perf: Improve throughput when vectored IO is not enabled (hyperium#712)

As discussed in hyperium#711, the
current implementation of sending data is suboptimal when vectored
I/O is not enabled: data frame's head is likely to be sent in a
separate TCP segment, whose payload is of only 9 bytes.

This PR adds some specialized implementaton for non-vectored I/O
case. In short, it sets a larget chain threhold, and also makes
sure a data frame's head is sent along with the beginning part of
the real data payload.

All existing unit tests passed. Also I take a look at the e2e
https://github.com/hyperium/hyper/blob/0.14.x/benches/end_to_end.rs
but realize that all the benchmarks there are for the case of
vectored I/O if the OS supports vectored I/O. There isn't a specific
case for non-vectored I/O so I am not sure how to proceed with
benchmark for performance evaluations.

Signed-off-by: Sven Pfennig <s.pfennig@reply.de>

Cargo.toml

@@ -48,7 +48,7 @@ unstable = []
 futures-core = { version = "0.3", default-features = false }
 futures-sink = { version = "0.3", default-features = false }
 futures-util = { version = "0.3", default-features = false }
-tokio-util_wasi = { version = "0.7.4", features = ["codec"] }
+tokio-util_wasi = { version = "0.7.4", features = ["codec", "io"] }
 tokio_wasi = {version = "1", features = ["io-util"] }
 bytes = "1"
 http = "0.2"

src/codec/framed_write.rs

@@ -7,8 +7,9 @@ use bytes::{Buf, BufMut, BytesMut};
 use std::pin::Pin;
 use std::task::{Context, Poll};
 use tokio::io::{AsyncRead, AsyncWrite, ReadBuf};
+use tokio_util::io::poll_write_buf;
 
-use std::io::{self, Cursor, IoSlice};
+use std::io::{self, Cursor};
 
 // A macro to get around a method needing to borrow &mut self
 macro_rules! limited_write_buf {
@@ -45,8 +46,11 @@ struct Encoder<B> {
     /// Max frame size, this is specified by the peer
     max_frame_size: FrameSize,
 
-    /// Whether or not the wrapped `AsyncWrite` supports vectored IO.
-    is_write_vectored: bool,
+    /// Chain payloads bigger than this.
+    chain_threshold: usize,
+
+    /// Min buffer required to attempt to write a frame
+    min_buffer_capacity: usize,
 }
 
 #[derive(Debug)]
@@ -61,22 +65,28 @@ enum Next<B> {
 /// frame that big.
 const DEFAULT_BUFFER_CAPACITY: usize = 16 * 1_024;
 
-/// Min buffer required to attempt to write a frame
-const MIN_BUFFER_CAPACITY: usize = frame::HEADER_LEN + CHAIN_THRESHOLD;
-
-/// Chain payloads bigger than this. The remote will never advertise a max frame
-/// size less than this (well, the spec says the max frame size can't be less
-/// than 16kb, so not even close).
+/// Chain payloads bigger than this when vectored I/O is enabled. The remote
+/// will never advertise a max frame size less than this (well, the spec says
+/// the max frame size can't be less than 16kb, so not even close).
 const CHAIN_THRESHOLD: usize = 256;
 
+/// Chain payloads bigger than this when vectored I/O is **not** enabled.
+/// A larger value in this scenario will reduce the number of small and
+/// fragmented data being sent, and hereby improve the throughput.
+const CHAIN_THRESHOLD_WITHOUT_VECTORED_IO: usize = 1024;
+
 // TODO: Make generic
 impl<T, B> FramedWrite<T, B>
 where
     T: AsyncWrite + Unpin,
     B: Buf,
 {
     pub fn new(inner: T) -> FramedWrite<T, B> {
-        let is_write_vectored = inner.is_write_vectored();
+        let chain_threshold = if inner.is_write_vectored() {
+            CHAIN_THRESHOLD
+        } else {
+            CHAIN_THRESHOLD_WITHOUT_VECTORED_IO
+        };
         FramedWrite {
             inner,
             encoder: Encoder {
@@ -85,7 +95,8 @@ where
                 next: None,
                 last_data_frame: None,
                 max_frame_size: frame::DEFAULT_MAX_FRAME_SIZE,
-                is_write_vectored,
+                chain_threshold,
+                min_buffer_capacity: chain_threshold + frame::HEADER_LEN,
             },
         }
     }
@@ -126,23 +137,17 @@ where
                     Some(Next::Data(ref mut frame)) => {
                         tracing::trace!(queued_data_frame = true);
                         let mut buf = (&mut self.encoder.buf).chain(frame.payload_mut());
-                        ready!(write(
-                            &mut self.inner,
-                            self.encoder.is_write_vectored,
-                            &mut buf,
-                            cx,
-                        ))?
+                        ready!(poll_write_buf(Pin::new(&mut self.inner), cx, &mut buf))?
                     }
                     _ => {
                         tracing::trace!(queued_data_frame = false);
-                        ready!(write(
-                            &mut self.inner,
-                            self.encoder.is_write_vectored,
-                            &mut self.encoder.buf,
+                        ready!(poll_write_buf(
+                            Pin::new(&mut self.inner),
                             cx,
+                            &mut self.encoder.buf
                         ))?
                     }
-                }
+                };
             }
 
             match self.encoder.unset_frame() {
@@ -165,30 +170,6 @@ where
     }
 }
 
-fn write<T, B>(
-    writer: &mut T,
-    is_write_vectored: bool,
-    buf: &mut B,
-    cx: &mut Context<'_>,
-) -> Poll<io::Result<()>>
-where
-    T: AsyncWrite + Unpin,
-    B: Buf,
-{
-    // TODO(eliza): when tokio-util 0.5.1 is released, this
-    // could just use `poll_write_buf`...
-    const MAX_IOVS: usize = 64;
-    let n = if is_write_vectored {
-        let mut bufs = [IoSlice::new(&[]); MAX_IOVS];
-        let cnt = buf.chunks_vectored(&mut bufs);
-        ready!(Pin::new(writer).poll_write_vectored(cx, &bufs[..cnt]))?
-    } else {
-        ready!(Pin::new(writer).poll_write(cx, buf.chunk()))?
-    };
-    buf.advance(n);
-    Ok(()).into()
-}
-
 #[must_use]
 enum ControlFlow {
     Continue,
@@ -240,12 +221,17 @@ where
                     return Err(PayloadTooBig);
                 }
 
-                if len >= CHAIN_THRESHOLD {
+                if len >= self.chain_threshold {
                     let head = v.head();
 
                     // Encode the frame head to the buffer
                     head.encode(len, self.buf.get_mut());
 
+                    if self.buf.get_ref().remaining() < self.chain_threshold {
+                        let extra_bytes = self.chain_threshold - self.buf.remaining();
+                        self.buf.get_mut().put(v.payload_mut().take(extra_bytes));
+                    }
+
                     // Save the data frame
                     self.next = Some(Next::Data(v));
                 } else {
@@ -305,7 +291,9 @@ where
     }
 
     fn has_capacity(&self) -> bool {
-        self.next.is_none() && self.buf.get_ref().remaining_mut() >= MIN_BUFFER_CAPACITY
+        self.next.is_none()
+            && (self.buf.get_ref().capacity() - self.buf.get_ref().len()
+                >= self.min_buffer_capacity)
     }
 
     fn is_empty(&self) -> bool {