Merge branch 'bug9093_023' into maint-0.2.3

changes/bug9093

@@ -0,0 +1,7 @@
+  o Minor features:
+    - Improve the circuit queue out-of-memory handler. Previously, when
+      we ran low on memory, we'd close whichever circuits had the most
+      queued cells. Now, we close those that have the *oldest* queued
+      cells, on the theory that those are most responsible for us
+      running low on memory. Based on analysis from a forthcoming paper
+      by Jansen, Tschorsch, Johnson, and Scheuermann. Fixes bug 9093.

src/or/circuitlist.c

@@ -1383,25 +1383,56 @@ n_cells_in_circ_queues(const circuit_t *c)
   return n;
 }
 
-/** helper to sort a list of circuit_q by total queue lengths, in descending
+/**
- * order. */
+ * Return the age of the oldest cell queued on <b>c</b>, in milliseconds.
+ * Return 0 if there are no cells queued on c.  Requires that <b>now</b> be
+ * the current time in milliseconds since the epoch, truncated.
+ *
+ * This function will return incorrect results if the oldest cell queued on
+ * the circuit is older than 2**32 msec (about 49 days) old.
+ */
+static uint32_t
+circuit_max_queued_cell_age(const circuit_t *c, uint32_t now)
+{
+  uint32_t age = 0;
+  if (c->n_conn_cells.head)
+    age = now - c->n_conn_cells.head->inserted_time;
+
+  if (! CIRCUIT_IS_ORIGIN(c)) {
+    const or_circuit_t *orcirc = TO_OR_CIRCUIT((circuit_t*)c);
+    if (orcirc->p_conn_cells.head) {
+      uint32_t age2 = now - orcirc->p_conn_cells.head->inserted_time;
+      if (age2 > age)
+        return age2;
+    }
+  }
+  return age;
+}
+
+/** Temporary variable for circuits_compare_by_oldest_queued_cell_ This is a
+ * kludge to work around the fact that qsort doesn't provide a way for
+ * comparison functions to take an extra argument. */
+static uint32_t circcomp_now_tmp;
+
+/** Helper to sort a list of circuit_t by age of oldest cell, in descending
+ * order. Requires that circcomp_now_tmp is set correctly. */
 static int
-circuits_compare_by_queue_len_(const void **a_, const void **b_)
+circuits_compare_by_oldest_queued_cell_(const void **a_, const void **b_)
 {
   const circuit_t *a = *a_;
   const circuit_t *b = *b_;
-  size_t a_n = n_cells_in_circ_queues(a);
+  uint32_t age_a = circuit_max_queued_cell_age(a, circcomp_now_tmp);
-  size_t b_n = n_cells_in_circ_queues(b);
+  uint32_t age_b = circuit_max_queued_cell_age(b, circcomp_now_tmp);
 
-  if (a_n < b_n)
+  if (age_a < age_b)
     return 1;
-  else if (a_n == b_n)
+  else if (age_a == age_b)
     return 0;
   else
     return -1;
 }
 
-#define FRACTION_OF_CIRCS_TO_RETAIN_ON_OOM 0.90
+#define FRACTION_OF_CELLS_TO_RETAIN_ON_OOM 0.90
 
 /** We're out of memory for cells, having allocated <b>current_allocation</b>
  * bytes' worth.  Kill the 'worst' circuits until we're under
@@ -1414,13 +1445,14 @@ circuits_handle_oom(size_t current_allocation)
   circuit_t *circ;
   size_t n_cells_removed=0, n_cells_to_remove;
   int n_circuits_killed=0;
+  struct timeval now;
   log_notice(LD_GENERAL, "We're low on memory.  Killing circuits with "
              "over-long queues. (This behavior is controlled by "
              "MaxMemInCellQueues.)");
 
   {
     size_t mem_target = (size_t)(get_options()->MaxMemInCellQueues *
-                                 FRACTION_OF_CIRCS_TO_RETAIN_ON_OOM);
+                                 FRACTION_OF_CELLS_TO_RETAIN_ON_OOM);
     size_t mem_to_recover;
     if (current_allocation <= mem_target)
       return;
@@ -1433,9 +1465,13 @@ circuits_handle_oom(size_t current_allocation)
   for (circ = global_circuitlist; circ; circ = circ->next)
     smartlist_add(circlist, circ);
 
+  /* Set circcomp_now_tmp so that the sort can work. */
+  tor_gettimeofday_cached(&now);
+  circcomp_now_tmp = (uint32_t)tv_to_msec(&now);
+
   /* This is O(n log n); there are faster algorithms we could use instead.
    * Let's hope this doesn't happen enough to be in the critical path. */
-  smartlist_sort(circlist, circuits_compare_by_queue_len_);
+  smartlist_sort(circlist, circuits_compare_by_oldest_queued_cell_);
 
   /* Okay, now the worst circuits are at the front of the list. Let's mark
    * them, and reclaim their storage aggressively. */

src/or/or.h

@@ -912,8 +912,13 @@ typedef struct var_cell_t {
 typedef struct packed_cell_t {
   struct packed_cell_t *next; /**< Next cell queued on this circuit. */
   char body[CELL_NETWORK_SIZE]; /**< Cell as packed for network. */
+  uint32_t inserted_time; /**< Time (in milliseconds since epoch, with high
+                           * bits truncated) when this cell was inserted. */
 } packed_cell_t;
 
+/* XXXX This next structure may be obsoleted by inserted_time in
+ * packed_cell_t */
+
 /** Number of cells added to a circuit queue including their insertion
  * time on 10 millisecond detail; used for buffer statistics. */
 typedef struct insertion_time_elem_t {

src/or/relay.c

@@ -1904,15 +1904,19 @@ cell_queue_append(cell_queue_t *queue, packed_cell_t *cell)
 void
 cell_queue_append_packed_copy(cell_queue_t *queue, const cell_t *cell)
 {
-  packed_cell_t *copy = packed_cell_copy(cell);
-  /* Remember the time when this cell was put in the queue. */
-  if (get_options()->CellStatistics) {
   struct timeval now;
+  packed_cell_t *copy = packed_cell_copy(cell);
+  tor_gettimeofday_cached(&now);
+  copy->inserted_time = (uint32_t)tv_to_msec(&now);
+
+  /* Remember the time when this cell was put in the queue. */
+  /*XXXX This may be obsoleted by inserted_time */
+  if (get_options()->CellStatistics) {
     uint32_t added;
     insertion_time_queue_t *it_queue = queue->insertion_times;
     if (!it_pool)
       it_pool = mp_pool_new(sizeof(insertion_time_elem_t), 1024);
-    tor_gettimeofday_cached(&now);
+
 #define SECONDS_IN_A_DAY 86400L
     added = (uint32_t)(((now.tv_sec % SECONDS_IN_A_DAY) * 100L)
             + ((uint32_t)now.tv_usec / (uint32_t)10000L));