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Improve VecCache under parallel frontend
This replaces the single Vec allocation with a series of progressively larger buckets. With the cfg for parallel enabled but with -Zthreads=1, this looks like a slight regression in i-count and cycle counts (<0.1%). With the parallel frontend at -Zthreads=4, this is an improvement (-5% wall-time from 5.788 to 5.4688 on libcore) than our current Lock-based approach, likely due to reducing the bouncing of the cache line holding the lock. At -Zthreads=32 it's a huge improvement (-46%: 8.829 -> 4.7319 seconds).
This commit is contained in:
Mark Rousskov
parent
b73478b6ee
commit
da58efb11d
@ -22,6 +22,7 @@
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#![feature(auto_traits)]
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#![feature(cfg_match)]
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#![feature(core_intrinsics)]
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#![feature(dropck_eyepatch)]
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#![feature(extend_one)]
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#![feature(file_buffered)]
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#![feature(hash_raw_entry)]
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@ -79,6 +80,7 @@ pub mod thinvec;
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pub mod transitive_relation;
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pub mod unhash;
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pub mod unord;
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pub mod vec_cache;
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pub mod work_queue;
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mod atomic_ref;
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324
compiler/rustc_data_structures/src/vec_cache.rs
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324
compiler/rustc_data_structures/src/vec_cache.rs
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@ -0,0 +1,324 @@
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//! VecCache maintains a mapping from K -> (V, I) pairing. K and I must be roughly u32-sized, and V
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//! must be Copy.
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//!
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//! VecCache supports efficient concurrent put/get across the key space, with write-once semantics
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//! (i.e., a given key can only be put once). Subsequent puts will panic.
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//!
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//! This is currently used for query caching.
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use std::fmt::Debug;
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use std::marker::PhantomData;
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use std::sync::atomic::{AtomicPtr, AtomicU32, AtomicUsize, Ordering};
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use rustc_index::Idx;
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struct Slot<V> {
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// We never construct &Slot<V> so it's fine for this to not be in an UnsafeCell.
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value: V,
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// This is both an index and a once-lock.
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//
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// 0: not yet initialized.
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// 1: lock held, initializing.
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// 2..u32::MAX - 2: initialized.
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index_and_lock: AtomicU32,
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}
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/// This uniquely identifies a single `Slot<V>` entry in the buckets map, and provides accessors for
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/// either getting the value or putting a value.
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#[derive(Copy, Clone, Debug)]
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struct SlotIndex {
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// the index of the bucket in VecCache (0 to 20)
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bucket_idx: usize,
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// number of entries in that bucket
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entries: usize,
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// the index of the slot within the bucket
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index_in_bucket: usize,
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}
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// This makes sure the counts are consistent with what we allocate, precomputing each bucket a
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// compile-time. Visiting all powers of two is enough to hit all the buckets.
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//
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// We confirm counts are accurate in the slot_index_exhaustive test.
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const ENTRIES_BY_BUCKET: [usize; 21] = {
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let mut entries = [0; 21];
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let mut key = 0;
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loop {
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let si = SlotIndex::from_index(key);
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entries[si.bucket_idx] = si.entries;
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if key == 0 {
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key = 1;
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} else if key == (1 << 31) {
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break;
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} else {
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key <<= 1;
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}
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}
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entries
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};
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impl SlotIndex {
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// This unpacks a flat u32 index into identifying which bucket it belongs to and the offset
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// within that bucket. As noted in the VecCache docs, buckets double in size with each index.
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// Typically that would mean 31 buckets (2^0 + 2^1 ... + 2^31 = u32::MAX - 1), but to reduce
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// the size of the VecCache struct and avoid uselessly small allocations, we instead have the
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// first bucket have 2**12 entries. To simplify the math, the second bucket also 2**12 entries,
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// and buckets double from there.
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//
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// We assert that [0, 2**32 - 1] uniquely map through this function to individual, consecutive
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// slots (see `slot_index_exhaustive` in tests).
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#[inline]
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const fn from_index(idx: u32) -> Self {
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let mut bucket = match idx.checked_ilog2() {
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Some(x) => x as usize,
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None => 0,
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};
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let entries;
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let running_sum;
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if bucket <= 11 {
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entries = 1 << 12;
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running_sum = 0;
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bucket = 0;
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} else {
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entries = 1 << bucket;
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running_sum = entries;
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bucket = bucket - 11;
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}
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SlotIndex { bucket_idx: bucket, entries, index_in_bucket: idx as usize - running_sum }
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}
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// SAFETY: Buckets must be managed solely by functions here (i.e., get/put on SlotIndex) and
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// `self` comes from SlotIndex::from_index
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#[inline]
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unsafe fn get<V: Copy>(&self, buckets: &[AtomicPtr<Slot<V>>; 21]) -> Option<(V, u32)> {
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// SAFETY: `bucket_idx` is ilog2(u32).saturating_sub(11), which is at most 21, i.e.,
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// in-bounds of buckets. See `from_index` for computation.
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let bucket = unsafe { buckets.get_unchecked(self.bucket_idx) };
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let ptr = bucket.load(Ordering::Acquire);
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// Bucket is not yet initialized: then we obviously won't find this entry in that bucket.
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if ptr.is_null() {
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return None;
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}
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assert!(self.index_in_bucket < self.entries);
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// SAFETY: `bucket` was allocated (so <= isize in total bytes) to hold `entries`, so this
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// must be inbounds.
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let slot = unsafe { ptr.add(self.index_in_bucket) };
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// SAFETY: initialized bucket has zeroed all memory within the bucket, so we are valid for
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// AtomicU32 access.
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let index_and_lock = unsafe { &(*slot).index_and_lock };
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let current = index_and_lock.load(Ordering::Acquire);
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let index = match current {
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0 => return None,
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// Treat "initializing" as actually just not initialized at all.
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// The only reason this is a separate state is that `complete` calls could race and
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// we can't allow that, but from load perspective there's no difference.
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1 => return None,
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_ => current - 2,
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};
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// SAFETY:
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// * slot is a valid pointer (buckets are always valid for the index we get).
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// * value is initialized since we saw a >= 2 index above.
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// * `V: Copy`, so safe to read.
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let value = unsafe { (*slot).value };
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Some((value, index))
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}
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fn bucket_ptr<V>(&self, bucket: &AtomicPtr<Slot<V>>) -> *mut Slot<V> {
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let ptr = bucket.load(Ordering::Acquire);
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if ptr.is_null() { self.initialize_bucket(bucket) } else { ptr }
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}
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#[cold]
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fn initialize_bucket<V>(&self, bucket: &AtomicPtr<Slot<V>>) -> *mut Slot<V> {
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static LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());
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// If we are initializing the bucket, then acquire a global lock.
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//
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// This path is quite cold, so it's cheap to use a global lock. This ensures that we never
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// have multiple allocations for the same bucket.
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let _allocator_guard = LOCK.lock().unwrap_or_else(|e| e.into_inner());
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let ptr = bucket.load(Ordering::Acquire);
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// OK, now under the allocator lock, if we're still null then it's definitely us that will
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// initialize this bucket.
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if ptr.is_null() {
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let bucket_layout =
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std::alloc::Layout::array::<Slot<V>>(self.entries as usize).unwrap();
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// This is more of a sanity check -- this code is very cold, so it's safe to pay a
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// little extra cost here.
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assert!(bucket_layout.size() > 0);
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// SAFETY: Just checked that size is non-zero.
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let allocated = unsafe { std::alloc::alloc_zeroed(bucket_layout).cast::<Slot<V>>() };
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if allocated.is_null() {
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std::alloc::handle_alloc_error(bucket_layout);
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}
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bucket.store(allocated, Ordering::Release);
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allocated
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} else {
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// Otherwise some other thread initialized this bucket after we took the lock. In that
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// case, just return early.
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ptr
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}
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}
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/// Returns true if this successfully put into the map.
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#[inline]
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fn put<V>(&self, buckets: &[AtomicPtr<Slot<V>>; 21], value: V, extra: u32) -> bool {
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// SAFETY: `bucket_idx` is ilog2(u32).saturating_sub(11), which is at most 21, i.e.,
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// in-bounds of buckets.
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let bucket = unsafe { buckets.get_unchecked(self.bucket_idx) };
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let ptr = self.bucket_ptr(bucket);
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assert!(self.index_in_bucket < self.entries);
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// SAFETY: `bucket` was allocated (so <= isize in total bytes) to hold `entries`, so this
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// must be inbounds.
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let slot = unsafe { ptr.add(self.index_in_bucket) };
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// SAFETY: initialized bucket has zeroed all memory within the bucket, so we are valid for
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// AtomicU32 access.
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let index_and_lock = unsafe { &(*slot).index_and_lock };
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match index_and_lock.compare_exchange(0, 1, Ordering::AcqRel, Ordering::Acquire) {
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Ok(_) => {
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// We have acquired the initialization lock. It is our job to write `value` and
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// then set the lock to the real index.
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unsafe {
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(&raw mut (*slot).value).write(value);
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}
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index_and_lock.store(extra.checked_add(2).unwrap(), Ordering::Release);
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true
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}
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// Treat "initializing" as the caller's fault. Callers are responsible for ensuring that
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// there are no races on initialization. In the compiler's current usage for query
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// caches, that's the "active query map" which ensures each query actually runs once
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// (even if concurrently started).
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Err(1) => panic!("caller raced calls to put()"),
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// This slot was already populated. Also ignore, currently this is the same as
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// "initializing".
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Err(_) => false,
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}
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}
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}
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pub struct VecCache<K: Idx, V, I> {
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// Entries per bucket:
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// Bucket 0: 4096 2^12
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// Bucket 1: 4096 2^12
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// Bucket 2: 8192
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// Bucket 3: 16384
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// ...
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// Bucket 19: 1073741824
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// Bucket 20: 2147483648
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// The total number of entries if all buckets are initialized is u32::MAX-1.
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buckets: [AtomicPtr<Slot<V>>; 21],
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// In the compiler's current usage these are only *read* during incremental and self-profiling.
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// They are an optimization over iterating the full buckets array.
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present: [AtomicPtr<Slot<()>>; 21],
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len: AtomicUsize,
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key: PhantomData<(K, I)>,
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}
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impl<K: Idx, V, I> Default for VecCache<K, V, I> {
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fn default() -> Self {
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VecCache {
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buckets: Default::default(),
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key: PhantomData,
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len: Default::default(),
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present: Default::default(),
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}
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}
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}
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// SAFETY: No access to `V` is made.
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unsafe impl<K: Idx, #[may_dangle] V, I> Drop for VecCache<K, V, I> {
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fn drop(&mut self) {
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// We have unique ownership, so no locks etc. are needed. Since `K` and `V` are both `Copy`,
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// we are also guaranteed to just need to deallocate any large arrays (not iterate over
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// contents).
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//
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// Confirm no need to deallocate invidual entries. Note that `V: Copy` is asserted on
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// insert/lookup but not necessarily construction, primarily to avoid annoyingly propagating
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// the bounds into struct definitions everywhere.
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assert!(!std::mem::needs_drop::<K>());
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assert!(!std::mem::needs_drop::<V>());
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for (idx, bucket) in self.buckets.iter().enumerate() {
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let bucket = bucket.load(Ordering::Acquire);
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if !bucket.is_null() {
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let layout = std::alloc::Layout::array::<Slot<V>>(ENTRIES_BY_BUCKET[idx]).unwrap();
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unsafe {
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std::alloc::dealloc(bucket.cast(), layout);
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}
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}
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}
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for (idx, bucket) in self.present.iter().enumerate() {
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let bucket = bucket.load(Ordering::Acquire);
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if !bucket.is_null() {
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let layout = std::alloc::Layout::array::<Slot<()>>(ENTRIES_BY_BUCKET[idx]).unwrap();
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unsafe {
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std::alloc::dealloc(bucket.cast(), layout);
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}
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}
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}
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}
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}
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impl<K, V, I> VecCache<K, V, I>
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where
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K: Eq + Idx + Copy + Debug,
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V: Copy,
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I: Idx + Copy,
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{
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#[inline(always)]
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pub fn lookup(&self, key: &K) -> Option<(V, I)> {
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let key = u32::try_from(key.index()).unwrap();
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let slot_idx = SlotIndex::from_index(key);
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match unsafe { slot_idx.get(&self.buckets) } {
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Some((value, idx)) => Some((value, I::new(idx as usize))),
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None => None,
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}
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}
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#[inline]
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pub fn complete(&self, key: K, value: V, index: I) {
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let key = u32::try_from(key.index()).unwrap();
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let slot_idx = SlotIndex::from_index(key);
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if slot_idx.put(&self.buckets, value, index.index() as u32) {
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let present_idx = self.len.fetch_add(1, Ordering::Relaxed);
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let slot = SlotIndex::from_index(present_idx as u32);
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// We should always be uniquely putting due to `len` fetch_add returning unique values.
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assert!(slot.put(&self.present, (), key));
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}
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}
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pub fn iter(&self, f: &mut dyn FnMut(&K, &V, I)) {
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for idx in 0..self.len.load(Ordering::Acquire) {
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let key = SlotIndex::from_index(idx as u32);
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match unsafe { key.get(&self.present) } {
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// This shouldn't happen in our current usage (iter is really only
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// used long after queries are done running), but if we hit this in practice it's
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// probably fine to just break early.
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None => unreachable!(),
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Some(((), key)) => {
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let key = K::new(key as usize);
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// unwrap() is OK: present entries are always written only after we put the real
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// entry.
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let value = self.lookup(&key).unwrap();
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f(&key, &value.0, value.1);
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}
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}
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}
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}
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}
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#[cfg(test)]
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mod tests;
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95
compiler/rustc_data_structures/src/vec_cache/tests.rs
Normal file
95
compiler/rustc_data_structures/src/vec_cache/tests.rs
Normal file
@ -0,0 +1,95 @@
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use super::*;
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#[test]
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#[cfg(not(miri))]
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fn vec_cache_empty() {
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let cache: VecCache<u32, u32, u32> = VecCache::default();
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for key in 0..u32::MAX {
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assert!(cache.lookup(&key).is_none());
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}
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}
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#[test]
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fn vec_cache_insert_and_check() {
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let cache: VecCache<u32, u32, u32> = VecCache::default();
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cache.complete(0, 1, 2);
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assert_eq!(cache.lookup(&0), Some((1, 2)));
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}
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#[test]
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fn sparse_inserts() {
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let cache: VecCache<u32, u8, u32> = VecCache::default();
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let end = if cfg!(target_pointer_width = "64") && cfg!(target_os = "linux") {
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// For paged memory, 64-bit systems we should be able to sparsely allocate all of the pages
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// needed for these inserts cheaply (without needing to actually have gigabytes of resident
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// memory).
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31
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} else {
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// Otherwise, still run the test but scaled back:
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//
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// Each slot is 5 bytes, so 2^25 entries (on non-virtual memory systems, like e.g. Windows) will
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// mean 160 megabytes of allocated memory. Going beyond that is probably not reasonable for
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// tests.
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25
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};
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for shift in 0..end {
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let key = 1u32 << shift;
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cache.complete(key, shift, key);
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assert_eq!(cache.lookup(&key), Some((shift, key)));
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}
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}
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#[test]
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fn concurrent_stress_check() {
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let cache: VecCache<u32, u32, u32> = VecCache::default();
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std::thread::scope(|s| {
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for idx in 0..100 {
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let cache = &cache;
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s.spawn(move || {
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cache.complete(idx, idx, idx);
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});
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}
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});
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for idx in 0..100 {
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assert_eq!(cache.lookup(&idx), Some((idx, idx)));
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}
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}
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#[test]
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fn slot_entries_table() {
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assert_eq!(ENTRIES_BY_BUCKET, [
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4096, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288, 1048576, 2097152, 4194304,
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8388608, 16777216, 33554432, 67108864, 134217728, 268435456, 536870912, 1073741824,
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2147483648
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]);
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}
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#[test]
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#[cfg(not(miri))]
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fn slot_index_exhaustive() {
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let mut buckets = [0u32; 21];
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for idx in 0..=u32::MAX {
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buckets[SlotIndex::from_index(idx).bucket_idx] += 1;
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}
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let mut prev = None::<SlotIndex>;
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for idx in 0..=u32::MAX {
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let slot_idx = SlotIndex::from_index(idx);
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if let Some(p) = prev {
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if p.bucket_idx == slot_idx.bucket_idx {
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assert_eq!(p.index_in_bucket + 1, slot_idx.index_in_bucket);
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} else {
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assert_eq!(slot_idx.index_in_bucket, 0);
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}
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} else {
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assert_eq!(idx, 0);
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assert_eq!(slot_idx.index_in_bucket, 0);
|
||||
assert_eq!(slot_idx.bucket_idx, 0);
|
||||
}
|
||||
|
||||
assert_eq!(buckets[slot_idx.bucket_idx], slot_idx.entries as u32);
|
||||
assert_eq!(ENTRIES_BY_BUCKET[slot_idx.bucket_idx], slot_idx.entries, "{}", idx);
|
||||
|
||||
prev = Some(slot_idx);
|
||||
}
|
||||
}
|
||||
@ -2,6 +2,7 @@
|
||||
|
||||
use rustc_hir::def_id::{CrateNum, DefId, LOCAL_CRATE, LocalDefId, LocalModDefId, ModDefId};
|
||||
use rustc_hir::hir_id::{HirId, OwnerId};
|
||||
use rustc_query_system::dep_graph::DepNodeIndex;
|
||||
use rustc_query_system::query::{DefIdCache, DefaultCache, SingleCache, VecCache};
|
||||
use rustc_span::symbol::{Ident, Symbol};
|
||||
use rustc_span::{DUMMY_SP, Span};
|
||||
@ -110,7 +111,7 @@ impl<'tcx> Key for mir::interpret::LitToConstInput<'tcx> {
|
||||
}
|
||||
|
||||
impl Key for CrateNum {
|
||||
type Cache<V> = VecCache<Self, V>;
|
||||
type Cache<V> = VecCache<Self, V, DepNodeIndex>;
|
||||
|
||||
fn default_span(&self, _: TyCtxt<'_>) -> Span {
|
||||
DUMMY_SP
|
||||
@ -127,7 +128,7 @@ impl AsLocalKey for CrateNum {
|
||||
}
|
||||
|
||||
impl Key for OwnerId {
|
||||
type Cache<V> = VecCache<Self, V>;
|
||||
type Cache<V> = VecCache<Self, V, DepNodeIndex>;
|
||||
|
||||
fn default_span(&self, tcx: TyCtxt<'_>) -> Span {
|
||||
self.to_def_id().default_span(tcx)
|
||||
@ -139,7 +140,7 @@ impl Key for OwnerId {
|
||||
}
|
||||
|
||||
impl Key for LocalDefId {
|
||||
type Cache<V> = VecCache<Self, V>;
|
||||
type Cache<V> = VecCache<Self, V, DepNodeIndex>;
|
||||
|
||||
fn default_span(&self, tcx: TyCtxt<'_>) -> Span {
|
||||
self.to_def_id().default_span(tcx)
|
||||
|
||||
@ -2,6 +2,7 @@
|
||||
#![allow(rustc::potential_query_instability, internal_features)]
|
||||
#![feature(assert_matches)]
|
||||
#![feature(core_intrinsics)]
|
||||
#![feature(dropck_eyepatch)]
|
||||
#![feature(hash_raw_entry)]
|
||||
#![feature(let_chains)]
|
||||
#![feature(min_specialization)]
|
||||
|
||||
@ -3,9 +3,10 @@ use std::hash::Hash;
|
||||
|
||||
use rustc_data_structures::fx::FxHashMap;
|
||||
use rustc_data_structures::sharded::{self, Sharded};
|
||||
use rustc_data_structures::sync::{Lock, OnceLock};
|
||||
use rustc_data_structures::sync::OnceLock;
|
||||
pub use rustc_data_structures::vec_cache::VecCache;
|
||||
use rustc_hir::def_id::LOCAL_CRATE;
|
||||
use rustc_index::{Idx, IndexVec};
|
||||
use rustc_index::Idx;
|
||||
use rustc_span::def_id::{DefId, DefIndex};
|
||||
|
||||
use crate::dep_graph::DepNodeIndex;
|
||||
@ -100,52 +101,10 @@ where
|
||||
}
|
||||
}
|
||||
|
||||
pub struct VecCache<K: Idx, V> {
|
||||
cache: Lock<IndexVec<K, Option<(V, DepNodeIndex)>>>,
|
||||
}
|
||||
|
||||
impl<K: Idx, V> Default for VecCache<K, V> {
|
||||
fn default() -> Self {
|
||||
VecCache { cache: Default::default() }
|
||||
}
|
||||
}
|
||||
|
||||
impl<K, V> QueryCache for VecCache<K, V>
|
||||
where
|
||||
K: Eq + Idx + Copy + Debug,
|
||||
V: Copy,
|
||||
{
|
||||
type Key = K;
|
||||
type Value = V;
|
||||
|
||||
#[inline(always)]
|
||||
fn lookup(&self, key: &K) -> Option<(V, DepNodeIndex)> {
|
||||
let lock = self.cache.lock();
|
||||
if let Some(Some(value)) = lock.get(*key) { Some(*value) } else { None }
|
||||
}
|
||||
|
||||
#[inline]
|
||||
fn complete(&self, key: K, value: V, index: DepNodeIndex) {
|
||||
let mut lock = self.cache.lock();
|
||||
lock.insert(key, (value, index));
|
||||
}
|
||||
|
||||
fn iter(&self, f: &mut dyn FnMut(&Self::Key, &Self::Value, DepNodeIndex)) {
|
||||
for (k, v) in self.cache.lock().iter_enumerated() {
|
||||
if let Some(v) = v {
|
||||
f(&k, &v.0, v.1);
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
pub struct DefIdCache<V> {
|
||||
/// Stores the local DefIds in a dense map. Local queries are much more often dense, so this is
|
||||
/// a win over hashing query keys at marginal memory cost (~5% at most) compared to FxHashMap.
|
||||
///
|
||||
/// The second element of the tuple is the set of keys actually present in the IndexVec, used
|
||||
/// for faster iteration in `iter()`.
|
||||
local: Lock<(IndexVec<DefIndex, Option<(V, DepNodeIndex)>>, Vec<DefIndex>)>,
|
||||
local: VecCache<DefIndex, V, DepNodeIndex>,
|
||||
foreign: DefaultCache<DefId, V>,
|
||||
}
|
||||
|
||||
@ -165,8 +124,7 @@ where
|
||||
#[inline(always)]
|
||||
fn lookup(&self, key: &DefId) -> Option<(V, DepNodeIndex)> {
|
||||
if key.krate == LOCAL_CRATE {
|
||||
let cache = self.local.lock();
|
||||
cache.0.get(key.index).and_then(|v| *v)
|
||||
self.local.lookup(&key.index)
|
||||
} else {
|
||||
self.foreign.lookup(key)
|
||||
}
|
||||
@ -175,27 +133,39 @@ where
|
||||
#[inline]
|
||||
fn complete(&self, key: DefId, value: V, index: DepNodeIndex) {
|
||||
if key.krate == LOCAL_CRATE {
|
||||
let mut cache = self.local.lock();
|
||||
let (cache, present) = &mut *cache;
|
||||
let slot = cache.ensure_contains_elem(key.index, Default::default);
|
||||
if slot.is_none() {
|
||||
// FIXME: Only store the present set when running in incremental mode. `iter` is not
|
||||
// used outside of saving caches to disk and self-profile.
|
||||
present.push(key.index);
|
||||
}
|
||||
*slot = Some((value, index));
|
||||
self.local.complete(key.index, value, index)
|
||||
} else {
|
||||
self.foreign.complete(key, value, index)
|
||||
}
|
||||
}
|
||||
|
||||
fn iter(&self, f: &mut dyn FnMut(&Self::Key, &Self::Value, DepNodeIndex)) {
|
||||
let guard = self.local.lock();
|
||||
let (cache, present) = &*guard;
|
||||
for &idx in present.iter() {
|
||||
let value = cache[idx].unwrap();
|
||||
f(&DefId { krate: LOCAL_CRATE, index: idx }, &value.0, value.1);
|
||||
}
|
||||
self.local.iter(&mut |key, value, index| {
|
||||
f(&DefId { krate: LOCAL_CRATE, index: *key }, value, index);
|
||||
});
|
||||
self.foreign.iter(f);
|
||||
}
|
||||
}
|
||||
|
||||
impl<K, V> QueryCache for VecCache<K, V, DepNodeIndex>
|
||||
where
|
||||
K: Idx + Eq + Hash + Copy + Debug,
|
||||
V: Copy,
|
||||
{
|
||||
type Key = K;
|
||||
type Value = V;
|
||||
|
||||
#[inline(always)]
|
||||
fn lookup(&self, key: &K) -> Option<(V, DepNodeIndex)> {
|
||||
self.lookup(key)
|
||||
}
|
||||
|
||||
#[inline]
|
||||
fn complete(&self, key: K, value: V, index: DepNodeIndex) {
|
||||
self.complete(key, value, index)
|
||||
}
|
||||
|
||||
fn iter(&self, f: &mut dyn FnMut(&Self::Key, &Self::Value, DepNodeIndex)) {
|
||||
self.iter(f)
|
||||
}
|
||||
}
|
||||
|
||||
Loading…
Reference in New Issue
Block a user