
+	printf("=== Pool state: st=%p\n", (void *) st);
+	for (i = 0; i < (int) NUM_POOLS; i++) {
+		pool_header *hdr = st->headers + i;
+
+		for (free = hdr->free, free_len = 0; free != NULL; free = free->next) {
+			free_len++;
+		}
+
+		printf("[%d]: size %ld, count %ld, used %ld, free list len %ld\n",
+		       i, (long) hdr->size, (long) hdr->count,
+		       (long) (hdr->count - free_len),
+		       (long) free_len);
+	}
+}
+
+void *duk_alloc_hybrid_init(void) {
+	pool_state *st;
+	size_t total_size, max_size;
+	int i, j;
+	char *p;
+
+	st = malloc(sizeof(pool_state));
+	if (!st) {
+		return NULL;
+	}
+	memset((void *) st, 0, sizeof(pool_state));
+	st->alloc_start = NULL;
+	st->alloc_end = NULL;
+
+	for (i = 0, total_size = 0, max_size = 0; i < (int) NUM_POOLS; i++) {
+		printf("Pool %d: size %ld, count %ld\n", i, (long) pool_sizes[i].size, (long) pool_sizes[i].count);
+		total_size += pool_sizes[i].size * pool_sizes[i].count;
+		if (pool_sizes[i].size > max_size) {
+			max_size = pool_sizes[i].size;
+		}
+	}
+	printf("Total size %ld, max pool size %ld\n", (long) total_size, (long) max_size);
+
+	st->alloc_start = malloc(total_size);
+	if (!st->alloc_start) {
+		free(st);
+		return NULL;
+	}
+	st->alloc_end = st->alloc_start + total_size;
+	st->pool_max_size = max_size;
+	memset((void *) st->alloc_start, 0, total_size);
+
+	for (i = 0, p = st->alloc_start; i < (int) NUM_POOLS; i++) {
+		pool_header *hdr = st->headers + i;
+
+		hdr->alloc_start = p;
+		hdr->alloc_end = p + pool_sizes[i].size * pool_sizes[i].count;
+		hdr->free = (pool_free_entry *) p;
+		hdr->size = pool_sizes[i].size;
+		hdr->count = pool_sizes[i].count;
+
+		for (j = 0; j < pool_sizes[i].count; j++) {
+			pool_free_entry *ent = (pool_free_entry *) p;
+			if (j == pool_sizes[i].count - 1) {
+				ent->next = (pool_free_entry *) NULL;
+			} else {
+				ent->next = (pool_free_entry *) (p + pool_sizes[i].size);
+			}
+			p += pool_sizes[i].size;
+		}
+	}
+
+	dump_pool_state(st);
+
+	/* Use 'st' as udata. */
+	return (void *) st;
+}
+
+void *duk_alloc_hybrid(void *udata, duk_size_t size) {
+	pool_state *st = (pool_state *) udata;
+	int i;
+	void *new_ptr;
+
+#if 0
+	dump_pool_state(st);
+#endif
+
+	if (size == 0) {
+		return NULL;
+	}
+	if (size > st->pool_max_size) {
+		printf("alloc fallback: %ld\n", (long) size);
+		return malloc(size);
+	}
+
+	for (i = 0; i < (int) NUM_POOLS; i++) {
+		pool_header *hdr = st->headers + i;
+		if (hdr->size < size) {
+			continue;
+		}
+
+		if (hdr->free) {
+#if 0
+			printf("alloc from pool: %ld -> pool size %ld\n", (long) size, (long) hdr->size);
+#endif
+			new_ptr = (void *) hdr->free;
+			hdr->free = hdr->free->next;
+			return new_ptr;
+		} else {
+			break;
+		}
+	}
+
+	printf("alloc fallback (out of pool): %ld\n", (long) size);
+	return malloc(size);
+}
+
+void *duk_realloc_hybrid(void *udata, void *ptr, duk_size_t size) {
+	pool_state *st = (pool_state *) udata;
+	void *new_ptr;
+	int i;
+
+#if 0
+	dump_pool_state(st);
+#endif
+
+	if (ADDR_IN_STATE_ALLOC(st, ptr)) {
+		/* 'ptr' cannot be NULL. */
+		for (i = 0; i < (int) NUM_POOLS; i++) {
+			pool_header *hdr = st->headers + i;
+			if (ADDR_IN_HEADER_ALLOC(hdr, ptr)) {
+				if (size <= hdr->size) {
+					/* Still fits, no shrink support. */
+#if 0
+					printf("realloc original from pool: still fits, size %ld, pool size %ld\n",
+					       (long) size, (long) hdr->size);
+#endif
+					return ptr;
+				}
+
+				new_ptr = duk_alloc_hybrid(udata, size);
+				if (!new_ptr) {
+					printf("realloc original from pool: needed larger size, failed to alloc\n");
+					return NULL;
+				}
+				memcpy(new_ptr, ptr, hdr->size);
+
+				((pool_free_entry *) ptr)->next = hdr->free;
+				hdr->free = (pool_free_entry *) ptr;
+#if 0
+				printf("realloc original from pool: size %ld, pool size %ld\n", (long) size, (long) hdr->size);
+#endif
+				return new_ptr;
+			}
+		}
+		printf("NEVER HERE\n");
+		return NULL;
+	} else if (ptr != NULL) {
+		if (size == 0) {
+			free(ptr);
+			return NULL;
+		} else {
+			printf("realloc fallback: size %ld\n", (long) size);
+			return realloc(ptr, size);
+		}
+	} else {
+#if 0
+		printf("realloc NULL ptr, call alloc: %ld\n", (long) size);
+#endif
+		return duk_alloc_hybrid(udata, size);
+	}
+}
+
+void duk_free_hybrid(void *udata, void *ptr) {
+	pool_state *st = (pool_state *) udata;
+	int i;
+
+#if 0
+	dump_pool_state(st);
+#endif
+
+	if (!ADDR_IN_STATE_ALLOC(st, ptr)) {
+		if (ptr == NULL) {
+			return;
+		}
+#if 0
+		printf("free out of pool: %p\n", (void *) ptr);
+#endif
+		free(ptr);
+		return;
+	}
+
+	for (i = 0; i < (int) NUM_POOLS; i++) {
+		pool_header *hdr = st->headers + i;
+		if (ADDR_IN_HEADER_ALLOC(hdr, ptr)) {
+			((pool_free_entry *) ptr)->next = hdr->free;
+			hdr->free = (pool_free_entry *) ptr;
+#if 0
+			printf("free from pool: %p\n", ptr);
+#endif
+			return;
+		}
+	}
+}

examples/alloc-hybrid/duk_alloc_hybrid.h

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+#ifndef DUK_ALLOC_HYBRID_H_INCLUDED
+#define DUK_ALLOC_HYBRID_H_INCLUDED
+
+#include "duktape.h"
+
+void *duk_alloc_hybrid_init(void);
+void *duk_alloc_hybrid(void *udata, duk_size_t size);
+void *duk_realloc_hybrid(void *udata, void *ptr, duk_size_t size);
+void duk_free_hybrid(void *udata, void *ptr);
+
+#endif  /* DUK_ALLOC_HYBRID_H_INCLUDED */