Possible connection window flow control improvements

Instead of incrementally doling out connection level flow control, use the fact that if a stream's initial_window_delta >= 0 then the application has made some commitment to read those bytes.
That means that we should target connection level flow control to be:

target_incoming_window = sum(stream_incoming_window_delta if stream_incoming_window_delta > 0) +
                         max(0, k * bdp - sum(abs(stream_incoming_window_delta) if stream_incoming_window_delta < 0))

4 files changed, 83 insertions, 21 deletions

diff --git a/src/core/ext/transport/chttp2/transport/chttp2_transport.c b/src/core/ext/transport/chttp2/transport/chttp2_transport.c
index 3ee5e976f8..93bae9f7bd 100644
--- a/src/core/ext/transport/chttp2/transport/chttp2_transport.c
+++ b/src/core/ext/transport/chttp2/transport/chttp2_transport.c
@@ -569,6 +569,14 @@ static void destroy_stream_locked(grpc_exec_ctx *exec_ctx, void *sp,
   GRPC_ERROR_UNREF(s->read_closed_error);
   GRPC_ERROR_UNREF(s->write_closed_error);
 
+  if (s->incoming_window_delta > 0) {
+    GRPC_CHTTP2_FLOW_DEBIT_STREAM_INCOMING_WINDOW_DELTA(
+        "destroy", t, s, s->incoming_window_delta);
+  } else if (s->incoming_window_delta < 0) {
+    GRPC_CHTTP2_FLOW_CREDIT_STREAM_INCOMING_WINDOW_DELTA(
+        "destroy", t, s, -s->incoming_window_delta);
+  }
+
   GRPC_CHTTP2_UNREF_TRANSPORT(exec_ctx, t, "stream");
 
   GPR_TIMER_END("destroy_stream", 0);
@@ -2054,8 +2062,8 @@ static void incoming_byte_stream_update_flow_control(grpc_exec_ctx *exec_ctx,
         (int64_t)have_already) {
       write_type = GRPC_CHTTP2_STREAM_WRITE_INITIATE_COVERED;
     }
-    GRPC_CHTTP2_FLOW_CREDIT_STREAM("op", t, s, incoming_window_delta,
-                                   add_max_recv_bytes);
+    GRPC_CHTTP2_FLOW_CREDIT_STREAM_INCOMING_WINDOW_DELTA("op", t, s,
+                                                         add_max_recv_bytes);
     GRPC_CHTTP2_FLOW_CREDIT_STREAM("op", t, s, announce_window,
                                    add_max_recv_bytes);
     if ((int64_t)s->incoming_window_delta + (int64_t)initial_window_size -
diff --git a/src/core/ext/transport/chttp2/transport/internal.h b/src/core/ext/transport/chttp2/transport/internal.h
index 075d421dd4..5d41f4bfda 100644
--- a/src/core/ext/transport/chttp2/transport/internal.h
+++ b/src/core/ext/transport/chttp2/transport/internal.h
@@ -271,10 +271,6 @@ struct grpc_chttp2_transport {
   /** data to write next write */
   grpc_slice_buffer qbuf;
 
-  /** window available to announce to peer */
-  int64_t announce_incoming_window;
-  /** how much window would we like to have for incoming_window */
-  uint32_t connection_window_target;
   /** how much data are we willing to buffer when the WRITE_BUFFER_HINT is set?
    */
   uint32_t write_buffer_size;
@@ -328,6 +324,16 @@ struct grpc_chttp2_transport {
 
   /** window available for peer to send to us */
   int64_t incoming_window;
+  /** calculating what we should give for incoming window:
+      we track the total amount of flow control over initial window size
+      across all streams: this is data that we want to receive right now (it
+      has an outstanding read)
+      and the total amount of flow control under initial window size across all
+      streams: this is data we've read early
+      we want to adjust incoming_window such that:
+      incoming_window = total_over - max(bdp - total_under, 0) */
+  int64_t stream_total_over_incoming_window;
+  int64_t stream_total_under_incoming_window;
 
   /* deframing */
   grpc_chttp2_deframe_transport_state deframe_state;
@@ -634,6 +640,44 @@ typedef enum {
   GRPC_CHTTP2_FLOW_CREDIT_COMMON(phase, dst_context, 0, dst_context, dst_var,  \
                                  amount)
 
+#define GRPC_CHTTP2_FLOW_STREAM_INCOMING_WINDOW_DELTA_PREUPDATE( \
+    phase, transport, dst_context)                               \
+  if (dst_context->incoming_window_delta < 0) {                  \
+    transport->stream_total_under_incoming_window +=             \
+        dst_context->incoming_window_delta;                      \
+  } else if (dst_context->incoming_window_delta > 0) {           \
+    transport->stream_total_over_incoming_window -=              \
+        dst_context->incoming_window_delta;                      \
+  }
+
+#define GRPC_CHTTP2_FLOW_STREAM_INCOMING_WINDOW_DELTA_POSTUPDATE( \
+    phase, transport, dst_context)                                \
+  if (dst_context->incoming_window_delta < 0) {                   \
+    transport->stream_total_under_incoming_window -=              \
+        dst_context->incoming_window_delta;                       \
+  } else if (dst_context->incoming_window_delta > 0) {            \
+    transport->stream_total_over_incoming_window +=               \
+        dst_context->incoming_window_delta;                       \
+  }
+
+#define GRPC_CHTTP2_FLOW_DEBIT_STREAM_INCOMING_WINDOW_DELTA(                 \
+    phase, transport, dst_context, amount)                                   \
+  GRPC_CHTTP2_FLOW_STREAM_INCOMING_WINDOW_DELTA_PREUPDATE(phase, transport,  \
+                                                          dst_context);      \
+  GRPC_CHTTP2_FLOW_DEBIT_STREAM(phase, transport, dst_context,               \
+                                incoming_window_delta, amount);              \
+  GRPC_CHTTP2_FLOW_STREAM_INCOMING_WINDOW_DELTA_POSTUPDATE(phase, transport, \
+                                                           dst_context);
+
+#define GRPC_CHTTP2_FLOW_CREDIT_STREAM_INCOMING_WINDOW_DELTA(                \
+    phase, transport, dst_context, amount)                                   \
+  GRPC_CHTTP2_FLOW_STREAM_INCOMING_WINDOW_DELTA_PREUPDATE(phase, transport,  \
+                                                          dst_context);      \
+  GRPC_CHTTP2_FLOW_CREDIT_STREAM(phase, transport, dst_context,              \
+                                 incoming_window_delta, amount);             \
+  GRPC_CHTTP2_FLOW_STREAM_INCOMING_WINDOW_DELTA_POSTUPDATE(phase, transport, \
+                                                           dst_context);
+
 #define GRPC_CHTTP2_FLOW_DEBIT_COMMON(phase, transport, id, dst_context,       \
                                       dst_var, amount)                         \
   do {                                                                         \
@@ -752,4 +796,6 @@ void grpc_chttp2_fail_pending_writes(grpc_exec_ctx *exec_ctx,
                                      grpc_chttp2_transport *t,
                                      grpc_chttp2_stream *s, grpc_error *error);
 
+uint32_t grpc_chttp2_target_incoming_window(grpc_chttp2_transport *t);
+
 #endif /* GRPC_CORE_EXT_TRANSPORT_CHTTP2_TRANSPORT_INTERNAL_H */
diff --git a/src/core/ext/transport/chttp2/transport/parsing.c b/src/core/ext/transport/chttp2/transport/parsing.c
index 24bd93067b..7ed00522c3 100644
--- a/src/core/ext/transport/chttp2/transport/parsing.c
+++ b/src/core/ext/transport/chttp2/transport/parsing.c
@@ -376,15 +376,6 @@ static grpc_error *update_incoming_window(grpc_exec_ctx *exec_ctx,
     return err;
   }
 
-  uint32_t target_incoming_window = GPR_MAX(
-      t->settings[GRPC_SENT_SETTINGS][GRPC_CHTTP2_SETTINGS_INITIAL_WINDOW_SIZE],
-      1024);
-  GRPC_CHTTP2_FLOW_DEBIT_TRANSPORT("parse", t, incoming_window,
-                                   incoming_frame_size);
-  if (t->incoming_window <= target_incoming_window / 2) {
-    grpc_chttp2_initiate_write(exec_ctx, t, false, "flow_control");
-  }
-
   if (s != NULL) {
     if (incoming_frame_size >
         s->incoming_window_delta +
@@ -402,8 +393,8 @@ static grpc_error *update_incoming_window(grpc_exec_ctx *exec_ctx,
       return err;
     }
 
-    GRPC_CHTTP2_FLOW_DEBIT_STREAM("parse", t, s, incoming_window_delta,
-                                  incoming_frame_size);
+    GRPC_CHTTP2_FLOW_DEBIT_STREAM_INCOMING_WINDOW_DELTA("parse", t, s,
+                                                        incoming_frame_size);
     if ((int64_t)t->settings[GRPC_SENT_SETTINGS]
                             [GRPC_CHTTP2_SETTINGS_INITIAL_WINDOW_SIZE] +
             (int64_t)s->incoming_window_delta - (int64_t)s->announce_window <=
@@ -417,6 +408,13 @@ static grpc_error *update_incoming_window(grpc_exec_ctx *exec_ctx,
     s->received_bytes += incoming_frame_size;
   }
 
+  uint32_t target_incoming_window = grpc_chttp2_target_incoming_window(t);
+  GRPC_CHTTP2_FLOW_DEBIT_TRANSPORT("parse", t, incoming_window,
+                                   incoming_frame_size);
+  if (t->incoming_window <= target_incoming_window / 2) {
+    grpc_chttp2_initiate_write(exec_ctx, t, false, "flow_control");
+  }
+
   return GRPC_ERROR_NONE;
 }
 
diff --git a/src/core/ext/transport/chttp2/transport/writing.c b/src/core/ext/transport/chttp2/transport/writing.c
index 05e6f59947..2b9d93cae7 100644
--- a/src/core/ext/transport/chttp2/transport/writing.c
+++ b/src/core/ext/transport/chttp2/transport/writing.c
@@ -152,6 +152,17 @@ static bool stream_ref_if_not_destroyed(gpr_refcount *r) {
   return true;
 }
 
+uint32_t grpc_chttp2_target_incoming_window(grpc_chttp2_transport *t) {
+  return (uint32_t)GPR_MAX(
+      (int64_t)((1u << 31) - 1),
+      t->stream_total_over_incoming_window +
+          (int64_t)GPR_MAX(
+              t->settings[GRPC_SENT_SETTINGS]
+                         [GRPC_CHTTP2_SETTINGS_INITIAL_WINDOW_SIZE] -
+                  t->stream_total_under_incoming_window,
+              0));
+}
+
 bool grpc_chttp2_begin_write(grpc_exec_ctx *exec_ctx,
                              grpc_chttp2_transport *t) {
   grpc_chttp2_stream *s;
@@ -310,13 +321,12 @@ bool grpc_chttp2_begin_write(grpc_exec_ctx *exec_ctx,
 
   /* if the grpc_chttp2_transport is ready to send a window update, do so here
      also; 3/4 is a magic number that will likely get tuned soon */
-  uint32_t target_incoming_window = GPR_MAX(
-      t->settings[GRPC_SENT_SETTINGS][GRPC_CHTTP2_SETTINGS_INITIAL_WINDOW_SIZE],
-      1024);
+  uint32_t target_incoming_window = grpc_chttp2_target_incoming_window(t);
   uint32_t threshold_to_send_transport_window_update =
       t->outbuf.count > 0 ? 3 * target_incoming_window / 4
                           : target_incoming_window / 2;
-  if (t->incoming_window <= threshold_to_send_transport_window_update) {
+  if (t->incoming_window <= threshold_to_send_transport_window_update &&
+      t->incoming_window != target_incoming_window) {
     maybe_initiate_ping(exec_ctx, t,
                         GRPC_CHTTP2_PING_BEFORE_TRANSPORT_WINDOW_UPDATE);
     uint32_t announced = (uint32_t)GPR_CLAMP(