feat(http2): add adaptive window size support using BDP (#2138)

This adds support for calculating the Bandwidth-delay product when using
HTTP2. When a DATA frame is received, a PING is sent to the remote.
While the PING acknoledgement is outstanding, the amount of bytes of all
received DATA frames is accumulated. Once we receive the PING
acknowledgement, we calculate the BDP based on the number of received
bytes and the round-trip-time of the PING. If we are near the current
maximum window size, the size is doubled.

It's disabled by default until tested more extensively.

Author: seanmonstar

src/body/body.rs

@@ -12,6 +12,7 @@ use http::HeaderMap;
 use http_body::{Body as HttpBody, SizeHint};
 
 use crate::common::{task, watch, Future, Never, Pin, Poll};
+use crate::proto::h2::bdp;
 use crate::proto::DecodedLength;
 use crate::upgrade::OnUpgrade;
 
@@ -61,6 +62,11 @@ struct Extra {
     /// connection yet.
     delayed_eof: Option<DelayEof>,
     on_upgrade: OnUpgrade,
+
+    /// Records bytes read to compute the BDP.
+    ///
+    /// Only used with `H2` variant.
+    h2_bdp: bdp::Sampler,
 }
 
 type DelayEofUntil = oneshot::Receiver<Never>;
@@ -175,11 +181,21 @@ impl Body {
         Body { kind, extra: None }
     }
 
-    pub(crate) fn h2(recv: h2::RecvStream, content_length: DecodedLength) -> Self {
-        Body::new(Kind::H2 {
+    pub(crate) fn h2(
+        recv: h2::RecvStream,
+        content_length: DecodedLength,
+        bdp: bdp::Sampler,
+    ) -> Self {
+        let mut body = Body::new(Kind::H2 {
             content_length,
             recv,
-        })
+        });
+
+        if bdp.is_enabled() {
+            body.extra_mut().h2_bdp = bdp;
+        }
+
+        body
     }
 
     pub(crate) fn set_on_upgrade(&mut self, upgrade: OnUpgrade) {
@@ -204,6 +220,7 @@ impl Body {
             Box::new(Extra {
                 delayed_eof: None,
                 on_upgrade: OnUpgrade::none(),
+                h2_bdp: bdp::disabled(),
             })
         })
     }
@@ -262,6 +279,9 @@ impl Body {
                 Some(Ok(bytes)) => {
                     let _ = h2.flow_control().release_capacity(bytes.len());
                     len.sub_if(bytes.len() as u64);
+                    if let Some(ref extra) = self.extra {
+                        extra.h2_bdp.sample(bytes.len());
+                    }
                     Poll::Ready(Some(Ok(bytes)))
                 }
                 Some(Err(e)) => Poll::Ready(Some(Err(crate::Error::new_body(e)))),

src/client/conn.rs

@@ -35,12 +35,6 @@ where
     H2(#[pin] proto::h2::ClientTask<B>),
 }
 
-// Our defaults are chosen for the "majority" case, which usually are not
-// resource constrained, and so the spec default of 64kb can be too limiting
-// for performance.
-const DEFAULT_HTTP2_CONN_WINDOW: u32 = 1024 * 1024 * 5; // 5mb
-const DEFAULT_HTTP2_STREAM_WINDOW: u32 = 1024 * 1024 * 2; // 2mb
-
 /// Returns a handshake future over some IO.
 ///
 /// This is a shortcut for `Builder::new().handshake(io)`.
@@ -82,7 +76,7 @@ pub struct Builder {
     h1_read_buf_exact_size: Option<usize>,
     h1_max_buf_size: Option<usize>,
     http2: bool,
-    h2_builder: h2::client::Builder,
+    h2_builder: proto::h2::client::Config,
 }
 
 /// A future returned by `SendRequest::send_request`.
@@ -420,20 +414,14 @@ impl Builder {
     /// Creates a new connection builder.
     #[inline]
     pub fn new() -> Builder {
-        let mut h2_builder = h2::client::Builder::default();
-        h2_builder
-            .initial_window_size(DEFAULT_HTTP2_STREAM_WINDOW)
-            .initial_connection_window_size(DEFAULT_HTTP2_CONN_WINDOW)
-            .enable_push(false);
-
         Builder {
             exec: Exec::Default,
             h1_writev: true,
             h1_read_buf_exact_size: None,
             h1_title_case_headers: false,
             h1_max_buf_size: None,
             http2: false,
-            h2_builder,
+            h2_builder: Default::default(),
         }
     }
 
@@ -491,7 +479,8 @@ impl Builder {
     /// [spec]: https://http2.github.io/http2-spec/#SETTINGS_INITIAL_WINDOW_SIZE
     pub fn http2_initial_stream_window_size(&mut self, sz: impl Into<Option<u32>>) -> &mut Self {
         if let Some(sz) = sz.into() {
-            self.h2_builder.initial_window_size(sz);
+            self.h2_builder.adaptive_window = false;
+            self.h2_builder.initial_stream_window_size = sz;
         }
         self
     }
@@ -506,7 +495,24 @@ impl Builder {
         sz: impl Into<Option<u32>>,
     ) -> &mut Self {
         if let Some(sz) = sz.into() {
-            self.h2_builder.initial_connection_window_size(sz);
+            self.h2_builder.adaptive_window = false;
+            self.h2_builder.initial_conn_window_size = sz;
+        }
+        self
+    }
+
+    /// Sets whether to use an adaptive flow control.
+    ///
+    /// Enabling this will override the limits set in
+    /// `http2_initial_stream_window_size` and
+    /// `http2_initial_connection_window_size`.
+    pub fn http2_adaptive_window(&mut self, enabled: bool) -> &mut Self {
+        use proto::h2::SPEC_WINDOW_SIZE;
+
+        self.h2_builder.adaptive_window = enabled;
+        if enabled {
+            self.h2_builder.initial_conn_window_size = SPEC_WINDOW_SIZE;
+            self.h2_builder.initial_stream_window_size = SPEC_WINDOW_SIZE;
         }
         self
     }

src/client/mod.rs

@@ -996,6 +996,16 @@ impl Builder {
         self
     }
 
+    /// Sets whether to use an adaptive flow control.
+    ///
+    /// Enabling this will override the limits set in
+    /// `http2_initial_stream_window_size` and
+    /// `http2_initial_connection_window_size`.
+    pub fn http2_adaptive_window(&mut self, enabled: bool) -> &mut Self {
+        self.conn_builder.http2_adaptive_window(enabled);
+        self
+    }
+
     /// Sets the maximum idle connection per host allowed in the pool.
     ///
     /// Default is `usize::MAX` (no limit).

src/proto/h2/bdp.rs

@@ -0,0 +1,190 @@
+// What should it do?
+//
+// # BDP Algorithm
+//
+// 1. When receiving a DATA frame, if a BDP ping isn't outstanding:
+//   1a. Record current time.
+//   1b. Send a BDP ping.
+// 2. Increment the number of received bytes.
+// 3. When the BDP ping ack is received:
+//   3a. Record duration from sent time.
+//   3b. Merge RTT with a running average.
+//   3c. Calculate bdp as bytes/rtt.
+//   3d. If bdp is over 2/3 max, set new max to bdp and update windows.
+//
+//
+// # Implementation
+//
+// - `hyper::Body::h2` variant includes a "bdp channel"
+//   - When the body's `poll_data` yields bytes, call `bdp.sample(bytes.len())`
+//
+
+use std::sync::{Arc, Mutex, Weak};
+use std::task::{self, Poll};
+use std::time::{Duration, Instant};
+
+use h2::{Ping, PingPong};
+
+type WindowSize = u32;
+
+/// Any higher than this likely will be hitting the TCP flow control.
+const BDP_LIMIT: usize = 1024 * 1024 * 16;
+
+pub(crate) fn disabled() -> Sampler {
+    Sampler {
+        shared: Weak::new(),
+    }
+}
+
+pub(super) fn channel(ping_pong: PingPong, initial_window: WindowSize) -> (Sampler, Estimator) {
+    let shared = Arc::new(Mutex::new(Shared {
+        bytes: 0,
+        ping_pong,
+        ping_sent: false,
+        sent_at: Instant::now(),
+    }));
+
+    (
+        Sampler {
+            shared: Arc::downgrade(&shared),
+        },
+        Estimator {
+            bdp: initial_window,
+            max_bandwidth: 0.0,
+            shared,
+            samples: 0,
+            rtt: 0.0,
+        },
+    )
+}
+
+#[derive(Clone)]
+pub(crate) struct Sampler {
+    shared: Weak<Mutex<Shared>>,
+}
+
+pub(super) struct Estimator {
+    shared: Arc<Mutex<Shared>>,
+
+    /// Current BDP in bytes
+    bdp: u32,
+    /// Largest bandwidth we've seen so far.
+    max_bandwidth: f64,
+    /// Count of samples made (ping sent and received)
+    samples: usize,
+    /// Round trip time in seconds
+    rtt: f64,
+}
+
+struct Shared {
+    bytes: usize,
+    ping_pong: PingPong,
+    ping_sent: bool,
+    sent_at: Instant,
+}
+
+impl Sampler {
+    pub(crate) fn sample(&self, bytes: usize) {
+        let shared = if let Some(shared) = self.shared.upgrade() {
+            shared
+        } else {
+            return;
+        };
+
+        let mut inner = shared.lock().unwrap();
+
+        if !inner.ping_sent {
+            if let Ok(()) = inner.ping_pong.send_ping(Ping::opaque()) {
+                inner.ping_sent = true;
+                inner.sent_at = Instant::now();
+                trace!("sending BDP ping");
+            } else {
+                return;
+            }
+        }
+
+        inner.bytes += bytes;
+    }
+
+    pub(crate) fn is_enabled(&self) -> bool {
+        self.shared.strong_count() > 0
+    }
+}
+
+impl Estimator {
+    pub(super) fn poll_estimate(&mut self, cx: &mut task::Context<'_>) -> Poll<WindowSize> {
+        let mut inner = self.shared.lock().unwrap();
+        if !inner.ping_sent {
+            // XXX: this doesn't register a waker...?
+            return Poll::Pending;
+        }
+
+        let (bytes, rtt) = match ready!(inner.ping_pong.poll_pong(cx)) {
+            Ok(_pong) => {
+                let rtt = inner.sent_at.elapsed();
+                let bytes = inner.bytes;
+                inner.bytes = 0;
+                inner.ping_sent = false;
+                self.samples += 1;
+                trace!("received BDP ack; bytes = {}, rtt = {:?}", bytes, rtt);
+                (bytes, rtt)
+            }
+            Err(e) => {
+                debug!("bdp pong error: {}", e);
+                return Poll::Pending;
+            }
+        };
+
+        drop(inner);
+
+        if let Some(bdp) = self.calculate(bytes, rtt) {
+            Poll::Ready(bdp)
+        } else {
+            // XXX: this doesn't register a waker...?
+            Poll::Pending
+        }
+    }
+
+    fn calculate(&mut self, bytes: usize, rtt: Duration) -> Option<WindowSize> {
+        // No need to do any math if we're at the limit.
+        if self.bdp as usize == BDP_LIMIT {
+            return None;
+        }
+
+        // average the rtt
+        let rtt = seconds(rtt);
+        if self.samples < 10 {
+            // Average the first 10 samples
+            self.rtt += (rtt - self.rtt) / (self.samples as f64);
+        } else {
+            self.rtt += (rtt - self.rtt) / 0.9;
+        }
+
+        // calculate the current bandwidth
+        let bw = (bytes as f64) / (self.rtt * 1.5);
+        trace!("current bandwidth = {:.1}B/s", bw);
+
+        if bw < self.max_bandwidth {
+            // not a faster bandwidth, so don't update
+            return None;
+        } else {
+            self.max_bandwidth = bw;
+        }
+
+        // if the current `bytes` sample is at least 2/3 the previous
+        // bdp, increase to double the current sample.
+        if (bytes as f64) >= (self.bdp as f64) * 0.66 {
+            self.bdp = (bytes * 2).min(BDP_LIMIT) as WindowSize;
+            trace!("BDP increased to {}", self.bdp);
+            Some(self.bdp)
+        } else {
+            None
+        }
+    }
+}
+
+fn seconds(dur: Duration) -> f64 {
+    const NANOS_PER_SEC: f64 = 1_000_000_000.0;
+    let secs = dur.as_secs() as f64;
+    secs + (dur.subsec_nanos() as f64) / NANOS_PER_SEC
+}