perf(lib): re-enable writev support (#2338)

Tokio's `AsyncWrite` trait once again has support for vectored writes in
Tokio 0.3.4 (see tokio-rs/tokio#3149).

This branch re-enables vectored writes in Hyper for HTTP/1. Using
vectored writes in HTTP/2 will require an upstream change in the `h2`
crate as well.

I've removed the adaptive write buffer implementation
that attempts to detect whether vectored IO is or is not available,
since the Tokio 0.3.4 `AsyncWrite` trait exposes this directly via the
`is_write_vectored` method. Now, we just ask the IO whether or not it
supports vectored writes, and configure the buffer accordingly. This
makes the implementation somewhat simpler.

This also removes `http1_writev()` methods from the builders. These are
no longer necessary, as Hyper can now determine whether or not
to use vectored writes based on `is_write_vectored`, rather than trying
to auto-detect it.

Closes #2320 

BREAKING CHANGE: Removed `http1_writev` methods from `client::Builder`,
  `client::conn::Builder`, `server::Builder`, and `server::conn::Builder`.
  
  Vectored writes are now enabled based on whether the `AsyncWrite`
  implementation in use supports them, rather than though adaptive
  detection. To explicitly disable vectored writes, users may wrap the IO
  in a newtype that implements `AsyncRead` and `AsyncWrite` and returns
  `false` from its `AsyncWrite::is_write_vectored` method.

src/client/conn.rs

@@ -82,7 +82,6 @@ where
 #[derive(Clone, Debug)]
 pub struct Builder {
     pub(super) exec: Exec,
-    h1_writev: Option<bool>,
     h1_title_case_headers: bool,
     h1_read_buf_exact_size: Option<usize>,
     h1_max_buf_size: Option<usize>,
@@ -453,7 +452,6 @@ impl Builder {
     pub fn new() -> Builder {
         Builder {
             exec: Exec::Default,
-            h1_writev: None,
             h1_read_buf_exact_size: None,
             h1_title_case_headers: false,
             h1_max_buf_size: None,
@@ -475,11 +473,6 @@ impl Builder {
         self
     }
 
-    pub(super) fn h1_writev(&mut self, enabled: bool) -> &mut Builder {
-        self.h1_writev = Some(enabled);
-        self
-    }
-
     pub(super) fn h1_title_case_headers(&mut self, enabled: bool) -> &mut Builder {
         self.h1_title_case_headers = enabled;
         self
@@ -663,13 +656,6 @@ impl Builder {
                 #[cfg(feature = "http1")]
                 Proto::Http1 => {
                     let mut conn = proto::Conn::new(io);
-                    if let Some(writev) = opts.h1_writev {
-                        if writev {
-                            conn.set_write_strategy_queue();
-                        } else {
-                            conn.set_write_strategy_flatten();
-                        }
-                    }
                     if opts.h1_title_case_headers {
                         conn.set_title_case_headers();
                     }

src/client/mod.rs

@@ -62,7 +62,7 @@ use http::{Method, Request, Response, Uri, Version};
 use self::connect::{sealed::Connect, Alpn, Connected, Connection};
 use self::pool::{Key as PoolKey, Pool, Poolable, Pooled, Reservation};
 use crate::body::{Body, HttpBody};
-use crate::common::{lazy as hyper_lazy, task, exec::BoxSendFuture, Future, Lazy, Pin, Poll};
+use crate::common::{exec::BoxSendFuture, lazy as hyper_lazy, task, Future, Lazy, Pin, Poll};
 use crate::rt::Executor;
 
 #[cfg(feature = "tcp")]
@@ -987,23 +987,6 @@ impl Builder {
 
     // HTTP/1 options
 
-    /// Set whether HTTP/1 connections should try to use vectored writes,
-    /// or always flatten into a single buffer.
-    ///
-    /// Note that setting this to false may mean more copies of body data,
-    /// but may also improve performance when an IO transport doesn't
-    /// support vectored writes well, such as most TLS implementations.
-    ///
-    /// Setting this to true will force hyper to use queued strategy
-    /// which may eliminate unnecessary cloning on some TLS backends
-    ///
-    /// Default is `auto`. In this mode hyper will try to guess which
-    /// mode to use
-    pub fn http1_writev(&mut self, val: bool) -> &mut Self {
-        self.conn_builder.h1_writev(val);
-        self
-    }
-
     /// Sets the exact size of the read buffer to *always* use.
     ///
     /// Note that setting this option unsets the `http1_max_buf_size` option.

src/common/io/mod.rs

@@ -1,3 +1,4 @@
 mod rewind;
 
 pub(crate) use self::rewind::Rewind;
+pub(crate) const MAX_WRITEV_BUFS: usize = 64;

src/common/io/rewind.rs

@@ -84,13 +84,25 @@ where
         Pin::new(&mut self.inner).poll_write(cx, buf)
     }
 
+    fn poll_write_vectored(
+        mut self: Pin<&mut Self>,
+        cx: &mut task::Context<'_>,
+        bufs: &[io::IoSlice<'_>],
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.inner).poll_write_vectored(cx, bufs)
+    }
+
     fn poll_flush(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<io::Result<()>> {
         Pin::new(&mut self.inner).poll_flush(cx)
     }
 
     fn poll_shutdown(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<io::Result<()>> {
         Pin::new(&mut self.inner).poll_shutdown(cx)
     }
+
+    fn is_write_vectored(&self) -> bool {
+        self.inner.is_write_vectored()
+    }
 }
 
 #[cfg(test)]

src/proto/h1/conn.rs

@@ -71,14 +71,6 @@ where
         self.io.set_read_buf_exact_size(sz);
     }
 
-    pub fn set_write_strategy_flatten(&mut self) {
-        self.io.set_write_strategy_flatten();
-    }
-
-    pub fn set_write_strategy_queue(&mut self) {
-        self.io.set_write_strategy_queue();
-    }
-
     #[cfg(feature = "client")]
     pub fn set_title_case_headers(&mut self) {
         self.state.title_case_headers = true;

src/proto/h1/io.rs

@@ -1,4 +1,3 @@
-use std::cell::Cell;
 use std::cmp;
 use std::fmt;
 use std::io::{self, IoSlice};
@@ -57,13 +56,14 @@ where
     B: Buf,
 {
     pub fn new(io: T) -> Buffered<T, B> {
+        let write_buf = WriteBuf::new(&io);
         Buffered {
             flush_pipeline: false,
             io,
             read_blocked: false,
             read_buf: BytesMut::with_capacity(0),
             read_buf_strategy: ReadStrategy::default(),
-            write_buf: WriteBuf::new(),
+            write_buf,
         }
     }
 
@@ -98,13 +98,6 @@ where
         self.write_buf.set_strategy(WriteStrategy::Flatten);
     }
 
-    pub fn set_write_strategy_queue(&mut self) {
-        // this should always be called only at construction time,
-        // so this assert is here to catch myself
-        debug_assert!(self.write_buf.queue.bufs_cnt() == 0);
-        self.write_buf.set_strategy(WriteStrategy::Queue);
-    }
-
     pub fn read_buf(&self) -> &[u8] {
         self.read_buf.as_ref()
     }
@@ -237,13 +230,13 @@ where
             if let WriteStrategy::Flatten = self.write_buf.strategy {
                 return self.poll_flush_flattened(cx);
             }
+
             loop {
-                // TODO(eliza): this basically ignores all of `WriteBuf`...put
-                // back vectored IO and `poll_write_buf` when the appropriate Tokio
-                // changes land...
-                let n = ready!(Pin::new(&mut self.io)
-                    // .poll_write_buf(cx, &mut self.write_buf.auto()))?;
-                    .poll_write(cx, self.write_buf.auto().bytes()))?;
+                let n = {
+                    let mut iovs = [IoSlice::new(&[]); crate::common::io::MAX_WRITEV_BUFS];
+                    let len = self.write_buf.bytes_vectored(&mut iovs);
+                    ready!(Pin::new(&mut self.io).poll_write_vectored(cx, &iovs[..len]))?
+                };
                 // TODO(eliza): we have to do this manually because
                 // `poll_write_buf` doesn't exist in Tokio 0.3 yet...when
                 // `poll_write_buf` comes back, the manual advance will need to leave!
@@ -462,12 +455,17 @@ pub(super) struct WriteBuf<B> {
 }
 
 impl<B: Buf> WriteBuf<B> {
-    fn new() -> WriteBuf<B> {
+    fn new(io: &impl AsyncWrite) -> WriteBuf<B> {
+        let strategy = if io.is_write_vectored() {
+            WriteStrategy::Queue
+        } else {
+            WriteStrategy::Flatten
+        };
         WriteBuf {
             headers: Cursor::new(Vec::with_capacity(INIT_BUFFER_SIZE)),
             max_buf_size: DEFAULT_MAX_BUFFER_SIZE,
             queue: BufList::new(),
-            strategy: WriteStrategy::Auto,
+            strategy,
         }
     }
 }
@@ -480,12 +478,6 @@ where
         self.strategy = strategy;
     }
 
-    // TODO(eliza): put back writev!
-    #[inline]
-    fn auto(&mut self) -> WriteBufAuto<'_, B> {
-        WriteBufAuto::new(self)
-    }
-
     pub(super) fn buffer<BB: Buf + Into<B>>(&mut self, mut buf: BB) {
         debug_assert!(buf.has_remaining());
         match self.strategy {
@@ -505,7 +497,7 @@ where
                     buf.advance(adv);
                 }
             }
-            WriteStrategy::Auto | WriteStrategy::Queue => {
+            WriteStrategy::Queue => {
                 self.queue.push(buf.into());
             }
         }
@@ -514,7 +506,7 @@ where
     fn can_buffer(&self) -> bool {
         match self.strategy {
             WriteStrategy::Flatten => self.remaining() < self.max_buf_size,
-            WriteStrategy::Auto | WriteStrategy::Queue => {
+            WriteStrategy::Queue => {
                 self.queue.bufs_cnt() < MAX_BUF_LIST_BUFFERS && self.remaining() < self.max_buf_size
             }
         }
@@ -573,65 +565,8 @@ impl<B: Buf> Buf for WriteBuf<B> {
     }
 }
 
-/// Detects when wrapped `WriteBuf` is used for vectored IO, and
-/// adjusts the `WriteBuf` strategy if not.
-struct WriteBufAuto<'a, B: Buf> {
-    bytes_called: Cell<bool>,
-    bytes_vec_called: Cell<bool>,
-    inner: &'a mut WriteBuf<B>,
-}
-
-impl<'a, B: Buf> WriteBufAuto<'a, B> {
-    fn new(inner: &'a mut WriteBuf<B>) -> WriteBufAuto<'a, B> {
-        WriteBufAuto {
-            bytes_called: Cell::new(false),
-            bytes_vec_called: Cell::new(false),
-            inner,
-        }
-    }
-}
-
-impl<'a, B: Buf> Buf for WriteBufAuto<'a, B> {
-    #[inline]
-    fn remaining(&self) -> usize {
-        self.inner.remaining()
-    }
-
-    #[inline]
-    fn bytes(&self) -> &[u8] {
-        self.bytes_called.set(true);
-        self.inner.bytes()
-    }
-
-    #[inline]
-    fn advance(&mut self, cnt: usize) {
-        self.inner.advance(cnt)
-    }
-
-    #[inline]
-    fn bytes_vectored<'t>(&'t self, dst: &mut [IoSlice<'t>]) -> usize {
-        self.bytes_vec_called.set(true);
-        self.inner.bytes_vectored(dst)
-    }
-}
-
-impl<'a, B: Buf + 'a> Drop for WriteBufAuto<'a, B> {
-    fn drop(&mut self) {
-        if let WriteStrategy::Auto = self.inner.strategy {
-            if self.bytes_vec_called.get() {
-                self.inner.strategy = WriteStrategy::Queue;
-            } else if self.bytes_called.get() {
-                trace!("detected no usage of vectored write, flattening");
-                self.inner.strategy = WriteStrategy::Flatten;
-                self.inner.headers.bytes.put(&mut self.inner.queue);
-            }
-        }
-    }
-}
-
 #[derive(Debug)]
 enum WriteStrategy {
-    Auto,
     Flatten,
     Queue,
 }
@@ -643,8 +578,8 @@ mod tests {
 
     use tokio_test::io::Builder as Mock;
 
-    #[cfg(feature = "nightly")]
-    use test::Bencher;
+    // #[cfg(feature = "nightly")]
+    // use test::Bencher;
 
     /*
     impl<T: Read> MemRead for AsyncIo<T> {
@@ -873,33 +808,6 @@ mod tests {
         buffered.flush().await.expect("flush");
     }
 
-    #[tokio::test]
-    async fn write_buf_auto_flatten() {
-        let _ = pretty_env_logger::try_init();
-
-        let mock = Mock::new()
-            // Expects write_buf to only consume first buffer
-            .write(b"hello ")
-            // And then the Auto strategy will have flattened
-            .write(b"world, it's hyper!")
-            .build();
-
-        let mut buffered = Buffered::<_, Cursor<Vec<u8>>>::new(mock);
-
-        // we have 4 buffers, but hope to detect that vectored IO isn't
-        // being used, and switch to flattening automatically,
-        // resulting in only 2 writes
-        buffered.headers_buf().extend(b"hello ");
-        buffered.buffer(Cursor::new(b"world, ".to_vec()));
-        buffered.buffer(Cursor::new(b"it's ".to_vec()));
-        buffered.buffer(Cursor::new(b"hyper!".to_vec()));
-        assert_eq!(buffered.write_buf.queue.bufs_cnt(), 3);
-
-        buffered.flush().await.expect("flush");
-
-        assert_eq!(buffered.write_buf.queue.bufs_cnt(), 0);
-    }
-
     #[tokio::test]
     async fn write_buf_queue_disable_auto() {
         let _ = pretty_env_logger::try_init();
@@ -928,19 +836,19 @@ mod tests {
         assert_eq!(buffered.write_buf.queue.bufs_cnt(), 0);
     }
 
-    #[cfg(feature = "nightly")]
-    #[bench]
-    fn bench_write_buf_flatten_buffer_chunk(b: &mut Bencher) {
-        let s = "Hello, World!";
-        b.bytes = s.len() as u64;
-
-        let mut write_buf = WriteBuf::<bytes::Bytes>::new();
-        write_buf.set_strategy(WriteStrategy::Flatten);
-        b.iter(|| {
-            let chunk = bytes::Bytes::from(s);
-            write_buf.buffer(chunk);
-            ::test::black_box(&write_buf);
-            write_buf.headers.bytes.clear();
-        })
-    }
+    // #[cfg(feature = "nightly")]
+    // #[bench]
+    // fn bench_write_buf_flatten_buffer_chunk(b: &mut Bencher) {
+    //     let s = "Hello, World!";
+    //     b.bytes = s.len() as u64;
+
+    //     let mut write_buf = WriteBuf::<bytes::Bytes>::new();
+    //     write_buf.set_strategy(WriteStrategy::Flatten);
+    //     b.iter(|| {
+    //         let chunk = bytes::Bytes::from(s);
+    //         write_buf.buffer(chunk);
+    //         ::test::black_box(&write_buf);
+    //         write_buf.headers.bytes.clear();
+    //     })
+    // }
 }