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perf(gpui): keep the profiler journal ring out of memory until it is written #11

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perf(gpui): keep the profiler journal ring out of memory until it is written #11

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23 changes: 11 additions & 12 deletions crates/gpui/src/profiler/journal.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -672,8 +672,11 @@ pub(crate) fn record_window_closed(window_id: WindowId) {

const SLOT_WRITER: usize = 1 << (usize::BITS - 1);
const SLOT_READER_MASK: usize = !SLOT_WRITER;
const EMPTY_SEQUENCE: u64 = u64::MAX;

/// A slot's `sequence` holds the published sequence plus one, so that a slot nothing has been
/// published to is all zeros. That lets the ring be allocated zeroed: the operating system backs
/// its pages as slots are first written, so a process that records a few entries (one that never
/// reads the journal) does not make the whole ring resident.
struct JournalSlot {
users: AtomicUsize,
sequence: AtomicU64,
Expand All @@ -685,14 +688,6 @@ struct JournalSlot {
unsafe impl Sync for JournalSlot {}

impl JournalSlot {
fn new() -> Self {
Self {
users: AtomicUsize::new(0),
sequence: AtomicU64::new(EMPTY_SEQUENCE),
entry: UnsafeCell::new(MaybeUninit::uninit()),
}
}

fn try_publish(&self, sequence: u64, entry: ForegroundJournalEntry) -> bool {
if self
.users
Expand All @@ -707,14 +702,15 @@ impl JournalSlot {
unsafe {
(*self.entry.get()).write(entry);
}
self.sequence.store(sequence, Ordering::Relaxed);
self.sequence
.store(sequence.wrapping_add(1), Ordering::Relaxed);
self.users.store(0, Ordering::Release);
true
}

fn try_read(&self, expected_sequence: u64) -> Option<ForegroundJournalEntry> {
let _guard = JournalSlotReadGuard::try_new(self)?;
if self.sequence.load(Ordering::Relaxed) != expected_sequence {
if self.sequence.load(Ordering::Relaxed) != expected_sequence.wrapping_add(1) {
return None;
}

Expand Down Expand Up @@ -780,8 +776,11 @@ struct JournalRing {
impl JournalRing {
fn new(capacity: usize) -> Self {
let capacity = capacity.max(1);
// SAFETY: every field of a slot is valid zeroed: `users` 0 is unowned, `sequence` 0 is
// empty, and the entry is `MaybeUninit`.
let slots = unsafe { Box::<[JournalSlot]>::new_zeroed_slice(capacity).assume_init() };
Self {
slots: (0..capacity).map(|_| JournalSlot::new()).collect(),
slots,
finalized: AtomicU64::new(0),
offered: AtomicU64::new(0),
}
Expand Down