• Dominik Inführ's avatar
    [heap] Safepointing with an atomic state · 17cba4f0
    Dominik Inführ authored
    To improve performance of parking, keep the thread state in an atomic
    variable instead of protecting it with a mutex.
    
    However the mutex was used e.g. to force Unpark() to block while the
    safepoint operation was still running. Therefore the safepoint algorithm
    has to change as well.
    
    Park() and Unpark() use CAS operation to transition the state.
    Safepoint() uses a relaxed load for checking whether a safepoint was
    requested. Since Safepoint(), Park() and Unpark() all have a slow path,
    there is no need for busy-waiting on the main thread.
    
    We need two more ThreadStates:
    * SafepointRequested: This state is set by GlobalSafepoint to force
        Running threads into the slow path on Safepoint() and Park(). This
        state also replaces the separate atomic<bool> safepoint_requested_
        field we used before.
    * ParkedSafepoint: This state is set by GlobalSafepoint as well to force
        parked threads into the slow path on Unpark().
    
    When stopping all threads, GlobalSafepoint transitions states from
    Running --> SafepointRequested and Parked --> ParkedSafepoint to force
    the slow path for all three methods. After performing the transition
    for each thread we know the exact number of Running threads and wait
    until each of them either reached a safepoint or parked itself.
    
    Design doc: https://docs.google.com/document/d/1p9klWyqT_AScAnK_PdHZTcNhZGzoBiYWPkUciIh2C58/edit?usp=sharing
    
    Bug: chromium:1177144, v8:10315
    Change-Id: I8697da915c7d18e2fb941f1bedf6181226408feb
    Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2704075Reviewed-by: 's avatarUlan Degenbaev <ulan@chromium.org>
    Reviewed-by: 's avatarMaya Lekova <mslekova@chromium.org>
    Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#73089}
    17cba4f0
local-heap.h 6.81 KB