diff --git a/c_src/async_nif.h b/c_src/async_nif.h
index 9f5b94f..c18106c 100644
--- a/c_src/async_nif.h
+++ b/c_src/async_nif.h
@@ -25,6 +25,7 @@ extern "C" {
 #endif
 
 #include <assert.h>
+
 #include "queue.h"
 #include "stats.h"
 
@@ -33,6 +34,7 @@ extern "C" {
 #endif
 
 #define ASYNC_NIF_MAX_WORKERS 1024
+#define ASYNC_NIF_MIN_WORKERS 2
 #define ASYNC_NIF_WORKER_QUEUE_SIZE 1000
 #define ASYNC_NIF_MAX_QUEUED_REQS ASYNC_NIF_WORKER_QUEUE_SIZE * ASYNC_NIF_MAX_WORKERS
 
@@ -43,17 +45,18 @@ struct async_nif_req_entry {
   void *args;
   void (*fn_work)(ErlNifEnv*, ERL_NIF_TERM, ErlNifPid*, unsigned int, void *);
   void (*fn_post)(void *);
+  uint64_t submitted;
   STAILQ_ENTRY(async_nif_req_entry) entries;
 };
 
 
 struct async_nif_work_queue {
   unsigned int num_workers;
+  unsigned int depth;
   ErlNifMutex *reqs_mutex;
   ErlNifCond *reqs_cnd;
   STAILQ_HEAD(reqs, async_nif_req_entry) reqs;
-  STAT_DEF(qwait);
-  STAT_DEF(wt);
+  STAT_DEF(work);
 };
 
 struct async_nif_worker_entry {
@@ -65,6 +68,8 @@ struct async_nif_worker_entry {
 };
 
 struct async_nif_state {
+  STAT_DEF(wait);
+  STAT_DEF(service);
   unsigned int shutdown;
   ErlNifMutex *we_mutex;
   unsigned int we_active;
@@ -103,10 +108,12 @@ struct async_nif_state {
     argc -= 1;                                                          \
     /* Note: !!! this assumes that the first element of priv_data is ours */ \
     struct async_nif_state *async_nif = *(struct async_nif_state**)enif_priv_data(env); \
-    if (async_nif->shutdown)                                            \
+    if (async_nif->shutdown) {                                          \
       return enif_make_tuple2(env, enif_make_atom(env, "error"),        \
                               enif_make_atom(env, "shutdown"));         \
+    }                                                                   \
     req = async_nif_reuse_req(async_nif);                               \
+    req->submitted = ts(ns);                                            \
     if (!req) {                                                         \
         return enif_make_tuple2(env, enif_make_atom(env, "error"),      \
                                 enif_make_atom(env, "eagain"));         \
@@ -137,7 +144,7 @@ struct async_nif_state {
       async_nif_recycle_req(req, async_nif);                           \
       enif_free(copy_of_args);                                         \
       return enif_make_tuple2(env, enif_make_atom(env, "error"),       \
-                              enif_make_atom(env, "eagain"));        \
+                              enif_make_atom(env, "eagain"));          \
     }                                                                  \
     return reply;                                                      \
   }
@@ -246,7 +253,6 @@ async_nif_start_worker(struct async_nif_state *async_nif, struct async_nif_work_
 
   enif_mutex_lock(async_nif->we_mutex);
 
-#if 0 // TODO:
   we = SLIST_FIRST(&async_nif->we_joining);
   while(we != NULL) {
     struct async_nif_worker_entry *n = SLIST_NEXT(we, entries);
@@ -257,7 +263,6 @@ async_nif_start_worker(struct async_nif_state *async_nif, struct async_nif_work_
     async_nif->we_active--;
     we = n;
   }
-#endif
 
   if (async_nif->we_active == ASYNC_NIF_MAX_WORKERS) {
       enif_mutex_unlock(async_nif->we_mutex);
@@ -289,9 +294,8 @@ async_nif_enqueue_req(struct async_nif_state* async_nif, struct async_nif_req_en
 {
   /* Identify the most appropriate worker for this request. */
   unsigned int i, qid = 0;
-  unsigned int n = async_nif->num_queues;
   struct async_nif_work_queue *q = NULL;
-  double avg_wait_across_q, avg_wt_service_time, avg_wait_this_q = 0;
+  double avg_depth;
 
   /* Either we're choosing a queue based on some affinity/hinted value or we
      need to select the next queue in the rotation and atomically update that
@@ -304,52 +308,66 @@ async_nif_enqueue_req(struct async_nif_state* async_nif, struct async_nif_req_en
       async_nif->next_q = qid;
   }
 
-  avg_wait_across_q = 0;
-  for (i = 0; i < async_nif->num_queues; i++) {
-      avg_wait_across_q += __stat_mean(async_nif->queues[i].qwait_stat);
-  }
-  if (avg_wait_across_q != 0)
-      avg_wait_across_q /= async_nif->num_queues;
-
   /* Now we inspect and interate across the set of queues trying to select one
      that isn't too full or too slow. */
-  do {
+  for (i = 0; i < async_nif->num_queues; i++) {
+      /* Compute the average queue depth not counting queues which are empty or
+         the queue we're considering right now. */
+      unsigned int j, n = 0;
+      for (j = 0; j < async_nif->num_queues; j++) {
+          if (j != qid && async_nif->queues[j].depth != 0) {
+              n++;
+              avg_depth += async_nif->queues[j].depth;
+          }
+      }
+      if (avg_depth != 0)
+          avg_depth /= n;
+
+      /* Lock this queue under consideration, then check for shutdown.  While
+         we hold this lock either a) we're shutting down so exit now or b) this
+         queue will be valid until we release the lock. */
       q = &async_nif->queues[qid];
       enif_mutex_lock(q->reqs_mutex);
-
-      /* Now that we hold the lock, check for shutdown.  As long as we hold
-         this lock either a) we're shutting down so exit now or b) this queue
-         will be valid until we release the lock. */
-
       if (async_nif->shutdown) {
           enif_mutex_unlock(q->reqs_mutex);
           return 0;
       }
 
-      avg_wait_this_q = __stat_mean(q->qwait_stat);
-      avg_wt_service_time = __stat_mean(q->wt_stat);
-      DPRINTF("q:%d w:%u %f/%f(%f) %c", qid, q->num_workers, avg_wait_this_q, avg_wait_across_q, avg_wt_service_time, avg_wait_this_q <= avg_wait_across_q ? 't' : 'f');
-      if (avg_wait_this_q <= avg_wait_across_q) break;
+      /* Try not to enqueue a request into a queue that isn't keeping up with
+         the request volume. */
+      if (q->depth <= avg_depth) break;
       else {
-	  enif_mutex_unlock(q->reqs_mutex);
-	  qid = (qid + 1) % async_nif->num_queues;
-	  q = &async_nif->queues[qid];
+          enif_mutex_unlock(q->reqs_mutex);
+          qid = (qid + 1) % async_nif->num_queues;
       }
-  } while(n-- > 0);
+  }
 
-  if (n == 0) return 0; // All queues are full, trigger eagain
+  /* If the for loop finished then we didn't find a suitable queue for this
+     request, meaning we're backed up so trigger eagain. */
+  if (i == async_nif->num_queues) {
+      enif_mutex_unlock(q->reqs_mutex);
+      return 0;
+  }
 
-  /* We hold the queue's lock, and we've seletect a reasonable queue for this
-     new request now check to make sure there are enough workers actively
-     processing requests on this queue. */
-  if (q->num_workers == 0 || avg_wait_this_q >= avg_wt_service_time) {
-      if (async_nif_start_worker(async_nif, q) == 0)
-	  q->num_workers++;
+  /* We've selected queue for this new request now check to make sure there are
+     enough workers actively processing requests on this queue. */
+  if (q->num_workers < ASYNC_NIF_MIN_WORKERS) {
+      if (async_nif_start_worker(async_nif, q) == 0) q->num_workers++;
+  } else {
+      /* If more the 1/4 of the time it takes to complete a request is spent in
+         waiting to be serviced then we need more worker threads to service
+         requests. */
+      double m, n;
+      m = __stat_mean(async_nif->wait_stat);
+      n = __stat_mean(q->work_stat);
+      DPRINTF("w: %f\ts: %f\t%f", m, n, n/(m+n));
+      if (m && n && n / (m+n)  < 0.75)
+          if (async_nif_start_worker(async_nif, q) == 0) q->num_workers++;
   }
 
   /* And finally add the request to the queue. */
-  __stat_tick(q->qwait_stat);
   STAILQ_INSERT_TAIL(&q->reqs, req, entries);
+  q->depth++;
 
   /* Build the term before releasing the lock so as not to race on the use of
      the req pointer (which will soon become invalid in another thread
@@ -385,36 +403,40 @@ async_nif_worker_fn(void *arg)
     }
     if (STAILQ_EMPTY(&q->reqs)) {
       /* Queue is empty so we wait for more work to arrive. */
-      if (q->num_workers > 2) {
-	  enif_mutex_unlock(q->reqs_mutex);
-	  break;
+      if (q->num_workers > ASYNC_NIF_MIN_WORKERS) {
+          enif_mutex_unlock(q->reqs_mutex);
+          break;
       } else {
-	  enif_cond_wait(q->reqs_cnd, q->reqs_mutex);
-	  goto check_again_for_work;
+          enif_cond_wait(q->reqs_cnd, q->reqs_mutex);
+          goto check_again_for_work;
       }
     } else {
       /* At this point the next req is ours to process and we hold the
          reqs_mutex lock.  Take the request off the queue. */
       req = STAILQ_FIRST(&q->reqs);
       STAILQ_REMOVE(&q->reqs, req, async_nif_req_entry, entries);
+      q->depth--;
 
       /* Ensure that there is at least one other worker thread watching this
          queue. */
       enif_cond_signal(q->reqs_cnd);
       enif_mutex_unlock(q->reqs_mutex);
 
-      __stat_tock(q->qwait_stat);
-
       /* Perform the work. */
-      __stat_tick(q->wt_stat);
+      uint64_t then = ts(q->work_stat->d.unit);
+      uint64_t wait = then - req->submitted;
       req->fn_work(req->env, req->ref, &req->pid, worker_id, req->args);
-      __stat_tock(q->wt_stat);
+      uint64_t work = ts(q->work_stat->d.unit) - then;
+      __stat_add(async_nif->wait_stat, wait);
+      __stat_add(q->work_stat, work);
+      __stat_add(async_nif->service_stat, wait + work);
 
       /* Now call the post-work cleanup function. */
       req->fn_post(req->args);
 
       /* Clean up req for reuse. */
       req->ref = 0;
+      req->submitted = 0;
       req->fn_work = 0;
       req->fn_post = 0;
       enif_free(req->args);
@@ -427,10 +449,6 @@ async_nif_worker_fn(void *arg)
   SLIST_INSERT_HEAD(&async_nif->we_joining, we, entries);
   enif_mutex_unlock(async_nif->we_mutex);
   q->num_workers--;
-  if (q->num_workers == 0) {
-      __stat_reset(q->qwait_stat);
-      __stat_reset(q->wt_stat);
-  }
   enif_thread_exit(0);
   return 0;
 }
@@ -445,12 +463,11 @@ async_nif_unload(ErlNifEnv *env, struct async_nif_state *async_nif)
   struct async_nif_worker_entry *we = NULL;
   UNUSED(env);
 
-  /* Signal the worker threads, stop what you're doing and exit.  To
-     ensure that we don't race with the enqueue() process we first
-     lock all the worker queues, then set shutdown to true, then
-     unlock.  The enqueue function will take the queue mutex, then
-     test for shutdown condition, then enqueue only if not shutting
-     down. */
+  /* Signal the worker threads, stop what you're doing and exit.  To ensure
+     that we don't race with the enqueue() process we first lock all the worker
+     queues, then set shutdown to true, then unlock.  The enqueue function will
+     take the queue mutex, then test for shutdown condition, then enqueue only
+     if not shutting down. */
   for (i = 0; i < num_queues; i++) {
       q = &async_nif->queues[i];
       enif_mutex_lock(q->reqs_mutex);
@@ -462,26 +479,25 @@ async_nif_unload(ErlNifEnv *env, struct async_nif_state *async_nif)
 
   for (i = 0; i < num_queues; i++) {
       q = &async_nif->queues[i];
-      __stat_print_histogram(async_nif->queues[i].qwait_stat, "wterl q-wait");
       enif_mutex_unlock(q->reqs_mutex);
   }
-  __stat_print_histogram(async_nif->queues[i].qwait_stat, "wterl service time");
+  __stat_print_histogram(async_nif->service_stat, "wterl");
 
   /* Join for the now exiting worker threads. */
   while(async_nif->we_active > 0) {
 
       for (i = 0; i < num_queues; i++)
-	  enif_cond_broadcast(async_nif->queues[i].reqs_cnd);
+          enif_cond_broadcast(async_nif->queues[i].reqs_cnd);
 
       we = SLIST_FIRST(&async_nif->we_joining);
       while(we != NULL) {
-	  struct async_nif_worker_entry *n = SLIST_NEXT(we, entries);
-	  SLIST_REMOVE_HEAD(&async_nif->we_joining, entries);
-	  void *exit_value = 0; /* We ignore the thread_join's exit value. */
-	  enif_thread_join(we->tid, &exit_value);
-	  enif_free(we);
-	  async_nif->we_active--;
-	  we = n;
+          struct async_nif_worker_entry *n = SLIST_NEXT(we, entries);
+          SLIST_REMOVE_HEAD(&async_nif->we_joining, entries);
+          void *exit_value = 0; /* We ignore the thread_join's exit value. */
+          enif_thread_join(we->tid, &exit_value);
+          enif_free(we);
+          async_nif->we_active--;
+          we = n;
       }
   }
   enif_mutex_destroy(async_nif->we_mutex);
@@ -570,14 +586,15 @@ async_nif_load()
   async_nif->recycled_req_mutex = enif_mutex_create(NULL);
   async_nif->we_mutex = enif_mutex_create(NULL);
   SLIST_INIT(&async_nif->we_joining);
+  async_nif->wait_stat = __stat_init(100000);
+  async_nif->service_stat = __stat_init(100000);
 
   for (i = 0; i < async_nif->num_queues; i++) {
       struct async_nif_work_queue *q = &async_nif->queues[i];
       STAILQ_INIT(&q->reqs);
       q->reqs_mutex = enif_mutex_create(NULL);
       q->reqs_cnd = enif_cond_create(NULL);
-      q->qwait_stat = __stat_init(100000);
-      q->wt_stat = __stat_init(100000);
+      q->work_stat = __stat_init(100000);
   }
   return async_nif;
 }
diff --git a/c_src/stats.c b/c_src/stats.c
index 9e6938c..d018c76 100644
--- a/c_src/stats.c
+++ b/c_src/stats.c
@@ -64,18 +64,20 @@ __stat_mean(struct stat *s)
     double mean;
 
     if (!s)
-	return 0.0;
+        return 0.0;
 
+    enif_mutex_lock(s->mutex);
     t = s->h;
     h = (s->h + 1) % s->num_samples;
     mean = 0;
 
     while (h != t) {
-	mean += s->samples[h];
-	h = (h + 1) % s->num_samples;
+        mean += s->samples[h];
+        h = (h + 1) % s->num_samples;
     }
     if (mean > 0)
-	mean /= (s->n < s->num_samples ? (double)s->n : (double)s->num_samples);
+        mean /= (s->n < s->num_samples ? (double)s->n : (double)s->num_samples);
+    enif_mutex_unlock(s->mutex);
     return mean;
 }
 
@@ -86,13 +88,14 @@ __stat_mean_log2(struct stat *s)
     double mean = 0.0;
 
     if (!s)
-	return 0.0;
+        return 0.0;
 
+    enif_mutex_lock(s->mutex);
     for (i = 0; i < 64; i++)
-	mean += (s->histogram[i] * i);
+        mean += (s->histogram[i] * i);
     if (mean > 0)
-	mean /= (s->n < s->num_samples ? s->n : s->num_samples);
-    DPRINTF("n: %u %llu %f", s->n < 64 ? s->n : 64, PRIuint64(s), mean);
+        mean /= (s->n < s->num_samples ? s->n : s->num_samples);
+    enif_mutex_unlock(s->mutex);
     return mean;
 }
 
@@ -102,10 +105,12 @@ __stat_tick(struct stat *s)
     uint64_t t;
 
     if (!s)
-	return 0.0;
+        return 0.0;
 
+    enif_mutex_lock(s->mutex);
     t = ts(s->d.unit);
     s->d.then = t;
+    enif_mutex_unlock(s->mutex);
     return t;
 }
 
@@ -113,45 +118,48 @@ void
 __stat_reset(struct stat *s)
 {
     if (!s)
-	return;
+        return;
 
+    enif_mutex_lock(s->mutex);
     s->h = 0;
     s->d.unit = ns;
     s->d.then = 0;
     memset(s->histogram, 0, sizeof(uint64_t) * 64);
     memset(s->samples, 0, sizeof(uint64_t) * s->num_samples);
+    enif_mutex_unlock(s->mutex);
 }
 
-uint64_t
-__stat_tock(struct stat *s)
+void
+__stat_add(struct stat *s, uint64_t elapsed)
 {
-    uint64_t now;
-    uint64_t elapsed;
     uint32_t i;
-    duration_t *d;
 
-    if (!s)
-	return 0.0;
-
-    d = &s->d;
-    now = ts(d->unit);
-    elapsed = now - d->then;
-    i = s->h;
+    enif_mutex_lock(s->mutex);
     if (s->n == s->num_samples) {
-	s->mean = (s->mean + __stat_mean(s)) / 2.0;
-	if (s->n >= 4294967295)
-	    __stat_reset(s);
+        s->mean = (s->mean + __stat_mean(s)) / 2.0;
+        if (s->n >= 4294967295) {
+            enif_mutex_unlock(s->mutex);
+            __stat_reset(s);
+            enif_mutex_lock(s->mutex);
+        }
     }
+    i = s->h;
     s->h = (s->h + 1) % s->num_samples;
     s->samples[i] = elapsed;
     if (elapsed != 0 && elapsed < s->min)
-	s->min = elapsed;
+        s->min = elapsed;
     if (elapsed > s->max)
-	s->max = elapsed;
+        s->max = elapsed;
     s->histogram[LOG2(elapsed)]++;
     s->n++;
-    d->then = ts(d->unit);
-    return elapsed;
+    enif_mutex_unlock(s->mutex);
+}
+
+void
+__stat_tock(struct stat *s)
+{
+    if (s)
+        __stat_add(s, ts(s->d.unit));
 }
 
 void
@@ -162,68 +170,73 @@ __stat_print_histogram(struct stat *s, const char *mod)
     double m;
 
     if (!s)
-	return;
+        return;
 
+    enif_mutex_lock(s->mutex);
     m = (s->mean + __stat_mean(s) / 2.0);
 
     fprintf(stderr, "%s:async_nif request latency histogram:\n", mod);
     for (i = 0; i < 64; i++) {
-	logs[i] = LOG2(s->histogram[i]);
-	if (logs[i] > max_log)
-	    max_log = logs[i];
+        logs[i] = LOG2(s->histogram[i]);
+        if (logs[i] > max_log)
+            max_log = logs[i];
     }
     for (i = max_log; i > 0; i--) {
-	if (!(i % 10))
-	    fprintf(stderr, "2^%2d ", i);
-	else
-	    fprintf(stderr, "     ");
-	for(j = 0; j < 64; j++)
-	    fprintf(stderr, logs[j] >= i ?  "•" : " ");
-	fprintf(stderr, "\n");
+        if (!(i % 10))
+            fprintf(stderr, "2^%2d ", i);
+        else
+            fprintf(stderr, "     ");
+        for(j = 0; j < 64; j++)
+            fprintf(stderr, logs[j] >= i ?  "•" : " ");
+        fprintf(stderr, "\n");
     }
     if (max_log == 0) {
-	fprintf(stderr, "[empty]\n");
+        fprintf(stderr, "[empty]\n");
     } else {
-	fprintf(stderr, "     ns        μs        ms        s         ks\n");
-	fprintf(stderr, "min: ");
-	if (s->min < 1000)
-	    fprintf(stderr, "%llu (ns)", PRIuint64(s->min));
-	else if (s->min < 1000000)
-	    fprintf(stderr, "%.2f (μs)", s->min / 1000.0);
-	else if (s->min < 1000000000)
-	    fprintf(stderr, "%.2f (ms)", s->min / 1000000.0);
-	else if (s->min < 1000000000000)
-	    fprintf(stderr, "%.2f (s)", s->min / 1000000000.0);
-	fprintf(stderr, "  max: ");
-	if (s->max < 1000)
-	    fprintf(stderr, "%llu (ns)", PRIuint64(s->max));
-	else if (s->max < 1000000)
-	    fprintf(stderr, "%.2f (μs)", s->max / 1000.0);
-	else if (s->max < 1000000000)
-	    fprintf(stderr, "%.2f (ms)", s->max / 1000000.0);
-	else if (s->max < 1000000000000)
-	    fprintf(stderr, "%.2f (s)", s->max / 1000000000.0);
-	fprintf(stderr, "  mean: ");
-	if (m < 1000)
-	    fprintf(stderr, "%.2f (ns)", m);
-	else if (m < 1000000)
-	    fprintf(stderr, "%.2f (μs)", m / 1000.0);
-	else if (m < 1000000000)
-	    fprintf(stderr, "%.2f (ms)", m / 1000000.0);
-	else if (m < 1000000000000)
-	    fprintf(stderr, "%.2f (s)", m / 1000000000.0);
-	fprintf(stderr, "\n");
+        fprintf(stderr, "     ns        μs        ms        s         ks\n");
+        fprintf(stderr, "min: ");
+        if (s->min < 1000)
+            fprintf(stderr, "%llu (ns)", PRIuint64(s->min));
+        else if (s->min < 1000000)
+            fprintf(stderr, "%.2f (μs)", s->min / 1000.0);
+        else if (s->min < 1000000000)
+            fprintf(stderr, "%.2f (ms)", s->min / 1000000.0);
+        else if (s->min < 1000000000000)
+            fprintf(stderr, "%.2f (s)", s->min / 1000000000.0);
+        fprintf(stderr, "  max: ");
+        if (s->max < 1000)
+            fprintf(stderr, "%llu (ns)", PRIuint64(s->max));
+        else if (s->max < 1000000)
+            fprintf(stderr, "%.2f (μs)", s->max / 1000.0);
+        else if (s->max < 1000000000)
+            fprintf(stderr, "%.2f (ms)", s->max / 1000000.0);
+        else if (s->max < 1000000000000)
+            fprintf(stderr, "%.2f (s)", s->max / 1000000000.0);
+        fprintf(stderr, "  mean: ");
+        if (m < 1000)
+            fprintf(stderr, "%.2f (ns)", m);
+        else if (m < 1000000)
+            fprintf(stderr, "%.2f (μs)", m / 1000.0);
+        else if (m < 1000000000)
+            fprintf(stderr, "%.2f (ms)", m / 1000000.0);
+        else if (m < 1000000000000)
+            fprintf(stderr, "%.2f (s)", m / 1000000000.0);
+        fprintf(stderr, "\n");
     }
     fflush(stderr);
+    enif_mutex_unlock(s->mutex);
 }
 
 void
 __stat_free(struct stat *s)
 {
     if (!s)
-	return;
+        return;
 
+    enif_mutex_lock(s->mutex);
     enif_free(s->samples);
+    enif_mutex_unlock(s->mutex);
+    enif_mutex_destroy(s->mutex);
     enif_free(s);
 }
 
@@ -232,7 +245,7 @@ __stat_init(uint32_t n)
 {
     struct stat *s = enif_alloc(sizeof(struct stat) + (sizeof(uint64_t) * n));
     if (!s)
-	return NULL;
+        return NULL;
     memset(s, 0, sizeof(struct stat) + (sizeof(uint64_t) * n));
     s->min = ~0;
     s->max = 0;
@@ -240,5 +253,6 @@ __stat_init(uint32_t n)
     s->h = 0;
     s->num_samples = n;
     s->d.unit = ns;
+    s->mutex = enif_mutex_create(NULL);
     return s;
 }
diff --git a/c_src/stats.h b/c_src/stats.h
index 0bdbe8e..f0e550f 100644
--- a/c_src/stats.h
+++ b/c_src/stats.h
@@ -30,6 +30,7 @@ extern "C" {
 #define STAT_DEF(name) struct stat *name ## _stat;
 
 struct stat {
+    ErlNifMutex *mutex;
     duration_t d;
     uint32_t h, n, num_samples;
     uint64_t min, max;
@@ -41,8 +42,9 @@ struct stat {
 extern double __stat_mean(struct stat *s);
 extern double __stat_mean_log2(struct stat *s);
 extern uint64_t __stat_tick(struct stat *s);
+extern void __stat_add(struct stat *s, uint64_t d);
 extern void __stat_reset(struct stat *s);
-extern uint64_t __stat_tock(struct stat *s);
+extern void __stat_tock(struct stat *s);
 extern void __stat_print_histogram(struct stat *s, const char *mod);
 extern void __stat_free(struct stat *s);
 extern struct stat *__stat_init(uint32_t n);
diff --git a/c_src/wterl.c b/c_src/wterl.c
index 7526ef3..c38654f 100644
--- a/c_src/wterl.c
+++ b/c_src/wterl.c
@@ -197,10 +197,12 @@ __ctx_cache_evict(WterlConnHandle *conn_handle)
     uint64_t now, elapsed;
     struct wterl_ctx *c, *n;
 
+#ifndef DEBUG
     if (conn_handle->cache_size < MAX_CACHE_SIZE)
         return 0;
+#endif
 
-    now = cpu_clock_ticks();
+    now = ts(ns);
 
     // Find the mean of the recorded times that items stayed in cache.
     mean = 0;
@@ -226,7 +228,8 @@ __ctx_cache_evict(WterlConnHandle *conn_handle)
         if (log > mean) {
             STAILQ_REMOVE(&conn_handle->cache, c, wterl_ctx, entries);
             DPRINTF("evicting: %llu", PRIuint64(c->sig));
-            c->session->close(c->session, NULL);
+            if (c->session)
+                c->session->close(c->session, NULL);
             enif_free(c);
             num_evicted++;
         }
@@ -256,22 +259,15 @@ __ctx_cache_find(WterlConnHandle *conn_handle, const uint64_t sig)
         if (c->sig == sig) { // TODO: hash collisions *will* lead to SEGVs
             // cache hit:
             STAILQ_REMOVE(&conn_handle->cache, c, wterl_ctx, entries);
-            conn_handle->histogram[__log2(cpu_clock_ticks() - c->tstamp)]++;
+            conn_handle->histogram[__log2(ts(ns) - c->tstamp)]++;
             conn_handle->histogram_count++;
             conn_handle->cache_size -= 1;
             break;
         }
         c = STAILQ_NEXT(c, entries);
     }
-#ifdef DEBUG
-    uint32_t sz = 0;
-    struct wterl_ctx *f;
-    STAILQ_FOREACH(f, &conn_handle->cache, entries) {
-	sz++;
-    }
-#endif
     enif_mutex_unlock(conn_handle->cache_mutex);
-    DPRINTF("cache_find: [%u:%u] %s (%p)", sz, conn_handle->cache_size, c ? "hit" : "miss", c);
+    DPRINTF("cache_find: [%u] %s (%p)", conn_handle->cache_size, c ? "hit" : "miss", c);
     return c;
 }
 
@@ -286,14 +282,14 @@ __ctx_cache_add(WterlConnHandle *conn_handle, struct wterl_ctx *c)
 {
     enif_mutex_lock(conn_handle->cache_mutex);
     __ctx_cache_evict(conn_handle);
-    c->tstamp = cpu_clock_ticks();
+    c->tstamp = ts(ns);
     STAILQ_INSERT_TAIL(&conn_handle->cache, c, entries);
     conn_handle->cache_size += 1;
 #ifdef DEBUG
     uint32_t sz = 0;
     struct wterl_ctx *f;
     STAILQ_FOREACH(f, &conn_handle->cache, entries) {
-	sz++;
+        sz++;
     }
 #endif
     enif_mutex_unlock(conn_handle->cache_mutex);
@@ -336,7 +332,7 @@ __retain_ctx(WterlConnHandle *conn_handle, uint32_t worker_id,
         hash = __str_hash(hash, session_config, l);
         crc = __crc32(crc, session_config, l);
         sig_len += l + 1;
-	DPRINTF("sig/1: %s", session_config);
+        DPRINTF("sig/1: %s", session_config);
     } else {
         sig_len += 1;
     }
@@ -344,7 +340,7 @@ __retain_ctx(WterlConnHandle *conn_handle, uint32_t worker_id,
         arg = va_arg(ap, const char *);
         if (arg) {
             l = __strlen(arg);
-	    DPRINTF("sig/args: %s", arg);
+            DPRINTF("sig/args: %s", arg);
             hash = __str_hash(hash, arg, l);
             crc = __crc32(crc, arg, l);
             sig_len += l + 1;
@@ -360,21 +356,21 @@ __retain_ctx(WterlConnHandle *conn_handle, uint32_t worker_id,
 
     c = conn_handle->mru_ctx[worker_id];
     if (CASPO(&conn_handle->mru_ctx[worker_id], c, 0) == c) {
-	if (c == 0) {
-	    // mru miss:
-	    DPRINTF("[%.4u] mru miss, empty", worker_id);
-	    *ctx = NULL;
-	} else {
-	    if (c->sig == sig) {
-		// mru hit:
-		DPRINTF("[%.4u] mru hit: %llu found", worker_id, PRIuint64(sig));
-		*ctx = c;
-	    } else {
-		// mru mismatch:
-		DPRINTF("[%.4u] mru miss: %llu != %llu", worker_id, PRIuint64(sig), PRIuint64(c->sig));
-		__ctx_cache_add(conn_handle, c);
-		*ctx = NULL;
-	    }
+        if (c == 0) {
+            // mru miss:
+            DPRINTF("[%.4u] mru miss, empty", worker_id);
+            *ctx = NULL;
+        } else {
+            if (c->sig == sig) {
+                // mru hit:
+                DPRINTF("[%.4u] mru hit: %llu found", worker_id, PRIuint64(sig));
+                *ctx = c;
+            } else {
+                // mru mismatch:
+                DPRINTF("[%.4u] mru miss: %llu != %llu", worker_id, PRIuint64(sig), PRIuint64(c->sig));
+                __ctx_cache_add(conn_handle, c);
+                *ctx = NULL;
+            }
         }
     }
 
@@ -2309,7 +2305,7 @@ static void __wterl_conn_dtor(ErlNifEnv* env, void* obj)
     WterlConnHandle *conn_handle = (WterlConnHandle *)obj;
 
     if (conn_handle->cache_mutex) {
-        DPRINTF("Non-NULL conn_handle (%p) to free", obj);
+        DPRINTF("conn_handle dtor free'ing (%p)", obj);
         enif_mutex_lock(conn_handle->cache_mutex);
         __close_all_sessions(conn_handle);
         conn_handle->conn->close(conn_handle->conn, NULL);
diff --git a/src/async_nif.hrl b/src/async_nif.hrl
index 9d0f215..9034d8a 100644
--- a/src/async_nif.hrl
+++ b/src/async_nif.hrl
@@ -26,9 +26,6 @@ async_nif_enqueue(R, F, A) ->
     case erlang:apply(F, [R|A]) of
         {ok, enqueued} ->
             receive
-                {R, {error, eagain}} ->
-                    %% Work unit was not queued, try again.
-                    async_nif_enqueue(R, F, A);
                 {R, {error, shutdown}=Error} ->
                     %% Work unit was queued, but not executed.
                     Error;
@@ -38,6 +35,11 @@ async_nif_enqueue(R, F, A) ->
                 {R, Reply} ->
                     Reply
             end;
+        {error, eagain} ->
+            %% Work unit was not queued, try again.
+            async_nif_enqueue(R, F, A);
+        %{error, enomem} ->
+        %{error, shutdown} ->
         Other ->
             Other
     end.
diff --git a/src/wterl.erl b/src/wterl.erl
index db44807..9045be2 100644
--- a/src/wterl.erl
+++ b/src/wterl.erl
@@ -524,7 +524,7 @@ set_event_handler_pid(Pid)
 -define(TEST_DATA_DIR, "test/wterl.basic").
 
 open_test_conn(DataDir) ->
-    open_test_conn(DataDir, [{create,true},{cache_size,"100MB"},{session_max, 8192}]).
+    open_test_conn(DataDir, [{create,true},{cache_size,"1GB"},{session_max, 8192}]).
 open_test_conn(DataDir, OpenConfig) ->
     {ok, CWD} = file:get_cwd(),
     rmdir:path(filename:join([CWD, DataDir])), %?cmd("rm -rf " ++ filename:join([CWD, DataDir])),
@@ -606,7 +606,7 @@ insert_delete_test() ->
 many_open_tables_test_() ->
     {timeout, 120,
      fun() ->
-	     ConnOpts = [{create,true},{cache_size,"100MB"},{session_max, 8192}],
+	     ConnOpts = [{create,true},{cache_size,"1GB"},{session_max, 8192}],
 	     DataDir = ?TEST_DATA_DIR,
 	     KeyGen =
 		 fun(X) ->
@@ -620,19 +620,31 @@ many_open_tables_test_() ->
 		 fun(X) ->
 			 "lsm:" ++ integer_to_list(X)
 		 end,
-	     N = 1000,
+	     NumTables = 16, N = 100,
 	     ConnRef = open_test_conn(DataDir, ConnOpts),
 	     Parent = self(),
-	     [wterl:create(ConnRef, TableNameGen(X), [{checksum, "uncompressed"}]) || X <- lists:seq(0, 128)],
+	     [ok = wterl:create(ConnRef, TableNameGen(X), [{checksum, "uncompressed"}]) || X <- lists:seq(0, NumTables)],
 	     [spawn(fun() ->
 			    TableName = TableNameGen(X),
-			    [wterl:put(ConnRef, TableName, KeyGen(P), ValGen()) || P <- lists:seq(1, N)],
-			    [wterl:get(ConnRef, TableName, KeyGen(P)) || P <- lists:seq(1, N)],
-			    [wterl:delete(ConnRef, TableName, KeyGen(P)) || P <- lists:seq(1, N)],
+			    [case wterl:put(ConnRef, TableName, KeyGen(P), ValGen()) of
+                                 ok -> ok;
+                                 {error, {enoent, _}} -> io:format("put failed, table missing ~p~n", [TableName])
+                             end || P <- lists:seq(1, N)],
+			    [case wterl:get(ConnRef, TableName, KeyGen(P)) of
+                                 {ok, _} -> ok;
+                                 {error, {enoent, _}} -> io:format("get failed, table missing ~p~n", [TableName])
+                             end || P <- lists:seq(1, N)],
+			    [case wterl:delete(ConnRef, TableName, KeyGen(P)) of
+                                 ok -> ok;
+                                 {error, {enoent, _}} -> io:format("delete failed, table missing ~p~n", [TableName])
+                             end || P <- lists:seq(1, N)],
 			    Parent ! done
-		    end) || X <- lists:seq(0, 128)],
-	     [wterl:drop(ConnRef, TableNameGen(X)) || X <- lists:seq(0, 128)],
-	     [receive done -> ok end || _ <- lists:seq(0, 128)],
+		    end) || X <- lists:seq(0, NumTables)],
+	     [receive done -> ok end || _ <- lists:seq(0, NumTables)],
+	     [case wterl:drop(ConnRef, TableNameGen(X)) of
+		  ok -> ok;
+		  {error, {enoent, _}} -> io:format("drop failed, table missing ~p~n", [TableNameGen(X)])
+	      end || X <- lists:seq(0, NumTables)],
 	     ok = wterl:connection_close(ConnRef)
      end}.