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io: reduce syscalls in poll_read
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As the [epoll documentation points out](https://man7.org/linux/man-pages/man7/epoll.7.html), a read that only partially fills a buffer is sufficient to show that the socket buffer has been drained.

We can take advantage of this by clearing readiness in this case, which seems to significantly improve performance under certain conditions.
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carllerche authored and Noah-Kennedy committed Jul 26, 2022
1 parent ffff9e6 commit b3d8fad
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Showing 2 changed files with 26 additions and 9 deletions.
1 change: 1 addition & 0 deletions tokio/src/io/driver/registration.rs
Original file line number Diff line number Diff line change
Expand Up @@ -115,6 +115,7 @@ impl Registration {

// Uses the poll path, requiring the caller to ensure mutual exclusion for
// correctness. Only the last task to call this function is notified.
#[cfg(not(all(target_arch = "wasm32", target_os = "wasi")))]
pub(crate) fn poll_read_io<R>(
&self,
cx: &mut Context<'_>,
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34 changes: 25 additions & 9 deletions tokio/src/io/poll_evented.rs
Original file line number Diff line number Diff line change
Expand Up @@ -153,16 +153,32 @@ feature! {
{
use std::io::Read;

let n = ready!(self.registration.poll_read_io(cx, || {
loop {
let evt = ready!(self.registration.poll_read_ready(cx))?;

let b = &mut *(buf.unfilled_mut() as *mut [std::mem::MaybeUninit<u8>] as *mut [u8]);
self.io.as_ref().unwrap().read(b)
}))?;

// Safety: We trust `TcpStream::read` to have filled up `n` bytes in the
// buffer.
buf.assume_init(n);
buf.advance(n);
Poll::Ready(Ok(()))
let len = b.len();

match self.io.as_ref().unwrap().read(b) {
Ok(n) => {
// if we read a partially full buffer, this is sufficient on unix to show
// that the socket buffer has been drained
if n > 0 && (cfg!(windows) || n < len) {
self.registration.clear_readiness(evt);
}

// Safety: We trust `TcpStream::read` to have filled up `n` bytes in the
// buffer.
buf.assume_init(n);
buf.advance(n);
return Poll::Ready(Ok(()));
},
Err(e) if e.kind() == io::ErrorKind::WouldBlock => {
self.registration.clear_readiness(evt);
}
Err(e) => return Poll::Ready(Err(e)),
}
}
}

pub(crate) fn poll_write<'a>(&'a self, cx: &mut Context<'_>, buf: &[u8]) -> Poll<io::Result<usize>>
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