diff --git a/jemalloc/doc/jemalloc.xml.in b/jemalloc/doc/jemalloc.xml.in
index 97893c16..2bde8904 100644
--- a/jemalloc/doc/jemalloc.xml.in
+++ b/jemalloc/doc/jemalloc.xml.in
@@ -1642,6 +1642,16 @@ malloc_conf = "xmalloc:true";]]></programlisting>
         </para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term>
+          <mallctl>stats.arenas.&lt;i&gt;.nthreads</mallctl>
+          (<type>unsigned</type>)
+          <literal>r-</literal>
+        </term>
+        <listitem><para>Number of threads currently assigned to
+        arena.</para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term>
           <mallctl>stats.arenas.&lt;i&gt;.pactive</mallctl>
diff --git a/jemalloc/include/jemalloc/internal/arena.h b/jemalloc/include/jemalloc/internal/arena.h
index 1744b45b..94b7f3d9 100644
--- a/jemalloc/include/jemalloc/internal/arena.h
+++ b/jemalloc/include/jemalloc/internal/arena.h
@@ -295,8 +295,18 @@ struct arena_s {
 	unsigned		ind;
 
 	/*
-	 * All non-bin-related operations on this arena require that lock be
-	 * locked.
+	 * Number of threads currently assigned to this arena.  This field is
+	 * protected by arenas_lock.
+	 */
+	unsigned		nthreads;
+
+	/*
+	 * There are three classes of arena operations from a locking
+	 * perspective:
+	 * 1) Thread asssignment (modifies nthreads) is protected by
+	 *    arenas_lock.
+	 * 2) Bin-related operations are protected by bin locks.
+	 * 3) Chunk- and run-related operations are protected by this mutex.
 	 */
 	malloc_mutex_t		lock;
 
diff --git a/jemalloc/include/jemalloc/internal/ctl.h b/jemalloc/include/jemalloc/internal/ctl.h
index 8776ad13..f1f5eb70 100644
--- a/jemalloc/include/jemalloc/internal/ctl.h
+++ b/jemalloc/include/jemalloc/internal/ctl.h
@@ -29,6 +29,7 @@ struct ctl_node_s {
 
 struct ctl_arena_stats_s {
 	bool			initialized;
+	unsigned		nthreads;
 	size_t			pactive;
 	size_t			pdirty;
 #ifdef JEMALLOC_STATS
diff --git a/jemalloc/include/jemalloc/internal/jemalloc_internal.h.in b/jemalloc/include/jemalloc/internal/jemalloc_internal.h.in
index a80fc7cd..a7472c00 100644
--- a/jemalloc/include/jemalloc/internal/jemalloc_internal.h.in
+++ b/jemalloc/include/jemalloc/internal/jemalloc_internal.h.in
@@ -293,6 +293,7 @@ extern size_t		lg_pagesize;
 extern unsigned		ncpus;
 
 extern malloc_mutex_t	arenas_lock; /* Protects arenas initialization. */
+extern pthread_key_t	arenas_tsd;
 #ifndef NO_TLS
 /*
  * Map of pthread_self() --> arenas[???], used for selecting an arena to use
@@ -302,9 +303,9 @@ extern __thread arena_t	*arenas_tls JEMALLOC_ATTR(tls_model("initial-exec"));
 #  define ARENA_GET()	arenas_tls
 #  define ARENA_SET(v)	do {						\
 	arenas_tls = (v);						\
+	pthread_setspecific(arenas_tsd, (void *)(v));			\
 } while (0)
 #else
-extern pthread_key_t	arenas_tsd;
 #  define ARENA_GET()	((arena_t *)pthread_getspecific(arenas_tsd))
 #  define ARENA_SET(v)	do {						\
 	pthread_setspecific(arenas_tsd, (void *)(v));			\
diff --git a/jemalloc/src/arena.c b/jemalloc/src/arena.c
index a1fa2a32..022f9ec3 100644
--- a/jemalloc/src/arena.c
+++ b/jemalloc/src/arena.c
@@ -2175,6 +2175,7 @@ arena_new(arena_t *arena, unsigned ind)
 	arena_bin_t *bin;
 
 	arena->ind = ind;
+	arena->nthreads = 0;
 
 	if (malloc_mutex_init(&arena->lock))
 		return (true);
diff --git a/jemalloc/src/ctl.c b/jemalloc/src/ctl.c
index c32e955e..b4f280dc 100644
--- a/jemalloc/src/ctl.c
+++ b/jemalloc/src/ctl.c
@@ -182,6 +182,7 @@ CTL_PROTO(stats_arenas_i_lruns_j_highruns)
 CTL_PROTO(stats_arenas_i_lruns_j_curruns)
 INDEX_PROTO(stats_arenas_i_lruns_j)
 #endif
+CTL_PROTO(stats_arenas_i_nthreads)
 CTL_PROTO(stats_arenas_i_pactive)
 CTL_PROTO(stats_arenas_i_pdirty)
 #ifdef JEMALLOC_STATS
@@ -434,6 +435,7 @@ static const ctl_node_t stats_arenas_i_lruns_node[] = {
 #endif
 
 static const ctl_node_t stats_arenas_i_node[] = {
+	{NAME("nthreads"),		CTL(stats_arenas_i_nthreads)},
 	{NAME("pactive"),		CTL(stats_arenas_i_pactive)},
 	{NAME("pdirty"),		CTL(stats_arenas_i_pdirty)}
 #ifdef JEMALLOC_STATS
@@ -620,6 +622,7 @@ ctl_arena_refresh(arena_t *arena, unsigned i)
 
 	ctl_arena_clear(astats);
 
+	sstats->nthreads += astats->nthreads;
 #ifdef JEMALLOC_STATS
 	ctl_arena_stats_amerge(astats, arena);
 	/* Merge into sum stats as well. */
@@ -657,10 +660,17 @@ ctl_refresh(void)
 	 * Clear sum stats, since they will be merged into by
 	 * ctl_arena_refresh().
 	 */
+	ctl_stats.arenas[narenas].nthreads = 0;
 	ctl_arena_clear(&ctl_stats.arenas[narenas]);
 
 	malloc_mutex_lock(&arenas_lock);
 	memcpy(tarenas, arenas, sizeof(arena_t *) * narenas);
+	for (i = 0; i < narenas; i++) {
+		if (arenas[i] != NULL)
+			ctl_stats.arenas[i].nthreads = arenas[i]->nthreads;
+		else
+			ctl_stats.arenas[i].nthreads = 0;
+	}
 	malloc_mutex_unlock(&arenas_lock);
 	for (i = 0; i < narenas; i++) {
 		bool initialized = (tarenas[i] != NULL);
@@ -1129,6 +1139,8 @@ thread_arena_ctl(const size_t *mib, size_t miblen, void *oldp, size_t *oldlenp,
 		malloc_mutex_lock(&arenas_lock);
 		if ((arena = arenas[newind]) == NULL)
 			arena = arenas_extend(newind);
+		arenas[oldind]->nthreads--;
+		arenas[newind]->nthreads++;
 		malloc_mutex_unlock(&arenas_lock);
 		if (arena == NULL) {
 			ret = EAGAIN;
@@ -1536,6 +1548,7 @@ stats_arenas_i_lruns_j_index(const size_t *mib, size_t miblen, size_t j)
 }
 
 #endif
+CTL_RO_GEN(stats_arenas_i_nthreads, ctl_stats.arenas[mib[2]].nthreads, unsigned)
 CTL_RO_GEN(stats_arenas_i_pactive, ctl_stats.arenas[mib[2]].pactive, size_t)
 CTL_RO_GEN(stats_arenas_i_pdirty, ctl_stats.arenas[mib[2]].pdirty, size_t)
 #ifdef JEMALLOC_STATS
diff --git a/jemalloc/src/jemalloc.c b/jemalloc/src/jemalloc.c
index 9f2fa92e..ecd521c9 100644
--- a/jemalloc/src/jemalloc.c
+++ b/jemalloc/src/jemalloc.c
@@ -7,12 +7,10 @@
 malloc_mutex_t		arenas_lock;
 arena_t			**arenas;
 unsigned		narenas;
-static unsigned		next_arena;
 
+pthread_key_t		arenas_tsd;
 #ifndef NO_TLS
 __thread arena_t	*arenas_tls JEMALLOC_ATTR(tls_model("initial-exec"));
-#else
-pthread_key_t		arenas_tsd;
 #endif
 
 #ifdef JEMALLOC_STATS
@@ -70,6 +68,7 @@ size_t	opt_narenas = 0;
 static void	wrtmessage(void *cbopaque, const char *s);
 static void	stats_print_atexit(void);
 static unsigned	malloc_ncpus(void);
+static void	arenas_cleanup(void *arg);
 #if (defined(JEMALLOC_STATS) && defined(NO_TLS))
 static void	thread_allocated_cleanup(void *arg);
 #endif
@@ -147,13 +146,53 @@ choose_arena_hard(void)
 	arena_t *ret;
 
 	if (narenas > 1) {
+		unsigned i, choose, first_null;
+
+		choose = 0;
+		first_null = narenas;
 		malloc_mutex_lock(&arenas_lock);
-		if ((ret = arenas[next_arena]) == NULL)
-			ret = arenas_extend(next_arena);
-		next_arena = (next_arena + 1) % narenas;
+		assert(arenas[i] != NULL);
+		for (i = 1; i < narenas; i++) {
+			if (arenas[i] != NULL) {
+				/*
+				 * Choose the first arena that has the lowest
+				 * number of threads assigned to it.
+				 */
+				if (arenas[i]->nthreads <
+				    arenas[choose]->nthreads)
+					choose = i;
+			} else if (first_null == narenas) {
+				/*
+				 * Record the index of the first uninitialized
+				 * arena, in case all extant arenas are in use.
+				 *
+				 * NB: It is possible for there to be
+				 * discontinuities in terms of initialized
+				 * versus uninitialized arenas, due to the
+				 * "thread.arena" mallctl.
+				 */
+				first_null = i;
+			}
+		}
+
+		if (arenas[choose] == 0 || first_null == narenas) {
+			/*
+			 * Use an unloaded arena, or the least loaded arena if
+			 * all arenas are already initialized.
+			 */
+			ret = arenas[choose];
+		} else {
+			/* Initialize a new arena. */
+			ret = arenas_extend(first_null);
+		}
+		ret->nthreads++;
 		malloc_mutex_unlock(&arenas_lock);
-	} else
+	} else {
 		ret = arenas[0];
+		malloc_mutex_lock(&arenas_lock);
+		ret->nthreads++;
+		malloc_mutex_unlock(&arenas_lock);
+	}
 
 	ARENA_SET(ret);
 
@@ -259,6 +298,16 @@ malloc_ncpus(void)
 	return (ret);
 }
 
+static void
+arenas_cleanup(void *arg)
+{
+	arena_t *arena = (arena_t *)arg;
+
+	malloc_mutex_lock(&arenas_lock);
+	arena->nthreads--;
+	malloc_mutex_unlock(&arenas_lock);
+}
+
 #if (defined(JEMALLOC_STATS) && defined(NO_TLS))
 static void
 thread_allocated_cleanup(void *arg)
@@ -737,6 +786,7 @@ malloc_init_hard(void)
 	 * threaded mode.
 	 */
 	ARENA_SET(arenas[0]);
+	arenas[0]->nthreads++;
 
 	if (malloc_mutex_init(&arenas_lock))
 		return (true);
@@ -779,14 +829,10 @@ malloc_init_hard(void)
 		malloc_write(")\n");
 	}
 
-	next_arena = (narenas > 0) ? 1 : 0;
-
-#ifdef NO_TLS
-	if (pthread_key_create(&arenas_tsd, NULL) != 0) {
+	if (pthread_key_create(&arenas_tsd, arenas_cleanup) != 0) {
 		malloc_mutex_unlock(&init_lock);
 		return (true);
 	}
-#endif
 
 	/* Allocate and initialize arenas. */
 	arenas = (arena_t **)base_alloc(sizeof(arena_t *) * narenas);
@@ -819,7 +865,6 @@ malloc_init_hard(void)
 	return (false);
 }
 
-
 #ifdef JEMALLOC_ZONE
 JEMALLOC_ATTR(constructor)
 void
diff --git a/jemalloc/src/stats.c b/jemalloc/src/stats.c
index 3dfe0d23..81105c45 100644
--- a/jemalloc/src/stats.c
+++ b/jemalloc/src/stats.c
@@ -319,6 +319,7 @@ static void
 stats_arena_print(void (*write_cb)(void *, const char *), void *cbopaque,
     unsigned i)
 {
+	unsigned nthreads;
 	size_t pagesize, pactive, pdirty, mapped;
 	uint64_t npurge, nmadvise, purged;
 	size_t small_allocated;
@@ -328,6 +329,9 @@ stats_arena_print(void (*write_cb)(void *, const char *), void *cbopaque,
 
 	CTL_GET("arenas.pagesize", &pagesize, size_t);
 
+	CTL_I_GET("stats.arenas.0.nthreads", &nthreads, unsigned);
+	malloc_cprintf(write_cb, cbopaque,
+	    "assigned threads: %u\n", nthreads);
 	CTL_I_GET("stats.arenas.0.pactive", &pactive, size_t);
 	CTL_I_GET("stats.arenas.0.pdirty", &pdirty, size_t);
 	CTL_I_GET("stats.arenas.0.npurge", &npurge, uint64_t);