8318986: Improve GenericWaitBarrier performance

shipilev · shipilev · commit 30462f9da40d · 2023-11-22T17:55:17.000Z
Reviewed-by: rehn, iwalulya, pchilanomate
diff --git a/src/hotspot/share/utilities/waitBarrier_generic.cpp b/src/hotspot/share/utilities/waitBarrier_generic.cpp
@@ -1,5 +1,6 @@
 /*
- * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2019, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -29,66 +30,228 @@
 #include "utilities/waitBarrier_generic.hpp"
 #include "utilities/spinYield.hpp"
 
+// Implements the striped semaphore wait barrier.
+//
+// To guarantee progress and safety, we need to make sure that new barrier tag
+// starts with the completely empty set of waiters and free semaphore. This
+// requires either waiting for all threads to leave wait() for current barrier
+// tag on disarm(), or waiting for all threads to leave the previous tag before
+// reusing the semaphore in arm().
+//
+// When there are multiple threads, it is normal for some threads to take
+// significant time to leave the barrier. Waiting for these threads introduces
+// stalls on barrier reuse.
+//
+// If we wait on disarm(), this stall is nearly guaranteed to happen if some threads
+// are de-scheduled by prior wait(). It would be especially bad if there are more
+// waiting threads than CPUs: every thread would need to wake up and register itself
+// as leaving, before we can unblock from disarm().
+//
+// If we wait on arm(), we can get lucky that most threads would be able to catch up,
+// exit wait(), and so we arrive to arm() with semaphore ready for reuse. However,
+// that is still insufficient in practice.
+//
+// Therefore, this implementation goes a step further and implements the _striped_
+// semaphores. We maintain several semaphores in cells. The barrier tags are assigned
+// to cells in some simple manner. Most of the current uses have sequential barrier
+// tags, so simple modulo works well. We then operate on a cell like we would operate
+// on a single semaphore: we wait at arm() for all threads to catch up before reusing
+// the cell. For the cost of maintaining just a few cells, we have enough window for
+// threads to catch up.
+//
+// The correctness is guaranteed by using a single atomic state variable per cell,
+// with updates always done with CASes:
+//
+//   [.......... barrier tag ..........][.......... waiters ..........]
+//  63                                  31                            0
+//
+// Cell starts with zero tag and zero waiters. Arming the cell swings barrier tag from
+// zero to some tag, while checking that no waiters have appeared. Disarming swings
+// the barrier tag back from tag to zero. Every waiter registers itself by incrementing
+// the "waiters", while checking that barrier tag is still the same. Every completing waiter
+// decrements the "waiters". When all waiters complete, a cell ends up in initial state,
+// ready to be armed again. This allows accurate tracking of how many signals
+// to issue and does not race with disarm.
+//
+// The implementation uses the strongest (default) barriers for extra safety, even
+// when not strictly required to do so for correctness. Extra barrier overhead is
+// dominated by the actual wait/notify latency anyway.
+//
+
 void GenericWaitBarrier::arm(int barrier_tag) {
-  assert(_barrier_tag == 0, "Already armed");
-  assert(_waiters == 0, "We left a thread hanging");
-  _barrier_tag = barrier_tag;
-  _waiters = 0;
+  assert(barrier_tag != 0, "Pre arm: Should be arming with armed value");
+  assert(Atomic::load(&_barrier_tag) == 0,
+         "Pre arm: Should not be already armed. Tag: %d",
+         Atomic::load(&_barrier_tag));
+  Atomic::release_store(&_barrier_tag, barrier_tag);
+
+  Cell &cell = tag_to_cell(barrier_tag);
+  cell.arm(barrier_tag);
+
+  // API specifies arm() must provide a trailing fence.
   OrderAccess::fence();
 }
 
-int GenericWaitBarrier::wake_if_needed() {
-  assert(_barrier_tag == 0, "Not disarmed");
-  int w = _waiters;
-  if (w == 0) {
-    // Load of _barrier_threads in caller must not pass the load of _waiters.
-    OrderAccess::loadload();
-    return 0;
-  }
-  assert(w > 0, "Bad counting");
-  // We need an exact count which never goes below zero,
-  // otherwise the semaphore may be signalled too many times.
-  if (Atomic::cmpxchg(&_waiters, w, w - 1) == w) {
-    _sem_barrier.signal();
-    return w - 1;
-  }
-  return w;
+void GenericWaitBarrier::disarm() {
+  int barrier_tag = Atomic::load_acquire(&_barrier_tag);
+  assert(barrier_tag != 0, "Pre disarm: Should be armed. Tag: %d", barrier_tag);
+  Atomic::release_store(&_barrier_tag, 0);
+
+  Cell &cell = tag_to_cell(barrier_tag);
+  cell.disarm(barrier_tag);
+
+  // API specifies disarm() must provide a trailing fence.
+  OrderAccess::fence();
 }
 
-void GenericWaitBarrier::disarm() {
-  assert(_barrier_tag != 0, "Not armed");
-  _barrier_tag = 0;
-  // Loads of _barrier_threads/_waiters must not float above disarm store and
-  // disarm store must not sink below.
+void GenericWaitBarrier::wait(int barrier_tag) {
+  assert(barrier_tag != 0, "Pre wait: Should be waiting on armed value");
+
+  Cell &cell = tag_to_cell(barrier_tag);
+  cell.wait(barrier_tag);
+
+  // API specifies wait() must provide a trailing fence.
   OrderAccess::fence();
-  int left;
+}
+
+void GenericWaitBarrier::Cell::arm(int32_t requested_tag) {
+  // Before we continue to arm, we need to make sure that all threads
+  // have left the previous cell.
+
+  int64_t state;
+
   SpinYield sp;
-  do {
-    left = GenericWaitBarrier::wake_if_needed();
-    if (left == 0 && _barrier_threads > 0) {
-      // There is no thread to wake but we still have barrier threads.
+  while (true) {
+    state = Atomic::load_acquire(&_state);
+    assert(decode_tag(state) == 0,
+           "Pre arm: Should not be armed. "
+           "Tag: " INT32_FORMAT "; Waiters: " INT32_FORMAT,
+           decode_tag(state), decode_waiters(state));
+    if (decode_waiters(state) == 0) {
+      break;
+    }
+    sp.wait();
+  }
+
+  // Try to swing cell to armed. This should always succeed after the check above.
+  int64_t new_state = encode(requested_tag, 0);
+  int64_t prev_state = Atomic::cmpxchg(&_state, state, new_state);
+  if (prev_state != state) {
+    fatal("Cannot arm the wait barrier. "
+          "Tag: " INT32_FORMAT "; Waiters: " INT32_FORMAT,
+          decode_tag(prev_state), decode_waiters(prev_state));
+  }
+}
+
+int GenericWaitBarrier::Cell::signal_if_needed(int max) {
+  int signals = 0;
+  while (true) {
+    int cur = Atomic::load_acquire(&_outstanding_wakeups);
+    if (cur == 0) {
+      // All done, no more waiters.
+      return 0;
+    }
+    assert(cur > 0, "Sanity");
+
+    int prev = Atomic::cmpxchg(&_outstanding_wakeups, cur, cur - 1);
+    if (prev != cur) {
+      // Contention, return to caller for early return or backoff.
+      return prev;
+    }
+
+    // Signal!
+    _sem.signal();
+
+    if (++signals >= max) {
+      // Signalled requested number of times, break out.
+      return prev;
+    }
+  }
+}
+
+void GenericWaitBarrier::Cell::disarm(int32_t expected_tag) {
+  int32_t waiters;
+
+  while (true) {
+    int64_t state = Atomic::load_acquire(&_state);
+    int32_t tag = decode_tag(state);
+    waiters = decode_waiters(state);
+
+    assert((tag == expected_tag) && (waiters >= 0),
+           "Mid disarm: Should be armed with expected tag and have sane waiters. "
+           "Tag: " INT32_FORMAT "; Waiters: " INT32_FORMAT,
+           tag, waiters);
+
+    int64_t new_state = encode(0, waiters);
+    if (Atomic::cmpxchg(&_state, state, new_state) == state) {
+      // Successfully disarmed.
+      break;
+    }
+  }
+
+  // Wake up waiters, if we have at least one.
+  // Allow other threads to assist with wakeups, if possible.
+  if (waiters > 0) {
+    Atomic::release_store(&_outstanding_wakeups, waiters);
+    SpinYield sp;
+    while (signal_if_needed(INT_MAX) > 0) {
       sp.wait();
     }
-    // We must loop here until there are no waiters or potential waiters.
-  } while (left > 0 || _barrier_threads > 0);
-  // API specifies disarm() must provide a trailing fence.
-  OrderAccess::fence();
+  }
+  assert(Atomic::load(&_outstanding_wakeups) == 0, "Post disarm: Should not have outstanding wakeups");
 }
 
-void GenericWaitBarrier::wait(int barrier_tag) {
-  assert(barrier_tag != 0, "Trying to wait on disarmed value");
-  if (barrier_tag != _barrier_tag) {
-    // API specifies wait() must provide a trailing fence.
-    OrderAccess::fence();
-    return;
+void GenericWaitBarrier::Cell::wait(int32_t expected_tag) {
+  // Try to register ourselves as pending waiter.
+  while (true) {
+    int64_t state = Atomic::load_acquire(&_state);
+    int32_t tag = decode_tag(state);
+    if (tag != expected_tag) {
+      // Cell tag had changed while waiting here. This means either the cell had
+      // been disarmed, or we are late and the cell was armed with a new tag.
+      // Exit without touching anything else.
+      return;
+    }
+    int32_t waiters = decode_waiters(state);
+
+    assert((tag == expected_tag) && (waiters >= 0 && waiters < INT32_MAX),
+           "Before wait: Should be armed with expected tag and waiters are in range. "
+           "Tag: " INT32_FORMAT "; Waiters: " INT32_FORMAT,
+           tag, waiters);
+
+    int64_t new_state = encode(tag, waiters + 1);
+    if (Atomic::cmpxchg(&_state, state, new_state) == state) {
+      // Success! Proceed to wait.
+      break;
+    }
   }
-  Atomic::add(&_barrier_threads, 1);
-  if (barrier_tag != 0 && barrier_tag == _barrier_tag) {
-    Atomic::add(&_waiters, 1);
-    _sem_barrier.wait();
-    // We help out with posting, but we need to do so before we decrement the
-    // _barrier_threads otherwise we might wake threads up in next wait.
-    GenericWaitBarrier::wake_if_needed();
+
+  // Wait for notification.
+  _sem.wait();
+
+  // Unblocked! We help out with waking up two siblings. This allows to avalanche
+  // the wakeups for many threads, even if some threads are lagging behind.
+  // Note that we can only do this *before* reporting back as completed waiter,
+  // otherwise we might prematurely wake up threads for another barrier tag.
+  // Current arm() sequence protects us from this trouble by waiting until all waiters
+  // leave.
+  signal_if_needed(2);
+
+  // Register ourselves as completed waiter before leaving.
+  while (true) {
+    int64_t state = Atomic::load_acquire(&_state);
+    int32_t tag = decode_tag(state);
+    int32_t waiters = decode_waiters(state);
+
+    assert((tag == 0) && (waiters > 0),
+           "After wait: Should be not armed and have non-complete waiters. "
+           "Tag: " INT32_FORMAT "; Waiters: " INT32_FORMAT,
+           tag, waiters);
+
+    int64_t new_state = encode(tag, waiters - 1);
+    if (Atomic::cmpxchg(&_state, state, new_state) == state) {
+      // Success!
+      break;
+    }
   }
-  Atomic::add(&_barrier_threads, -1);
 }
diff --git a/src/hotspot/share/utilities/waitBarrier_generic.hpp b/src/hotspot/share/utilities/waitBarrier_generic.hpp
@@ -26,29 +26,79 @@
 #define SHARE_UTILITIES_WAITBARRIER_GENERIC_HPP
 
 #include "memory/allocation.hpp"
+#include "memory/padded.hpp"
 #include "runtime/semaphore.hpp"
 #include "utilities/globalDefinitions.hpp"
 
-// In addition to the barrier tag, it uses two counters to keep the semaphore
-// count correct and not leave any late thread waiting.
 class GenericWaitBarrier : public CHeapObj<mtInternal> {
+private:
+  class Cell : public CHeapObj<mtInternal> {
+  private:
+    // Pad out the cells to avoid interference between the cells.
+    // This would insulate from stalls when adjacent cells have returning
+    // workers and contend over the cache line for current latency-critical
+    // cell.
+    DEFINE_PAD_MINUS_SIZE(0, DEFAULT_CACHE_LINE_SIZE, 0);
+
+    Semaphore _sem;
+
+    // Cell state, tracks the arming + waiters status
+    volatile int64_t _state;
+
+    // Wakeups to deliver for current waiters
+    volatile int _outstanding_wakeups;
+
+    int signal_if_needed(int max);
+
+    static int64_t encode(int32_t barrier_tag, int32_t waiters) {
+      int64_t val = (((int64_t) barrier_tag) << 32) |
+                    (((int64_t) waiters) & 0xFFFFFFFF);
+      assert(decode_tag(val) == barrier_tag, "Encoding is reversible");
+      assert(decode_waiters(val) == waiters, "Encoding is reversible");
+      return val;
+    }
+
+    static int32_t decode_tag(int64_t value) {
+      return (int32_t)(value >> 32);
+    }
+
+    static int32_t decode_waiters(int64_t value) {
+      return (int32_t)(value & 0xFFFFFFFF);
+    }
+
+  public:
+    Cell() : _sem(0), _state(encode(0, 0)), _outstanding_wakeups(0) {}
+    NONCOPYABLE(Cell);
+
+    void arm(int32_t requested_tag);
+    void disarm(int32_t expected_tag);
+    void wait(int32_t expected_tag);
+  };
+
+  // Should be enough for most uses without exploding the footprint.
+  static constexpr int CELLS_COUNT = 16;
+
+  Cell _cells[CELLS_COUNT];
+
+  // Trailing padding to protect the last cell.
+  DEFINE_PAD_MINUS_SIZE(0, DEFAULT_CACHE_LINE_SIZE, 0);
+
   volatile int _barrier_tag;
-  // The number of threads waiting on or about to wait on the semaphore.
-  volatile int _waiters;
-  // The number of threads in the wait path, before or after the tag check.
-  // These threads can become waiters.
-  volatile int _barrier_threads;
-  Semaphore _sem_barrier;
+
+  // Trailing padding to insulate the rest of the barrier from adjacent
+  // data structures. The leading padding is not needed, as cell padding
+  // handles this for us.
+  DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, 0);
 
   NONCOPYABLE(GenericWaitBarrier);
 
-  int wake_if_needed();
+  Cell& tag_to_cell(int tag) { return _cells[tag & (CELLS_COUNT - 1)]; }
 
- public:
-  GenericWaitBarrier() : _barrier_tag(0), _waiters(0), _barrier_threads(0), _sem_barrier(0) {}
+public:
+  GenericWaitBarrier() : _cells(), _barrier_tag(0) {}
   ~GenericWaitBarrier() {}
 
-  const char* description() { return "semaphore"; }
+  const char* description() { return "striped semaphore"; }
 
   void arm(int barrier_tag);
   void disarm();
