diff --git a/src/test/include.am b/src/test/include.am
index 294b9f309c..667bbf5368 100644
--- a/src/test/include.am
+++ b/src/test/include.am
@@ -123,6 +123,7 @@ src_test_test_SOURCES += \
 	src/test/test_checkdir.c \
 	src/test/test_circuitlist.c \
 	src/test/test_circuitmux.c \
+	src/test/test_circuitmux_ewma.c \
 	src/test/test_circuitbuild.c \
 	src/test/test_circuituse.c \
 	src/test/test_circuitstats.c \
diff --git a/src/test/test.c b/src/test/test.c
index 6dbec26fa8..c4227dca50 100644
--- a/src/test/test.c
+++ b/src/test/test.c
@@ -836,6 +836,7 @@ struct testgroup_t testgroups[] = {
   { "circuitpadding/", circuitpadding_tests },
   { "circuitlist/", circuitlist_tests },
   { "circuitmux/", circuitmux_tests },
+  { "circuitmux_ewma/", circuitmux_ewma_tests },
   { "circuitstats/", circuitstats_tests },
   { "circuituse/", circuituse_tests },
   { "compat/libevent/", compat_libevent_tests },
diff --git a/src/test/test.h b/src/test/test.h
index feaa13a3a5..b2dc552c82 100644
--- a/src/test/test.h
+++ b/src/test/test.h
@@ -199,6 +199,7 @@ extern struct testcase_t checkdir_tests[];
 extern struct testcase_t circuitbuild_tests[];
 extern struct testcase_t circuitlist_tests[];
 extern struct testcase_t circuitmux_tests[];
+extern struct testcase_t circuitmux_ewma_tests[];
 extern struct testcase_t circuitstats_tests[];
 extern struct testcase_t circuituse_tests[];
 extern struct testcase_t compat_libevent_tests[];
diff --git a/src/test/test_circuitmux_ewma.c b/src/test/test_circuitmux_ewma.c
new file mode 100644
index 0000000000..8b3edf2b06
--- /dev/null
+++ b/src/test/test_circuitmux_ewma.c
@@ -0,0 +1,228 @@
+/* Copyright (c) 2013-2019, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#define CIRCUITMUX_PRIVATE
+#define CIRCUITMUX_EWMA_PRIVATE
+
+#include "core/or/or.h"
+#include "core/or/circuitmux.h"
+#include "core/or/circuitmux_ewma.h"
+
+#include "test/fakechans.h"
+#include "test/fakecircs.h"
+#include "test/test.h"
+
+static void
+test_cmux_ewma_active_circuit(void *arg)
+{
+  circuitmux_t cmux; /* garbage */
+  circuitmux_policy_data_t *pol_data = NULL;
+  circuit_t circ; /* garbage */
+  circuitmux_policy_circ_data_t *circ_data = NULL;
+
+  (void) arg;
+
+  pol_data = ewma_policy.alloc_cmux_data(&cmux);
+  tt_assert(pol_data);
+  circ_data = ewma_policy.alloc_circ_data(&cmux, pol_data, &circ,
+                                          CELL_DIRECTION_OUT, 42);
+  tt_assert(circ_data);
+
+  /* Get EWMA specific objects. */
+
+  /* Make circuit active. */
+  ewma_policy.notify_circ_active(&cmux, pol_data, &circ, circ_data);
+
+  circuit_t *entry = ewma_policy.pick_active_circuit(&cmux, pol_data);
+  tt_mem_op(entry, OP_EQ, &circ, sizeof(circ));
+
+ done:
+  ewma_policy.free_circ_data(&cmux, pol_data, &circ, circ_data);
+  ewma_policy.free_cmux_data(&cmux, pol_data);
+}
+
+static void
+test_cmux_ewma_xmit_cell(void *arg)
+{
+  circuitmux_t cmux; /* garbage */
+  circuitmux_policy_data_t *pol_data = NULL;
+  circuit_t circ; /* garbage */
+  circuitmux_policy_circ_data_t *circ_data = NULL;
+  ewma_policy_data_t *ewma_pol_data;
+  ewma_policy_circ_data_t *ewma_data;
+  double old_cell_count;
+
+  (void) arg;
+
+  pol_data = ewma_policy.alloc_cmux_data(&cmux);
+  tt_assert(pol_data);
+  circ_data = ewma_policy.alloc_circ_data(&cmux, pol_data, &circ,
+                                          CELL_DIRECTION_OUT, 42);
+  tt_assert(circ_data);
+  ewma_pol_data = TO_EWMA_POL_DATA(pol_data);
+  ewma_data = TO_EWMA_POL_CIRC_DATA(circ_data);
+
+  /* Make circuit active. */
+  ewma_policy.notify_circ_active(&cmux, pol_data, &circ, circ_data);
+
+  /* Move back in time the last time we calibrated so we scale the active
+   * circuit when emitting a cell. */
+  ewma_pol_data->active_circuit_pqueue_last_recalibrated -= 100;
+  ewma_data->cell_ewma.last_adjusted_tick =
+    ewma_pol_data->active_circuit_pqueue_last_recalibrated;
+
+  /* Grab old cell count. */
+  old_cell_count = ewma_data->cell_ewma.cell_count;
+
+  ewma_policy.notify_xmit_cells(&cmux, pol_data, &circ, circ_data, 1);
+
+  /* Our old cell count should be lower to what we have since we just emitted
+   * a cell and thus we scale. */
+  tt_double_op(old_cell_count, OP_LT, ewma_data->cell_ewma.cell_count);
+
+ done:
+  ewma_policy.free_circ_data(&cmux, pol_data, &circ, circ_data);
+  ewma_policy.free_cmux_data(&cmux, pol_data);
+}
+
+static void
+test_cmux_ewma_notify_circ(void *arg)
+{
+  circuitmux_t cmux; /* garbage */
+  circuitmux_policy_data_t *pol_data = NULL;
+  circuit_t circ; /* garbage */
+  circuitmux_policy_circ_data_t *circ_data = NULL;
+  const ewma_policy_data_t *ewma_pol_data;
+
+  (void) arg;
+
+  pol_data = ewma_policy.alloc_cmux_data(&cmux);
+  tt_assert(pol_data);
+  circ_data = ewma_policy.alloc_circ_data(&cmux, pol_data, &circ,
+                                          CELL_DIRECTION_OUT, 42);
+  tt_assert(circ_data);
+
+  /* Currently, notify_circ_active() ignores cmux and circ. They can not be
+   * NULL so it is fine to pass garbage. */
+  ewma_policy.notify_circ_active(&cmux, pol_data, &circ, circ_data);
+
+  /* We should have an active circuit in the queue so its EWMA value can be
+   * tracked. */
+  ewma_pol_data = TO_EWMA_POL_DATA(pol_data);
+  tt_int_op(smartlist_len(ewma_pol_data->active_circuit_pqueue), OP_EQ, 1);
+  tt_uint_op(ewma_pol_data->active_circuit_pqueue_last_recalibrated, OP_NE, 0);
+
+  ewma_policy.notify_circ_inactive(&cmux, pol_data, &circ, circ_data);
+  /* Should be removed from the active queue. */
+  ewma_pol_data = TO_EWMA_POL_DATA(pol_data);
+  tt_int_op(smartlist_len(ewma_pol_data->active_circuit_pqueue), OP_EQ, 0);
+  tt_uint_op(ewma_pol_data->active_circuit_pqueue_last_recalibrated, OP_NE, 0);
+
+ done:
+  ewma_policy.free_circ_data(&cmux, pol_data, &circ, circ_data);
+  ewma_policy.free_cmux_data(&cmux, pol_data);
+}
+
+static void
+test_cmux_ewma_policy_circ_data(void *arg)
+{
+  circuitmux_t cmux; /* garbage */
+  circuitmux_policy_data_t pol_data; /* garbage */
+  circuit_t circ; /* garbage */
+  circuitmux_policy_circ_data_t *circ_data = NULL;
+  const ewma_policy_circ_data_t *ewma_data;
+
+  (void) arg;
+
+  /* Currently, alloc_circ_data() ignores every parameter _except_ the cell
+   * direction so it is OK to pass garbage. They can not be NULL. */
+  circ_data = ewma_policy.alloc_circ_data(&cmux, &pol_data, &circ,
+                                          CELL_DIRECTION_OUT, 42);
+  tt_assert(circ_data);
+  tt_uint_op(circ_data->magic, OP_EQ, EWMA_POL_CIRC_DATA_MAGIC);
+
+  ewma_data = TO_EWMA_POL_CIRC_DATA(circ_data);
+  tt_mem_op(ewma_data->circ, OP_EQ, &circ, sizeof(circuit_t));
+  tt_double_op(ewma_data->cell_ewma.cell_count, OP_LE, 0.0);
+  tt_int_op(ewma_data->cell_ewma.heap_index, OP_EQ, -1);
+  tt_uint_op(ewma_data->cell_ewma.is_for_p_chan, OP_EQ, 0);
+  ewma_policy.free_circ_data(&cmux, &pol_data, &circ, circ_data);
+
+  circ_data = ewma_policy.alloc_circ_data(&cmux, &pol_data, &circ,
+                                          CELL_DIRECTION_IN, 42);
+  tt_assert(circ_data);
+  tt_uint_op(circ_data->magic, OP_EQ, EWMA_POL_CIRC_DATA_MAGIC);
+
+  ewma_data = TO_EWMA_POL_CIRC_DATA(circ_data);
+  tt_mem_op(ewma_data->circ, OP_EQ, &circ, sizeof(circuit_t));
+  tt_double_op(ewma_data->cell_ewma.cell_count, OP_LE, 0.0);
+  tt_int_op(ewma_data->cell_ewma.heap_index, OP_EQ, -1);
+  tt_uint_op(ewma_data->cell_ewma.is_for_p_chan, OP_EQ, 1);
+
+ done:
+  ewma_policy.free_circ_data(&cmux, &pol_data, &circ, circ_data);
+}
+
+static void
+test_cmux_ewma_policy_data(void *arg)
+{
+  circuitmux_t cmux; /* garbage. */
+  circuitmux_policy_data_t *pol_data = NULL;
+  const ewma_policy_data_t *ewma_pol_data;
+
+  (void) arg;
+
+  pol_data = ewma_policy.alloc_cmux_data(&cmux);
+  tt_assert(pol_data);
+  tt_uint_op(pol_data->magic, OP_EQ, EWMA_POL_DATA_MAGIC);
+
+  /* Test EWMA object. */
+  ewma_pol_data = TO_EWMA_POL_DATA(pol_data);
+  tt_assert(ewma_pol_data->active_circuit_pqueue);
+  tt_uint_op(ewma_pol_data->active_circuit_pqueue_last_recalibrated, OP_NE, 0);
+
+ done:
+  ewma_policy.free_cmux_data(&cmux, pol_data);
+}
+
+static void *
+cmux_ewma_setup_test(const struct testcase_t *tc)
+{
+  static int whatever;
+
+  (void) tc;
+
+  cell_ewma_initialize_ticks();
+  cmux_ewma_set_options(NULL, NULL);
+
+  return &whatever;
+}
+
+static int
+cmux_ewma_cleanup_test(const struct testcase_t *tc, void *ptr)
+{
+  (void) tc;
+  (void) ptr;
+
+  circuitmux_ewma_free_all();
+
+  return 1;
+}
+
+static struct testcase_setup_t cmux_ewma_test_setup = {
+  .setup_fn = cmux_ewma_setup_test,
+  .cleanup_fn = cmux_ewma_cleanup_test,
+};
+
+#define TEST_CMUX_EWMA(name) \
+  { #name, test_cmux_ewma_##name, TT_FORK, &cmux_ewma_test_setup, NULL }
+
+struct testcase_t circuitmux_ewma_tests[] = {
+  TEST_CMUX_EWMA(active_circuit),
+  TEST_CMUX_EWMA(policy_data),
+  TEST_CMUX_EWMA(policy_circ_data),
+  TEST_CMUX_EWMA(notify_circ),
+  TEST_CMUX_EWMA(xmit_cell),
+
+  END_OF_TESTCASES
+};