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Diffstat (limited to 'kamon-core/src/legacy-main/java/kamon/jsr166/Striped64.java')
| -rw-r--r-- | kamon-core/src/legacy-main/java/kamon/jsr166/Striped64.java | 371 |
1 files changed, 371 insertions, 0 deletions
diff --git a/kamon-core/src/legacy-main/java/kamon/jsr166/Striped64.java b/kamon-core/src/legacy-main/java/kamon/jsr166/Striped64.java new file mode 100644 index 00000000..8fbfa4ba --- /dev/null +++ b/kamon-core/src/legacy-main/java/kamon/jsr166/Striped64.java @@ -0,0 +1,371 @@ +/* + +Note: this was copied from Doug Lea's CVS repository + http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/ + +Striped64.java version 1.8 + +*/ + + +/* + * Written by Doug Lea with assistance from members of JCP JSR-166 + * Expert Group and released to the public domain, as explained at + * http://creativecommons.org/publicdomain/zero/1.0/ + */ + +package kamon.jsr166; + +import java.util.Random; + +/** + * A package-local class holding common representation and mechanics + * for classes supporting dynamic striping on 64bit values. The class + * extends Number so that concrete subclasses must publicly do so. + */ +abstract class Striped64 extends Number { + /* + * This class maintains a lazily-initialized table of atomically + * updated variables, plus an extra "base" field. The table size + * is a power of two. Indexing uses masked per-thread hash codes. + * Nearly all declarations in this class are package-private, + * accessed directly by subclasses. + * + * Table entries are of class Cell; a variant of AtomicLong padded + * to reduce cache contention on most processors. Padding is + * overkill for most Atomics because they are usually irregularly + * scattered in memory and thus don't interfere much with each + * other. But Atomic objects residing in arrays will tend to be + * placed adjacent to each other, and so will most often share + * cache lines (with a huge negative performance impact) without + * this precaution. + * + * In part because Cells are relatively large, we avoid creating + * them until they are needed. When there is no contention, all + * updates are made to the base field. Upon first contention (a + * failed CAS on base update), the table is initialized to size 2. + * The table size is doubled upon further contention until + * reaching the nearest power of two greater than or equal to the + * number of CPUS. Table slots remain empty (null) until they are + * needed. + * + * A single spinlock ("busy") is used for initializing and + * resizing the table, as well as populating slots with new Cells. + * There is no need for a blocking lock; when the lock is not + * available, threads try other slots (or the base). During these + * retries, there is increased contention and reduced locality, + * which is still better than alternatives. + * + * Per-thread hash codes are initialized to random values. + * Contention and/or table collisions are indicated by failed + * CASes when performing an update operation (see method + * retryUpdate). Upon a collision, if the table size is less than + * the capacity, it is doubled in size unless some other thread + * holds the lock. If a hashed slot is empty, and lock is + * available, a new Cell is created. Otherwise, if the slot + * exists, a CAS is tried. Retries proceed by "double hashing", + * using a secondary hash (Marsaglia XorShift) to try to find a + * free slot. + * + * The table size is capped because, when there are more threads + * than CPUs, supposing that each thread were bound to a CPU, + * there would exist a perfect hash function mapping threads to + * slots that eliminates collisions. When we reach capacity, we + * search for this mapping by randomly varying the hash codes of + * colliding threads. Because search is random, and collisions + * only become known via CAS failures, convergence can be slow, + * and because threads are typically not bound to CPUS forever, + * may not occur at all. However, despite these limitations, + * observed contention rates are typically low in these cases. + * + * It is possible for a Cell to become unused when threads that + * once hashed to it terminate, as well as in the case where + * doubling the table causes no thread to hash to it under + * expanded mask. We do not try to detect or remove such cells, + * under the assumption that for long-running instances, observed + * contention levels will recur, so the cells will eventually be + * needed again; and for short-lived ones, it does not matter. + */ + + /** + * Padded variant of AtomicLong supporting only raw accesses plus CAS. + * The value field is placed between pads, hoping that the JVM doesn't + * reorder them. + * + * JVM intrinsics note: It would be possible to use a release-only + * form of CAS here, if it were provided. + */ + static final class Cell { + volatile long p0, p1, p2, p3, p4, p5, p6; + volatile long value; + volatile long q0, q1, q2, q3, q4, q5, q6; + Cell(long x) { value = x; } + + final boolean cas(long cmp, long val) { + return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val); + } + + final long getAndSet(long val) { + long v; + do { + v = UNSAFE.getLongVolatile(this, valueOffset); + } while (!UNSAFE.compareAndSwapLong(this, valueOffset, v, val)); + return v; + } + + // Unsafe mechanics + private static final sun.misc.Unsafe UNSAFE; + private static final long valueOffset; + static { + try { + UNSAFE = getUnsafe(); + Class<?> ak = Cell.class; + valueOffset = UNSAFE.objectFieldOffset + (ak.getDeclaredField("value")); + } catch (Exception e) { + throw new Error(e); + } + } + + } + + /** + * Holder for the thread-local hash code. The code is initially + * random, but may be set to a different value upon collisions. + */ + static final class HashCode { + static final Random rng = new Random(); + int code; + HashCode() { + int h = rng.nextInt(); // Avoid zero to allow xorShift rehash + code = (h == 0) ? 1 : h; + } + } + + /** + * The corresponding ThreadLocal class + */ + static final class ThreadHashCode extends ThreadLocal<HashCode> { + public HashCode initialValue() { return new HashCode(); } + } + + /** + * Static per-thread hash codes. Shared across all instances to + * reduce ThreadLocal pollution and because adjustments due to + * collisions in one table are likely to be appropriate for + * others. + */ + static final ThreadHashCode threadHashCode = new ThreadHashCode(); + + /** Number of CPUS, to place bound on table size */ + static final int NCPU = Runtime.getRuntime().availableProcessors(); + + /** + * Table of cells. When non-null, size is a power of 2. + */ + transient volatile Cell[] cells; + + /** + * Base value, used mainly when there is no contention, but also as + * a fallback during table initialization races. Updated via CAS. + */ + transient volatile long base; + + /** + * Spinlock (locked via CAS) used when resizing and/or creating Cells. + */ + transient volatile int busy; + + /** + * Package-private default constructor + */ + Striped64() { + } + + /** + * CASes the base field. + */ + final boolean casBase(long cmp, long val) { + return UNSAFE.compareAndSwapLong(this, baseOffset, cmp, val); + } + + /** + * CASes the base field. + */ + final long getAndSetBase(long val) { + long v; + do { + v = UNSAFE.getLongVolatile(this, baseOffset); + } while (!UNSAFE.compareAndSwapLong(this, baseOffset, v, val)); + return v; + } + + /** + * CASes the busy field from 0 to 1 to acquire lock. + */ + final boolean casBusy() { + return UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1); + } + + /** + * Computes the function of current and new value. Subclasses + * should open-code this update function for most uses, but the + * virtualized form is needed within retryUpdate. + * + * @param currentValue the current value (of either base or a cell) + * @param newValue the argument from a user update call + * @return result of the update function + */ + abstract long fn(long currentValue, long newValue); + + /** + * Handles cases of updates involving initialization, resizing, + * creating new Cells, and/or contention. See above for + * explanation. This method suffers the usual non-modularity + * problems of optimistic retry code, relying on rechecked sets of + * reads. + * + * @param x the value + * @param hc the hash code holder + * @param wasUncontended false if CAS failed before call + */ + final void retryUpdate(long x, HashCode hc, boolean wasUncontended) { + int h = hc.code; + boolean collide = false; // True if last slot nonempty + for (;;) { + Cell[] as; Cell a; int n; long v; + if ((as = cells) != null && (n = as.length) > 0) { + if ((a = as[(n - 1) & h]) == null) { + if (busy == 0) { // Try to attach new Cell + Cell r = new Cell(x); // Optimistically create + if (busy == 0 && casBusy()) { + boolean created = false; + try { // Recheck under lock + Cell[] rs; int m, j; + if ((rs = cells) != null && + (m = rs.length) > 0 && + rs[j = (m - 1) & h] == null) { + rs[j] = r; + created = true; + } + } finally { + busy = 0; + } + if (created) + break; + continue; // Slot is now non-empty + } + } + collide = false; + } + else if (!wasUncontended) // CAS already known to fail + wasUncontended = true; // Continue after rehash + else if (a.cas(v = a.value, fn(v, x))) + break; + else if (n >= NCPU || cells != as) + collide = false; // At max size or stale + else if (!collide) + collide = true; + else if (busy == 0 && casBusy()) { + try { + if (cells == as) { // Expand table unless stale + Cell[] rs = new Cell[n << 1]; + for (int i = 0; i < n; ++i) + rs[i] = as[i]; + cells = rs; + } + } finally { + busy = 0; + } + collide = false; + continue; // Retry with expanded table + } + h ^= h << 13; // Rehash + h ^= h >>> 17; + h ^= h << 5; + } + else if (busy == 0 && cells == as && casBusy()) { + boolean init = false; + try { // Initialize table + if (cells == as) { + Cell[] rs = new Cell[2]; + rs[h & 1] = new Cell(x); + cells = rs; + init = true; + } + } finally { + busy = 0; + } + if (init) + break; + } + else if (casBase(v = base, fn(v, x))) + break; // Fall back on using base + } + hc.code = h; // Record index for next time + } + + + /** + * Sets base and all cells to the given value. + */ + final void internalReset(long initialValue) { + Cell[] as = cells; + base = initialValue; + if (as != null) { + int n = as.length; + for (int i = 0; i < n; ++i) { + Cell a = as[i]; + if (a != null) + a.value = initialValue; + } + } + } + + // Unsafe mechanics + private static final sun.misc.Unsafe UNSAFE; + private static final long baseOffset; + private static final long busyOffset; + static { + try { + UNSAFE = getUnsafe(); + Class<?> sk = Striped64.class; + baseOffset = UNSAFE.objectFieldOffset + (sk.getDeclaredField("base")); + busyOffset = UNSAFE.objectFieldOffset + (sk.getDeclaredField("busy")); + } catch (Exception e) { + throw new Error(e); + } + } + + /** + * Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. + * Replace with a simple call to Unsafe.getUnsafe when integrating + * into a jdk. + * + * @return a sun.misc.Unsafe + */ + private static sun.misc.Unsafe getUnsafe() { + try { + return sun.misc.Unsafe.getUnsafe(); + } catch (SecurityException tryReflectionInstead) {} + try { + return java.security.AccessController.doPrivileged + (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() { + public sun.misc.Unsafe run() throws Exception { + Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class; + for (java.lang.reflect.Field f : k.getDeclaredFields()) { + f.setAccessible(true); + Object x = f.get(null); + if (k.isInstance(x)) + return k.cast(x); + } + throw new NoSuchFieldError("the Unsafe"); + }}); + } catch (java.security.PrivilegedActionException e) { + throw new RuntimeException("Could not initialize intrinsics", + e.getCause()); + } + } +} |