Change interface for configuring cell ewma algorithm.

The rule is now: take the value from the CircuitPriorityHalflife
config option if it is set.  If it zero, disable the cell_ewma
algorithm.  If it is set, use it to calculate the scaling factor.
If it is not set, look for a CircPriorityHalflifeMsec parameter in the
consensus networkstatus.  If *that* is zero, then disable the cell_ewma
algorithm; if it is set, use it to calculate the scaling factor.
If it is not set at all, disable the algorithm.

src/or/config.c

@@ -167,6 +167,7 @@ static config_var_t _option_vars[] = {
   V(CircuitBuildTimeout,         INTERVAL, "0"),
   V(CircuitIdleTimeout,          INTERVAL, "1 hour"),
   V(CircuitStreamTimeout,        INTERVAL, "0"),
+  V(CircuitPriorityHalflife,     DOUBLE,  "-100.0"), /*negative:'Use default'*/
   V(ClientDNSRejectInternalAddresses, BOOL,"1"),
   V(ClientOnly,                  BOOL,     "0"),
   V(ConsensusParams,             STRING,   NULL),
@@ -212,6 +213,7 @@ static config_var_t _option_vars[] = {
   V(ExitPolicyRejectPrivate,     BOOL,     "1"),
   V(ExitPortStatistics,          BOOL,     "0"),
   V(ExtraInfoStatistics,         BOOL,     "0"),
+
   V(FallbackNetworkstatusFile,   FILENAME,
     SHARE_DATADIR PATH_SEPARATOR "tor" PATH_SEPARATOR "fallback-consensus"),
   V(FascistFirewall,             BOOL,     "0"),
@@ -356,10 +358,6 @@ static config_var_t _option_vars[] = {
       NULL),
   V(MinUptimeHidServDirectoryV2, INTERVAL, "24 hours"),
 
-  /* Options for EWMA selection of circuit to write from */
-  VAR("EWMASignificance", DOUBLE, EWMASignificance, "-1.0"),
-  VAR("EWMAInterval", DOUBLE, EWMAInterval, "-1.0"),
-
   { NULL, CONFIG_TYPE_OBSOLETE, 0, NULL }
 };
 
@@ -1380,7 +1378,7 @@ options_act(or_options_t *old_options)
     configure_accounting(time(NULL));
 
   /* Change the cell EWMA settings */
-  cell_ewma_set_scale_factor(options);
+  cell_ewma_set_scale_factor(options, networkstatus_get_latest_consensus());
 
   /* Check for transitions that need action. */
   if (old_options) {

src/or/networkstatus.c

@@ -1679,6 +1679,7 @@ networkstatus_set_current_consensus(const char *consensus,
     update_consensus_networkstatus_fetch_time(now);
     dirvote_recalculate_timing(get_options(), now);
     routerstatus_list_update_named_server_map();
+    cell_ewma_set_scale_factor(get_options(), current_consensus);
   }
 
   if (!from_cache) {

src/or/or.h

@@ -2783,13 +2783,20 @@ typedef struct {
    * to make this false. */
   int ReloadTorrcOnSIGHUP;
 
-  /* The EWMA parameters for circuit selection within a connection.
-   * The most recent EWMAInterval seconds will account for an
-   * EWMASignificance (between 0 and 1) portion of the weight.
-   * If these values are negative, use the global defaults (soon to be
-   * set in the consensus). */
-  double EWMASignificance;
-  double EWMAInterval;
+  /* The main parameter for picking circuits within a connection.
+   *
+   * If this value is positive, when picking a cell to relay on a connection,
+   * we always relay from the circuit whose weighted cell count is lowest.
+   * Cells are weighted exponentially such that if one cell is sent
+   * 'CircuitPriorityHalflife' seconds before another, it counts for half as
+   * much.
+   *
+   * If this value is zero, we're disabling the cell-EWMA algorithm.
+   *
+   * If this value is negative, we're using the default approach
+   * according to either Tor or a parameter set in the consensus.
+   */
+  double CircuitPriorityHalflife;
 
 } or_options_t;
 
@@ -4491,7 +4498,8 @@ const char *decode_address_from_payload(tor_addr_t *addr_out,
                                         const char *payload,
                                         int payload_len);
 unsigned cell_ewma_get_tick(void);
-void cell_ewma_set_scale_factor(or_options_t *options);
+void cell_ewma_set_scale_factor(or_options_t *options,
+                                networkstatus_t *consensus);
 
 /********************************* rephist.c ***************************/
 

src/or/relay.c

@@ -1814,8 +1814,8 @@ cell_ewma_to_circuit(cell_ewma_t *ewma)
 #define EWMA_TICK_LEN 10
 
 /** The default per-tick scale factor, if it hasn't been overridden by a
- * consensus or a configuration setting. */
-#define EWMA_DEFAULT_SCALE_FACTOR 0.9
+ * consensus or a configuration setting.  zero means "disabled". */
+#define EWMA_DEFAULT_HALFLIFE 0.0
 
 /** Given a timeval <b>now</b>, compute the cell_ewma tick in which it occurs
  * and the fraction of the tick that has elapsed between the start of the tick
@@ -1846,24 +1846,51 @@ cell_ewma_get_tick(void)
 /** The per-tick scale factor to be used when computing cell-count EWMA
  * values.  (A cell sent N ticks before the start of the current tick
  * has value ewma_scale_factor ** N.)
- *
- * If ewma_scale_factor is <= 0, the EWMA algorithm is disabled.
  */
-static double ewma_scale_factor = EWMA_DEFAULT_SCALE_FACTOR;
+static double ewma_scale_factor = 0.1;
+static int ewma_enabled = 0;
+
+#define EPSILON 0.00001
+#define LOG_ONEHALF -0.69314718055994529
 
 /** Adjust the global cell scale factor based on <b>options</b> */
 void
-cell_ewma_set_scale_factor(or_options_t *options)
+cell_ewma_set_scale_factor(or_options_t *options, networkstatus_t *consensus)
 {
-  double f;
-  if (options->EWMAInterval > 0.0001) {
-    f = pow(options->EWMASignificance,
-            EWMA_TICK_LEN / options->EWMAInterval);
+  int32_t halflife_ms;
+  double halflife;
+  const char *source;
+  if (options && options->CircuitPriorityHalflife >= -EPSILON) {
+    halflife = options->CircuitPriorityHalflife;
+    source = "CircuitPriorityHalflife in configuration";
+  } else if (consensus &&
+             (halflife_ms = networkstatus_get_param(
+                   consensus, "CircPriorityHalflifeMsec", -1) >= 0)) {
+    halflife = ((double)halflife_ms)/1000.0;
+    source = "CircPriorityHalflifeMsec in consensus";
   } else {
-    f = EWMA_DEFAULT_SCALE_FACTOR;
+    halflife = EWMA_DEFAULT_HALFLIFE;
+    source = "Default value";
   }
 
-  ewma_scale_factor = f;
+  if (halflife <= EPSILON) {
+    /* The cell EWMA algorithm is disabled. */
+    ewma_scale_factor = 0.1;
+    ewma_enabled = 0;
+    log_info(LD_OR,
+             "Disabled cell_ewma algorithm because of value in %s",
+             source);
+  } else {
+    /* convert halflife into halflife-per-tick. */
+    halflife /= EWMA_TICK_LEN;
+    /* compute per-tick scale factor. */
+    ewma_scale_factor = exp( LOG_ONEHALF / halflife );
+    ewma_enabled = 1;
+    log_info(LD_OR,
+             "Enabled cell_ewma algorithm because of value in %s; "
+             "scale factor is %lf per %d seconds",
+             source, ewma_scale_factor, EWMA_TICK_LEN);
+  }
 }
 
 /** Return the multiplier necessary to convert the value of a cell sent in
@@ -2108,7 +2135,7 @@ connection_or_flush_from_first_active_circuit(or_connection_t *conn, int max,
   assert_active_circuits_ok_paranoid(conn);
 
   /* See if we're doing the ewma circuit selection algorithm. */
-  if (ewma_scale_factor > 0.0) {
+  if (ewma_enabled) {
     unsigned tick;
     double fractional_tick;
     tor_gettimeofday_cached(&now_hires);