[pacer] spawn 2 full-sized packets at once

h2o · Mar 9, 2024 · fc7e0bd · fc7e0bd
1 parent 73fd022
commit fc7e0bd
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Showing 2 changed files with 25 additions and 26 deletions.
diff --git a/include/quicly/pacer.h b/include/quicly/pacer.h
@@ -33,13 +33,7 @@ extern "C" {
 /**
  * Simple pacer. The design guarantees that the formula below is met for any given pacer-restricted period:
  *
- *   flow_rate * duration + burst_credit <= bytes_sent < flow_rate * duration + burst_credit + mtu
- *
- * where `burst_credit` is defined as:
- *
- *   burst_credit = max((9 * mtu + 1) - flow_rate, 0)
- *
- * and that the sender never sends more than max(10*mtu, flow_rate) per every time slice.
+ *   flow_rate * duration + 8 * mtu <= bytes_sent < flow_rate * duration + 10 * mtu
  */
 typedef struct st_quicly_pacer_t {
     /**
@@ -52,7 +46,8 @@ typedef struct st_quicly_pacer_t {
     size_t bytes_sent;
 } quicly_pacer_t;
 
-#define QUICLY_PACER_CALC_BURST_BYTES(mtu) ((size_t)(mtu)*9 + 1)
+#define QUICLY_PACER_BURST_LOW 8   /* lower bound in packets */
+#define QUICLY_PACER_BURST_HIGH 10 /* high bound in packets */
 
 /**
  * resets the pacer
@@ -86,7 +81,7 @@ inline void quicly_pacer_reset(quicly_pacer_t *pacer)
 inline int64_t quicly_pacer_can_send_at(quicly_pacer_t *pacer, uint32_t bytes_per_msec, uint16_t mtu)
 {
     /* return "now" if we have room in current msec */
-    size_t burst_size = QUICLY_PACER_CALC_BURST_BYTES(mtu);
+    size_t burst_size = QUICLY_PACER_BURST_LOW * mtu + 1;
     size_t burst_credit = burst_size > bytes_per_msec ? burst_size - bytes_per_msec : 0;
     if (pacer->bytes_sent < bytes_per_msec + burst_credit)
         return 0;
@@ -106,8 +101,8 @@ inline uint64_t quicly_pacer_get_window(quicly_pacer_t *pacer, int64_t now, uint
     if (now < can_send_at)
         return 0;
 
-    /* Calculate burst window, as max(10mtu, bytes_per_msec) */
-    size_t burst_window = QUICLY_PACER_CALC_BURST_BYTES(mtu);
+    /* Calculate the upper bound of burst window (the size is later rounded up) */
+    size_t burst_window = (QUICLY_PACER_BURST_HIGH - 1) * mtu + 1;
     if (burst_window < bytes_per_msec)
         burst_window = bytes_per_msec;
 
@@ -118,11 +113,18 @@ inline uint64_t quicly_pacer_get_window(quicly_pacer_t *pacer, int64_t now, uint
     uint64_t window, delta = (now - pacer->at) * bytes_per_msec;
     if (pacer->bytes_sent > delta) {
         pacer->bytes_sent -= delta;
-        window = burst_window > pacer->bytes_sent ? burst_window - pacer->bytes_sent : 1;
+        if (burst_window > pacer->bytes_sent) {
+            window = (burst_window - pacer->bytes_sent + mtu - 1) / mtu;
+            if (window < 2)
+                window = 2;
+        } else {
+            window = 2;
+        }
     } else {
         pacer->bytes_sent = 0;
-        window = burst_window;
+        window = (burst_window + mtu - 1) / mtu;
     }
+    window *= mtu;
 
     pacer->at = now;
 

diff --git a/t/pacer.c b/t/pacer.c
@@ -108,26 +108,23 @@ static void test_medium(void)
 
 static void test_slow(void)
 {
-    const uint32_t bytes_per_msec = 500; /* one packet every 2.4ms */
+    const uint32_t bytes_per_msec = 700;
     quicly_pacer_t pacer;
     int64_t now = 1;
 
     quicly_pacer_reset(&pacer);
 
     now = test_pattern(&pacer, now, bytes_per_msec,
                        (const struct pattern[]){
-                           {1, 10 * mtu, 10 * mtu}, /* borrow 1199 bytes */
-                           {4, 1 * mtu, 1 * mtu},   /* borrowing 899 bytes, after 3ms */
-                           {6, 1 * mtu, 1 * mtu},   /* borrowing 1099 bytes */
-                           {9, 1 * mtu, 1 * mtu},   /* borrowing 799 bytes, after 3ms  */
-                           {11, 1 * mtu, 1 * mtu},  /* borrowing 999 bytes */
-                           {13, 1 * mtu, 1 * mtu},  /* borrowing 1199 bytes */
-                           {16, 1 * mtu, 1 * mtu},  /* borrowing 899 bytes, after 3ms */
-                           {18, 1 * mtu, 1 * mtu},  /* borrowing 1099 bytes */
-                           {21, 1 * mtu, 1 * mtu},  /* borrowing 799 bytes, after 3ms  */
-                           {23, 1 * mtu, 1 * mtu},  /* borrowing 999 bytes */
-                           {25, 1 * mtu, 1 * mtu},  /* borrowing 1199 bytes */
-                           {28, 1 * mtu, 1 * mtu},  /* borrowing 899 bytes, after 3ms */
+                           {1, 10 * mtu, 10 * mtu}, /* borrow 12000 bytes */
+                           {5, 2 * mtu, 2 * mtu},   /* borrowing 11600 bytes after 4ms */
+                           {8, 2 * mtu, 2 * mtu},   /* borrowing 11900 bytes after 3ms */
+                           {12, 2 * mtu, 2 * mtu},  /* borrowing 11500 bytes after 4ms */
+                           {15, 2 * mtu, 2 * mtu},  /* borrowing 11800 bytes after 3ms */
+                           {19, 2 * mtu, 2 * mtu},  /* borrowing 11400 bytes after 4ms */
+                           {22, 2 * mtu, 2 * mtu},  /* borrowing 11700 bytes after 3ms */
+                           {25, 2 * mtu, 2 * mtu},  /* borrowing 12000 bytes after 3ms */
+                           {29, 2 * mtu, 2 * mtu},  /* borrowing 11600 bytes after 4ms */
                            {0},
                        });
 }