servo/components/style/global_style_data.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

//! Global style data

use crate::context::StyleSystemOptions;
#[cfg(feature = "gecko")]
use crate::gecko_bindings::bindings;
use crate::parallel::STYLE_THREAD_STACK_SIZE_KB;
use crate::shared_lock::SharedRwLock;
use crate::thread_state;
use parking_lot::{RwLock, RwLockReadGuard};
use rayon;
use std::env;
use std::sync::atomic::{AtomicUsize, Ordering};

/// Global style data
pub struct GlobalStyleData {
    /// Shared RWLock for CSSOM objects
    pub shared_lock: SharedRwLock,

    /// Global style system options determined by env vars.
    pub options: StyleSystemOptions,
}

/// Global thread pool.
pub struct StyleThreadPool {
    /// How many threads parallel styling can use.
    pub num_threads: usize,

    /// The parallel styling thread pool.
    ///
    /// For leak-checking purposes, we want to terminate the thread-pool, which
    /// waits for all the async jobs to complete. Thus the RwLock.
    style_thread_pool: RwLock<Option<rayon::ThreadPool>>,
}

fn thread_name(index: usize) -> String {
    format!("StyleThread#{}", index)
}

// A counter so that we can wait for shutdown of all threads. See
// StyleThreadPool::shutdown.
static ALIVE_WORKER_THREADS: AtomicUsize = AtomicUsize::new(0);

fn thread_startup(_index: usize) {
    ALIVE_WORKER_THREADS.fetch_add(1, Ordering::Relaxed);
    thread_state::initialize_layout_worker_thread();
    #[cfg(feature = "gecko")]
    unsafe {
        use std::ffi::CString;

        bindings::Gecko_SetJemallocThreadLocalArena(true);
        let name = thread_name(_index);
        let name = CString::new(name).unwrap();
        // Gecko_RegisterProfilerThread copies the passed name here.
        bindings::Gecko_RegisterProfilerThread(name.as_ptr());
    }
}

fn thread_shutdown(_: usize) {
    #[cfg(feature = "gecko")]
    unsafe {
        bindings::Gecko_UnregisterProfilerThread();
        bindings::Gecko_SetJemallocThreadLocalArena(false);
    }
    ALIVE_WORKER_THREADS.fetch_sub(1, Ordering::Relaxed);
}

impl StyleThreadPool {
    /// Shuts down the thread pool, waiting for all work to complete.
    pub fn shutdown() {
        if ALIVE_WORKER_THREADS.load(Ordering::Relaxed) == 0 {
            return;
        }
        {
            // Drop the pool.
            let _ = STYLE_THREAD_POOL.style_thread_pool.write().take();
        }
        // Spin until all our threads are done. This will usually be pretty
        // fast, as on shutdown there should be basically no threads left
        // running.
        //
        // This still _technically_ doesn't give us the guarantee of TLS
        // destructors running on the worker threads. For that we'd need help
        // from rayon to properly join the threads.
        //
        // See https://github.com/rayon-rs/rayon/issues/688
        //
        // So we instead intentionally leak TLS stuff (see BLOOM_KEY and co) for
        // now until that's fixed.
        while ALIVE_WORKER_THREADS.load(Ordering::Relaxed) != 0 {
            std::thread::yield_now();
        }
    }

    /// Returns a reference to the thread pool.
    ///
    /// We only really want to give read-only access to the pool, except
    /// for shutdown().
    pub fn pool(&self) -> RwLockReadGuard<Option<rayon::ThreadPool>> {
        self.style_thread_pool.read()
    }
}

lazy_static! {
    /// Global thread pool
    pub static ref STYLE_THREAD_POOL: StyleThreadPool = {
        let stylo_threads = env::var("STYLO_THREADS")
            .map(|s| s.parse::<usize>().expect("invalid STYLO_THREADS value"));
        let mut num_threads = match stylo_threads {
            Ok(num) => num,
            #[cfg(feature = "servo")]
            _ => {
                use servo_config::pref;
                // We always set this pref on startup, before layout or script
                // have had a chance of accessing (and thus creating) the
                // thread-pool.
                pref!(layout.threads) as usize
            }
            #[cfg(feature = "gecko")]
            _ => {
                // The default heuristic is num_virtual_cores * .75. This gives
                // us three threads on a hyper-threaded dual core, and six
                // threads on a hyper-threaded quad core. The performance
                // benefit of additional threads seems to level off at around
                // six, so we cap it there on many-core machines
                // (see bug 1431285 comment 14).
                use num_cpus;
                use std::cmp;
                cmp::min(cmp::max(num_cpus::get() * 3 / 4, 1), 6)
            }
        };

        // If num_threads is one, there's no point in creating a thread pool, so
        // force it to zero.
        //
        // We allow developers to force a one-thread pool for testing via a
        // special environmental variable.
        if num_threads == 1 {
            let force_pool = env::var("FORCE_STYLO_THREAD_POOL")
                .ok().map_or(false, |s| s.parse::<usize>().expect("invalid FORCE_STYLO_THREAD_POOL value") == 1);
            if !force_pool {
                num_threads = 0;
            }
        }

        let pool = if num_threads < 1 {
            None
        } else {
            let workers = rayon::ThreadPoolBuilder::new()
                .num_threads(num_threads)
                .thread_name(thread_name)
                .start_handler(thread_startup)
                .exit_handler(thread_shutdown)
                .stack_size(STYLE_THREAD_STACK_SIZE_KB * 1024)
                .build();
            workers.ok()
        };

        StyleThreadPool {
            num_threads,
            style_thread_pool: RwLock::new(pool),
        }
    };

    /// Global style data
    pub static ref GLOBAL_STYLE_DATA: GlobalStyleData = GlobalStyleData {
        shared_lock: SharedRwLock::new_leaked(),
        options: StyleSystemOptions::default(),
    };
}