perf(http1): implement an adaptive read buffer strategy

The default read strategy for HTTP/1 connections is now adaptive. It
increases or decreases the size of the read buffer depending on the
number of bytes that are received in a `read` call. If a transport
continuously fills the read buffer, it will continue to grow (up to the
`max_buf_size`), allowing for reading faster. If the transport
consistently only fills a portion of the read buffer, it will be shrunk.

This doesn't provide much benefit to small requests/responses, but
benchmarks show it to be a noticeable improvement to throughput when
streaming larger bodies.

Closes #1708

src/proto/h1/io.rs

@@ -1,4 +1,5 @@
 use std::cell::Cell;
+use std::cmp;
 use std::collections::VecDeque;
 use std::fmt;
 use std::io;
@@ -60,9 +61,7 @@ where
             io: io,
             read_blocked: false,
             read_buf: BytesMut::with_capacity(0),
-            read_buf_strategy: ReadStrategy::Adaptive {
-                max: DEFAULT_MAX_BUFFER_SIZE,
-            },
+            read_buf_strategy: ReadStrategy::default(),
             write_buf: WriteBuf::new(),
         }
     }
@@ -81,9 +80,7 @@ where
             "The max_buf_size cannot be smaller than {}.",
             MINIMUM_MAX_BUFFER_SIZE,
         );
-        self.read_buf_strategy = ReadStrategy::Adaptive {
-            max,
-        };
+        self.read_buf_strategy = ReadStrategy::with_max(max);
         self.write_buf.max_buf_size = max;
     }
 
@@ -149,18 +146,11 @@ where
                     debug!("parsed {} headers", msg.head.headers.len());
                     return Ok(Async::Ready(msg))
                 },
-                None => match self.read_buf_strategy {
-                    ReadStrategy::Adaptive { max } => {
-                        if self.read_buf.len() >= max {
-                            debug!("max_buf_size ({}) reached, closing", max);
-                            return Err(::Error::new_too_large());
-                        }
-                    },
-                    ReadStrategy::Exact(exact) => {
-                        if self.read_buf.len() >= exact {
-                            debug!("exact buf size ({}) filled, closing", exact);
-                            return Err(::Error::new_too_large());
-                        }
+                None => {
+                    let max = self.read_buf_strategy.max();
+                    if self.read_buf.len() >= max {
+                        debug!("max_buf_size ({}) reached, closing", max);
+                        return Err(::Error::new_too_large());
                     }
                 },
             }
@@ -177,22 +167,15 @@ where
     pub fn read_from_io(&mut self) -> Poll<usize, io::Error> {
         use bytes::BufMut;
         self.read_blocked = false;
-        match self.read_buf_strategy {
-            ReadStrategy::Adaptive { .. } => {
-                if self.read_buf.remaining_mut() < INIT_BUFFER_SIZE {
-                    self.read_buf.reserve(INIT_BUFFER_SIZE);
-                }
-            },
-            ReadStrategy::Exact(exact) => {
-                if self.read_buf.capacity() < exact {
-                    self.read_buf.reserve(exact);
-                }
-            },
+        let next = self.read_buf_strategy.next();
+        if self.read_buf.remaining_mut() < next {
+            self.read_buf.reserve(next);
         }
         self.io.read_buf(&mut self.read_buf).map(|ok| {
             match ok {
                 Async::Ready(n) => {
                     debug!("read {} bytes", n);
+                    self.read_buf_strategy.record(n);
                     Async::Ready(n)
                 },
                 Async::NotReady => {
@@ -285,11 +268,82 @@ where
 #[derive(Clone, Copy, Debug)]
 enum ReadStrategy {
     Adaptive {
+        decrease_now: bool,
+        next: usize,
         max: usize
     },
     Exact(usize),
 }
 
+impl ReadStrategy {
+    fn with_max(max: usize) -> ReadStrategy {
+        ReadStrategy::Adaptive {
+            decrease_now: false,
+            next: INIT_BUFFER_SIZE,
+            max,
+        }
+    }
+
+    fn next(&self) -> usize {
+        match *self {
+            ReadStrategy::Adaptive { next, .. } => next,
+            ReadStrategy::Exact(exact) => exact,
+        }
+    }
+
+    fn max(&self) -> usize {
+        match *self {
+            ReadStrategy::Adaptive { max, .. } => max,
+            ReadStrategy::Exact(exact) => exact,
+        }
+    }
+
+    fn record(&mut self, bytes_read: usize) {
+        match *self {
+            ReadStrategy::Adaptive { ref mut decrease_now, ref mut next, max, .. } => {
+                if bytes_read >= *next {
+                    *next = cmp::min(incr_power_of_two(*next), max);
+                    *decrease_now = false;
+                } else {
+                    let decr_to = prev_power_of_two(*next);
+                    if bytes_read < decr_to {
+                        if *decrease_now {
+                            *next = cmp::max(decr_to, INIT_BUFFER_SIZE);
+                            *decrease_now = false;
+                        } else {
+                            // Decreasing is a two "record" process.
+                            *decrease_now = true;
+                        }
+                    } else {
+                        // A read within the current range should cancel
+                        // a potential decrease, since we just saw proof
+                        // that we still need this size.
+                        *decrease_now = false;
+                    }
+                }
+            },
+            _ => (),
+        }
+    }
+}
+
+fn incr_power_of_two(n: usize) -> usize {
+    n.saturating_mul(2)
+}
+
+fn prev_power_of_two(n: usize) -> usize {
+    // Only way this shift can underflow is if n is less than 4.
+    // (Which would means `usize::MAX >> 64` and underflowed!)
+    debug_assert!(n >= 4);
+    (::std::usize::MAX >> (n.leading_zeros() + 2)) + 1
+}
+
+impl Default for ReadStrategy {
+    fn default() -> ReadStrategy {
+        ReadStrategy::with_max(DEFAULT_MAX_BUFFER_SIZE)
+    }
+}
+
 #[derive(Clone)]
 pub struct Cursor<T> {
     bytes: T,
@@ -637,6 +691,97 @@ mod tests {
         assert!(buffered.io.blocked());
     }
 
+    #[test]
+    fn read_strategy_adaptive_increments() {
+        let mut strategy = ReadStrategy::default();
+        assert_eq!(strategy.next(), 8192);
+
+        // Grows if record == next
+        strategy.record(8192);
+        assert_eq!(strategy.next(), 16384);
+
+        strategy.record(16384);
+        assert_eq!(strategy.next(), 32768);
+
+        // Enormous records still increment at same rate
+        strategy.record(::std::usize::MAX);
+        assert_eq!(strategy.next(), 65536);
+
+        let max = strategy.max();
+        while strategy.next() < max {
+            strategy.record(max);
+        }
+
+        assert_eq!(strategy.next(), max, "never goes over max");
+        strategy.record(max + 1);
+        assert_eq!(strategy.next(), max, "never goes over max");
+    }
+
+    #[test]
+    fn read_strategy_adaptive_decrements() {
+        let mut strategy = ReadStrategy::default();
+        strategy.record(8192);
+        assert_eq!(strategy.next(), 16384);
+
+        strategy.record(1);
+        assert_eq!(strategy.next(), 16384, "first smaller record doesn't decrement yet");
+        strategy.record(8192);
+        assert_eq!(strategy.next(), 16384, "record was with range");
+
+        strategy.record(1);
+        assert_eq!(strategy.next(), 16384, "in-range record should make this the 'first' again");
+
+        strategy.record(1);
+        assert_eq!(strategy.next(), 8192, "second smaller record decrements");
+
+        strategy.record(1);
+        assert_eq!(strategy.next(), 8192, "first doesn't decrement");
+        strategy.record(1);
+        assert_eq!(strategy.next(), 8192, "doesn't decrement under minimum");
+    }
+
+    #[test]
+    fn read_strategy_adaptive_stays_the_same() {
+        let mut strategy = ReadStrategy::default();
+        strategy.record(8192);
+        assert_eq!(strategy.next(), 16384);
+
+        strategy.record(8193);
+        assert_eq!(strategy.next(), 16384, "first smaller record doesn't decrement yet");
+
+        strategy.record(8193);
+        assert_eq!(strategy.next(), 16384, "with current step does not decrement");
+    }
+
+    #[test]
+    fn read_strategy_adaptive_max_fuzz() {
+        fn fuzz(max: usize) {
+            let mut strategy = ReadStrategy::with_max(max);
+            while strategy.next() < max {
+                strategy.record(::std::usize::MAX);
+            }
+            let mut next = strategy.next();
+            while next > 8192 {
+                strategy.record(1);
+                strategy.record(1);
+                next = strategy.next();
+                assert!(
+                    next.is_power_of_two(),
+                    "decrement should be powers of two: {} (max = {})",
+                    next,
+                    max,
+                );
+            }
+        }
+
+        let mut max = 8192;
+        while max < ::std::usize::MAX {
+            fuzz(max);
+            max = (max / 2).saturating_mul(3);
+        }
+        fuzz(::std::usize::MAX);
+    }
+
     #[test]
     #[should_panic]
     fn write_buf_requires_non_empty_bufs() {