Use first-fit rather than first-best-fit run/chunk allocation.

This tends to more effectively pack active memory toward low addresses.
However, additional tree searches are required in many cases, so whether
this change stands the test of time will depend on real-world
benchmarks.

include/jemalloc/internal/arena.h

@@ -312,7 +312,7 @@ struct arena_s {
 
 	/*
 	 * Size/address-ordered tree of this arena's available runs.  The tree
-	 * is used for first-best-fit run allocation.
+	 * is used for first-fit run allocation.
 	 */
 	arena_avail_tree_t	runs_avail;
 

src/arena.c

@@ -907,23 +907,55 @@ arena_chunk_ralloc_huge_expand(arena_t *arena, void *chunk, size_t oldsize,
 	return (err);
 }
 
+/* Do first-fit run selection. */
 static arena_run_t *
-arena_run_alloc_large_helper(arena_t *arena, size_t size, bool zero)
+arena_run_first_fit(arena_t *arena, size_t size)
 {
-	arena_chunk_map_misc_t *miscelm;
-	arena_chunk_map_misc_t *key;
-	size_t usize;
+	arena_run_t *run;
+	index_t index, max_index;
+
+	assert(size == s2u(size));
+	assert(size == PAGE_CEILING(size));
+
+	/*
+	 * Iterate over all size classes that are at least large enough to
+	 * satisfy the request, search for the lowest run of each size class,
+	 * and choose the lowest of the runs found.
+	 */
+	run = NULL;
+	for (index = size2index(size), max_index = size2index(arena_maxclass);
+	    index <= max_index;) {
+		arena_run_t *currun;
+		arena_chunk_t *currun_chunk;
+		size_t currun_pageind, currun_size;
+		size_t usize = PAGE_CEILING(index2size(index));
+		arena_chunk_map_misc_t *key = (arena_chunk_map_misc_t *)(usize |
+		    CHUNK_MAP_KEY);
+		arena_chunk_map_misc_t *miscelm =
+		    arena_avail_tree_nsearch(&arena->runs_avail, key);
+		if (miscelm == NULL)
+			break;
+		currun = &miscelm->run;
+		if (run == NULL || (uintptr_t)currun < (uintptr_t)run)
+			run = currun;
+		currun_chunk = (arena_chunk_t *)CHUNK_ADDR2BASE(currun);
+		currun_pageind = arena_miscelm_to_pageind(miscelm);
+		currun_size = arena_mapbits_unallocated_size_get(currun_chunk,
+		    currun_pageind);
+		assert(size2index(currun_size) + 1 > index);
+		index = size2index(currun_size) + 1;
+	}
 
-	usize = s2u(size);
-	key = (arena_chunk_map_misc_t *)(usize | CHUNK_MAP_KEY);
-	miscelm = arena_avail_tree_nsearch(&arena->runs_avail, key);
-	if (miscelm != NULL) {
-		arena_run_t *run = &miscelm->run;
-		arena_run_split_large(arena, &miscelm->run, size, zero);
 	return (run);
 }
 
-	return (NULL);
+static arena_run_t *
+arena_run_alloc_large_helper(arena_t *arena, size_t size, bool zero)
+{
+	arena_run_t *run = arena_run_first_fit(arena, s2u(size));
+	if (run != NULL)
+		arena_run_split_large(arena, run, size, zero);
+	return (run);
 }
 
 static arena_run_t *
@@ -961,22 +993,12 @@ arena_run_alloc_large(arena_t *arena, size_t size, bool zero)
 static arena_run_t *
 arena_run_alloc_small_helper(arena_t *arena, size_t size, index_t binind)
 {
-	arena_run_t *run;
-	arena_chunk_map_misc_t *miscelm;
-	arena_chunk_map_misc_t *key;
-
-	assert(size == s2u(size));
-	key = (arena_chunk_map_misc_t *)(PAGE_CEILING(size) | CHUNK_MAP_KEY);
-	miscelm = arena_avail_tree_nsearch(&arena->runs_avail, key);
-	if (miscelm != NULL) {
-		run = &miscelm->run;
+	arena_run_t *run = arena_run_first_fit(arena, PAGE_CEILING(size));
+	if (run != NULL)
 		arena_run_split_small(arena, run, size, binind);
 	return (run);
 }
 
-	return (NULL);
-}
-
 static arena_run_t *
 arena_run_alloc_small(arena_t *arena, size_t size, index_t binind)
 {

src/chunk.c

@@ -62,6 +62,39 @@ chunk_deregister(const void *chunk, const extent_node_t *node)
 	}
 }
 
+/* Do first-fit chunk selection. */
+static extent_node_t *
+chunk_first_fit(arena_t *arena, extent_tree_t *chunks_szad, size_t size)
+{
+	extent_node_t *node;
+	index_t index;
+
+	assert(size == CHUNK_CEILING(size));
+
+	/*
+	 * Iterate over all size classes that are at least large enough to
+	 * satisfy the request, search for the lowest chunk of each size class,
+	 * and choose the lowest of the chunks found.
+	 */
+	node = NULL;
+	for (index = size2index(size); index < NSIZES;) {
+		extent_node_t *curnode;
+		extent_node_t key;
+		extent_node_init(&key, arena, NULL,
+		    CHUNK_CEILING(index2size(index)), false);
+		curnode = extent_tree_szad_nsearch(chunks_szad, &key);
+		if (curnode == NULL)
+			break;
+		if (node == NULL || (uintptr_t)extent_node_addr_get(curnode) <
+		    (uintptr_t)extent_node_addr_get(node))
+			node = curnode;
+		assert(size2index(extent_node_size_get(curnode)) + 1 > index);
+		index = size2index(extent_node_size_get(curnode)) + 1;
+	}
+
+	return (node);
+}
+
 static void *
 chunk_recycle(arena_t *arena, extent_tree_t *chunks_szad,
     extent_tree_t *chunks_ad, bool cache, void *new_addr, size_t size,
@@ -69,7 +102,6 @@ chunk_recycle(arena_t *arena, extent_tree_t *chunks_szad,
 {
 	void *ret;
 	extent_node_t *node;
-	extent_node_t key;
 	size_t alloc_size, leadsize, trailsize;
 	bool zeroed;
 
@@ -80,10 +112,13 @@ chunk_recycle(arena_t *arena, extent_tree_t *chunks_szad,
 	/* Beware size_t wrap-around. */
 	if (alloc_size < size)
 		return (NULL);
-	extent_node_init(&key, arena, new_addr, alloc_size, false);
 	malloc_mutex_lock(&arena->chunks_mtx);
-	node = (new_addr != NULL) ? extent_tree_ad_search(chunks_ad, &key) :
-	    extent_tree_szad_nsearch(chunks_szad, &key);
+	if (new_addr != NULL || size == chunksize) {
+		extent_node_t key;
+		extent_node_init(&key, arena, new_addr, alloc_size, false);
+		node = extent_tree_ad_search(chunks_ad, &key);
+	} else
+		node = chunk_first_fit(arena, chunks_szad, alloc_size);
 	if (node == NULL || (new_addr != NULL && extent_node_size_get(node) <
 	    size)) {
 		malloc_mutex_unlock(&arena->chunks_mtx);