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Reduce allocations in poll(2) implementation #1705

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Reduce allocations in poll(2) implementation #1705

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02c5a2a
Reuse alloction for closed_raw_fds in poll(2) Selector
Thomasdezeeuw Aug 4, 2023
7b3afc2
Small formatting cleanups
Thomasdezeeuw Aug 4, 2023
ec27b19
Reuse allocation for pending_removal in poll(2) impl
Thomasdezeeuw Aug 4, 2023
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114 changes: 61 additions & 53 deletions src/sys/unix/selector/poll.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,18 +3,19 @@
// Permission to use this code has been granted by original author:
// https://github.com/tokio-rs/mio/pull/1602#issuecomment-1218441031

use crate::sys::unix::selector::LOWEST_FD;
use crate::sys::unix::waker::WakerInternal;
use crate::{Interest, Token};
use std::collections::HashMap;
use std::fmt::{Debug, Formatter};
use std::mem::swap;
use std::os::unix::io::{AsRawFd, RawFd};
use std::sync::atomic::AtomicBool;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
use std::sync::{Arc, Condvar, Mutex};
use std::time::Duration;
use std::{cmp, fmt, io};

use crate::sys::unix::selector::LOWEST_FD;
use crate::sys::unix::waker::WakerInternal;
use crate::{Interest, Token};

/// Unique id for use as `SelectorId`.
#[cfg(debug_assertions)]
static NEXT_ID: AtomicUsize = AtomicUsize::new(1);
Expand All @@ -25,34 +26,36 @@ pub struct Selector {
/// Whether this selector currently has an associated waker.
#[cfg(debug_assertions)]
has_waker: AtomicBool,
/// Buffer used in [`SelectorState::select`].
fd_bufs0: Vec<RawFd>,
fd_bufs1: Vec<RawFd>,
}

impl Selector {
pub fn new() -> io::Result<Selector> {
let state = SelectorState::new()?;

Ok(Selector {
state: Arc::new(state),
state: Arc::new(SelectorState::new()?),
#[cfg(debug_assertions)]
has_waker: AtomicBool::new(false),
fd_bufs0: Vec::new(),
fd_bufs1: Vec::new(),
})
}

pub fn try_clone(&self) -> io::Result<Selector> {
// Just to keep the compiler happy :)
let _ = LOWEST_FD;

let state = self.state.clone();

let _ = LOWEST_FD; // Just to keep the compiler happy :)
Ok(Selector {
state,
state: self.state.clone(),
#[cfg(debug_assertions)]
has_waker: AtomicBool::new(self.has_waker.load(Ordering::Acquire)),
fd_bufs0: Vec::new(),
fd_bufs1: Vec::new(),
})
}

pub fn select(&self, events: &mut Events, timeout: Option<Duration>) -> io::Result<()> {
self.state.select(events, timeout)
pub fn select(&mut self, events: &mut Events, timeout: Option<Duration>) -> io::Result<()> {
self.state
.select(events, timeout, &mut self.fd_bufs0, &mut self.fd_bufs1)
}

pub fn register(&self, fd: RawFd, token: Token, interests: Interest) -> io::Result<()> {
Expand Down Expand Up @@ -85,6 +88,7 @@ impl Selector {
pub fn wake(&self, token: Token) -> io::Result<()> {
self.state.wake(token)
}

cfg_io_source! {
#[cfg(debug_assertions)]
pub fn id(&self) -> usize {
Expand All @@ -98,33 +102,30 @@ impl Selector {
struct SelectorState {
/// File descriptors to poll.
fds: Mutex<Fds>,

/// File descriptors which will be removed before the next poll call.
///
/// When a file descriptor is deregistered while a poll is running, we need to filter
/// out all removed descriptors after that poll is finished running.
/// When a file descriptor is deregistered while a poll is running, we need
/// to filter out all removed descriptors after that poll is finished
/// running.
pending_removal: Mutex<Vec<RawFd>>,

/// Token associated with Waker that have recently asked to wake. This will
/// cause a synthetic behaviour where on any wakeup we add all pending tokens
/// to the list of emitted events.
/// cause a synthetic behaviour where on any wakeup we add all pending
/// tokens to the list of emitted events.
pending_wake_token: Mutex<Option<Token>>,

/// Data is written to this to wake up the current instance of `wait`, which can occur when the
/// user notifies it (in which case `notified` would have been set) or when an operation needs
/// to occur (in which case `waiting_operations` would have been incremented).
/// Data is written to this to wake up the current instance of `wait`, which
/// can occur when the user notifies it (in which case `notified` would have
/// been set) or when an operation needs to occur (in which case
/// `waiting_operations` would have been incremented).
notify_waker: WakerInternal,

/// The number of operations (`add`, `modify` or `delete`) that are currently waiting on the
/// mutex to become free. When this is nonzero, `wait` must be suspended until it reaches zero
/// again.
/// The number of operations (`add`, `modify` or `delete`) that are
/// currently waiting on the mutex to become free. When this is nonzero,
/// `wait` must be suspended until it reaches zero again.
waiting_operations: AtomicUsize,
/// The condition variable that gets notified when `waiting_operations` reaches zero or
/// `notified` becomes true.
/// The condition variable that gets notified when `waiting_operations`
/// reaches zero or `notified` becomes true.
///
/// This is used with the `fds` mutex.
operations_complete: Condvar,

/// This selectors id.
#[cfg(debug_assertions)]
#[allow(dead_code)]
Expand All @@ -136,15 +137,16 @@ struct SelectorState {
struct Fds {
/// The list of `pollfds` taken by poll.
///
/// The first file descriptor is always present and is used to notify the poller.
/// The first file descriptor is always present and is used to notify the
/// poller.
poll_fds: Vec<PollFd>,
/// The map of each file descriptor to data associated with it. This does not include the file
/// descriptors created by the internal notify waker.
/// The map of each file descriptor to data associated with it. This does
/// not include the file descriptors created by the internal notify waker.
fd_data: HashMap<RawFd, FdData>,
}

/// Transparent wrapper around `libc::pollfd`, used to support `Debug` derives without adding the
/// `extra_traits` feature of `libc`.
/// Transparent wrapper around `libc::pollfd`, used to support `Debug` derives
/// without adding the `extra_traits` feature of `libc`.
#[repr(transparent)]
#[derive(Clone)]
struct PollFd(libc::pollfd);
Expand All @@ -166,16 +168,15 @@ struct FdData {
poll_fds_index: usize,
/// The key of the `Event` associated with this file descriptor.
token: Token,
/// Used to communicate with IoSourceState when we need to internally deregister
/// based on a closed fd.
/// Used to communicate with IoSourceState when we need to internally
/// deregister based on a closed fd.
shared_record: Arc<RegistrationRecord>,
}

impl SelectorState {
pub fn new() -> io::Result<SelectorState> {
let notify_waker = WakerInternal::new()?;

Ok(Self {
Ok(SelectorState {
fds: Mutex::new(Fds {
poll_fds: vec![PollFd(libc::pollfd {
fd: notify_waker.as_raw_fd(),
Expand All @@ -194,17 +195,19 @@ impl SelectorState {
})
}

pub fn select(&self, events: &mut Events, timeout: Option<Duration>) -> io::Result<()> {
pub fn select(
&self,
events: &mut Events,
timeout: Option<Duration>,
closed_raw_fds: &mut Vec<RawFd>,
pending_removal: &mut Vec<RawFd>,
) -> io::Result<()> {
events.clear();
closed_raw_fds.clear();
pending_removal.clear();

let mut fds = self.fds.lock().unwrap();

// Keep track of fds that receive POLLHUP or POLLERR (i.e. won't receive further
// events) and internally deregister them before they are externally deregister'd. See
// IoSourceState below to track how the external deregister call will be handled
// when this state occurs.
let mut closed_raw_fds = Vec::new();

loop {
// Complete all current operations.
loop {
Expand Down Expand Up @@ -240,7 +243,7 @@ impl SelectorState {
// We now check whether this poll was performed with descriptors which were pending
// for removal and filter out any matching.
let mut pending_removal_guard = self.pending_removal.lock().unwrap();
let mut pending_removal = std::mem::replace(pending_removal_guard.as_mut(), Vec::new());
swap(pending_removal_guard.as_mut(), pending_removal);
drop(pending_removal_guard);

// Store the events if there were any.
Expand All @@ -258,7 +261,7 @@ impl SelectorState {
}

for fd_data in fds.fd_data.values_mut() {
let PollFd(poll_fd) = &mut fds.poll_fds[fd_data.poll_fds_index];
let poll_fd = &mut fds.poll_fds[fd_data.poll_fds_index].0;

if pending_removal.contains(&poll_fd.fd) {
// Fd was removed while poll was running
Expand All @@ -272,6 +275,12 @@ impl SelectorState {
events: poll_fd.revents,
});

// Keep track of fds that receive POLLHUP or POLLERR
// (i.e. won't receive further events) and internally
// deregister them before they are externally
// deregistered. See IoSourceState below to track how
// the external deregister call will be handled when
// this state occurs.
if poll_fd.revents & (libc::POLLHUP | libc::POLLERR) != 0 {
pending_removal.push(poll_fd.fd);
closed_raw_fds.push(poll_fd.fd);
Expand Down Expand Up @@ -299,7 +308,6 @@ impl SelectorState {

drop(fds);
let _ = self.deregister_all(&closed_raw_fds);

Ok(())
}

Expand Down Expand Up @@ -665,7 +673,7 @@ cfg_io_source! {

pub fn do_io<T, F, R>(&self, f: F, io: &T) -> io::Result<R>
where
F: FnOnce(&T) -> io::Result<R>,
F: FnOnce(&T) -> io::Result<R>,
{
let result = f(io);

Expand Down