Add opt.lg_extent_max_active_fit

When allocating from dirty extents (which we always prefer if available), large
active extents can get split even if the new allocation is much smaller, in
which case the introduced fragmentation causes high long term damage.  This new
option controls the threshold to reuse and split an existing active extent.  We
avoid using a large extent for much smaller sizes, in order to reduce
fragmentation.  In some workload, adding the threshold improves virtual memory
usage by >10x.

doc/jemalloc.xml.in

@@ -1069,6 +1069,22 @@ mallctl("arena." STRINGIFY(MALLCTL_ARENAS_ALL) ".decay",
         for related dynamic control options.</para></listitem>
       </varlistentry>
 
+      <varlistentry id="opt.lg_extent_max_active_fit">
+        <term>
+          <mallctl>opt.lg_extent_max_active_fit</mallctl>
+          (<type>size_t</type>)
+          <literal>r-</literal>
+        </term>
+        <listitem><para>When reusing dirty extents, this determines the (log
+        base 2 of the) maximum ratio between the size of the active extent
+        selected (to split off from) and the size of the requested allocation.
+        This prevents the splitting of large active extents for smaller
+        allocations, which can reduce fragmentation over the long run
+        (especially for non-active extents).  Lower value may reduce
+        fragmentation, at the cost of extra active extents.  The default value
+        is 6, which gives a maximum ratio of 64 (2^6).</para></listitem>
+      </varlistentry>
+
       <varlistentry id="opt.stats_print">
         <term>
           <mallctl>opt.stats_print</mallctl>

include/jemalloc/internal/extent_externs.h

@@ -6,6 +6,8 @@
 #include "jemalloc/internal/ph.h"
 #include "jemalloc/internal/rtree.h"
 
+extern size_t opt_lg_extent_max_active_fit;
+
 extern rtree_t extents_rtree;
 extern const extent_hooks_t extent_hooks_default;
 extern mutex_pool_t extent_mutex_pool;

include/jemalloc/internal/extent_types.h

@@ -8,4 +8,10 @@ typedef struct extents_s extents_t;
 
 #define EXTENT_GROW_MAX_PIND (NPSIZES - 1)
 
+/*
+ * When reuse (and split) an active extent, (1U << opt_lg_extent_max_active_fit)
+ * is the max ratio between the size of the active extent and the new extent.
+ */
+#define LG_EXTENT_MAX_ACTIVE_FIT_DEFAULT 6
+
 #endif /* JEMALLOC_INTERNAL_EXTENT_TYPES_H */

src/ctl.c

@@ -95,6 +95,7 @@ CTL_PROTO(opt_zero)
 CTL_PROTO(opt_utrace)
 CTL_PROTO(opt_xmalloc)
 CTL_PROTO(opt_tcache)
+CTL_PROTO(opt_lg_extent_max_active_fit)
 CTL_PROTO(opt_lg_tcache_max)
 CTL_PROTO(opt_prof)
 CTL_PROTO(opt_prof_prefix)
@@ -293,6 +294,7 @@ static const ctl_named_node_t opt_node[] = {
 	{NAME("utrace"),	CTL(opt_utrace)},
 	{NAME("xmalloc"),	CTL(opt_xmalloc)},
 	{NAME("tcache"),	CTL(opt_tcache)},
+	{NAME("lg_extent_max_active_fit"), CTL(opt_lg_extent_max_active_fit)},
 	{NAME("lg_tcache_max"),	CTL(opt_lg_tcache_max)},
 	{NAME("prof"),		CTL(opt_prof)},
 	{NAME("prof_prefix"),	CTL(opt_prof_prefix)},
@@ -1597,6 +1599,8 @@ CTL_RO_NL_CGEN(config_fill, opt_zero, opt_zero, bool)
 CTL_RO_NL_CGEN(config_utrace, opt_utrace, opt_utrace, bool)
 CTL_RO_NL_CGEN(config_xmalloc, opt_xmalloc, opt_xmalloc, bool)
 CTL_RO_NL_GEN(opt_tcache, opt_tcache, bool)
+CTL_RO_NL_GEN(opt_lg_extent_max_active_fit, opt_lg_extent_max_active_fit,
+    size_t)
 CTL_RO_NL_GEN(opt_lg_tcache_max, opt_lg_tcache_max, ssize_t)
 CTL_RO_NL_CGEN(config_prof, opt_prof, opt_prof, bool)
 CTL_RO_NL_CGEN(config_prof, opt_prof_prefix, opt_prof_prefix, const char *)

src/extent.c

@@ -17,6 +17,8 @@ rtree_t		extents_rtree;
 /* Keyed by the address of the extent_t being protected. */
 mutex_pool_t	extent_mutex_pool;
 
+size_t opt_lg_extent_max_active_fit = LG_EXTENT_MAX_ACTIVE_FIT_DEFAULT;
+
 static const bitmap_info_t extents_bitmap_info =
     BITMAP_INFO_INITIALIZER(NPSIZES+1);
 
@@ -369,6 +371,13 @@ extents_best_fit_locked(tsdn_t *tsdn, arena_t *arena, extents_t *extents,
 	pszind_t i = (pszind_t)bitmap_ffu(extents->bitmap, &extents_bitmap_info,
 	    (size_t)pind);
 	if (i < NPSIZES+1) {
+		/*
+		 * In order to reduce fragmentation, avoid reusing and splitting
+		 * large extents for much smaller sizes.
+		 */
+		if ((sz_pind2sz(i) >> opt_lg_extent_max_active_fit) > size) {
+			return NULL;
+		}
 		assert(!extent_heap_empty(&extents->heaps[i]));
 		extent_t *extent = extent_heap_first(&extents->heaps[i]);
 		assert(extent_size_get(extent) >= size);

src/jemalloc.c

@@ -1146,6 +1146,9 @@ malloc_conf_init(void) {
 				CONF_HANDLE_BOOL(opt_xmalloc, "xmalloc")
 			}
 			CONF_HANDLE_BOOL(opt_tcache, "tcache")
+			CONF_HANDLE_SIZE_T(opt_lg_extent_max_active_fit,
+			    "lg_extent_max_active_fit", 0,
+			    (sizeof(size_t) << 3), yes, yes, false)
 			CONF_HANDLE_SSIZE_T(opt_lg_tcache_max, "lg_tcache_max",
 			    -1, (sizeof(size_t) << 3) - 1)
 			if (strncmp("percpu_arena", k, klen) == 0) {

test/unit/mallctl.c

@@ -172,6 +172,7 @@ TEST_BEGIN(test_mallctl_opt) {
 	TEST_MALLCTL_OPT(bool, utrace, utrace);
 	TEST_MALLCTL_OPT(bool, xmalloc, xmalloc);
 	TEST_MALLCTL_OPT(bool, tcache, always);
+	TEST_MALLCTL_OPT(size_t, lg_extent_max_active_fit, always);
 	TEST_MALLCTL_OPT(size_t, lg_tcache_max, always);
 	TEST_MALLCTL_OPT(bool, prof, prof);
 	TEST_MALLCTL_OPT(const char *, prof_prefix, prof);