runtime: replace Semacquire/Semrelease implementation

1. The implementation uses distributed hash table of waitlists instead of a centralized one.
  It significantly improves scalability for uncontended semaphores.
2. The implementation provides wait-free fast-path for signalers.
3. The implementation uses less locks (1 lock/unlock instead of 5 for Semacquire).
4. runtime·ready() call is moved out of critical section.
5. Semacquire() does not call semwake().
Benchmark results on HP Z600 (2 x Xeon E5620, 8 HT cores, 2.40GHz)
are as follows:
benchmark                                        old ns/op    new ns/op    delta
runtime_test.BenchmarkSemaUncontended                58.20        36.30  -37.63%
runtime_test.BenchmarkSemaUncontended-2             199.00        18.30  -90.80%
runtime_test.BenchmarkSemaUncontended-4             327.00         9.20  -97.19%
runtime_test.BenchmarkSemaUncontended-8             491.00         5.32  -98.92%
runtime_test.BenchmarkSemaUncontended-16            946.00         4.18  -99.56%

runtime_test.BenchmarkSemaSyntNonblock               59.00        36.80  -37.63%
runtime_test.BenchmarkSemaSyntNonblock-2            167.00       138.00  -17.37%
runtime_test.BenchmarkSemaSyntNonblock-4            333.00       129.00  -61.26%
runtime_test.BenchmarkSemaSyntNonblock-8            464.00       130.00  -71.98%
runtime_test.BenchmarkSemaSyntNonblock-16          1015.00       136.00  -86.60%

runtime_test.BenchmarkSemaSyntBlock                  58.80        36.70  -37.59%
runtime_test.BenchmarkSemaSyntBlock-2               294.00       149.00  -49.32%
runtime_test.BenchmarkSemaSyntBlock-4               333.00       177.00  -46.85%
runtime_test.BenchmarkSemaSyntBlock-8               471.00       221.00  -53.08%
runtime_test.BenchmarkSemaSyntBlock-16              990.00       227.00  -77.07%

runtime_test.BenchmarkSemaWorkNonblock              829.00       832.00   +0.36%
runtime_test.BenchmarkSemaWorkNonblock-2            425.00       419.00   -1.41%
runtime_test.BenchmarkSemaWorkNonblock-4            308.00       220.00  -28.57%
runtime_test.BenchmarkSemaWorkNonblock-8            394.00       147.00  -62.69%
runtime_test.BenchmarkSemaWorkNonblock-16          1510.00       149.00  -90.13%

runtime_test.BenchmarkSemaWorkBlock                 828.00       813.00   -1.81%
runtime_test.BenchmarkSemaWorkBlock-2               428.00       436.00   +1.87%
runtime_test.BenchmarkSemaWorkBlock-4               232.00       219.00   -5.60%
runtime_test.BenchmarkSemaWorkBlock-8               392.00       251.00  -35.97%
runtime_test.BenchmarkSemaWorkBlock-16             1524.00       298.00  -80.45%

sync_test.BenchmarkMutexUncontended                  24.10        24.00   -0.41%
sync_test.BenchmarkMutexUncontended-2                12.00        12.00   +0.00%
sync_test.BenchmarkMutexUncontended-4                 6.25         6.17   -1.28%
sync_test.BenchmarkMutexUncontended-8                 3.43         3.34   -2.62%
sync_test.BenchmarkMutexUncontended-16                2.34         2.32   -0.85%

sync_test.BenchmarkMutex                             24.70        24.70   +0.00%
sync_test.BenchmarkMutex-2                          208.00        99.50  -52.16%
sync_test.BenchmarkMutex-4                         2744.00       256.00  -90.67%
sync_test.BenchmarkMutex-8                         5137.00       556.00  -89.18%
sync_test.BenchmarkMutex-16                        5368.00      1284.00  -76.08%

sync_test.BenchmarkMutexSlack                        24.70        25.00   +1.21%
sync_test.BenchmarkMutexSlack-2                    1094.00       186.00  -83.00%
sync_test.BenchmarkMutexSlack-4                    3430.00       402.00  -88.28%
sync_test.BenchmarkMutexSlack-8                    5051.00      1066.00  -78.90%
sync_test.BenchmarkMutexSlack-16                   6806.00      1363.00  -79.97%

sync_test.BenchmarkMutexWork                        793.00       792.00   -0.13%
sync_test.BenchmarkMutexWork-2                      398.00       398.00   +0.00%
sync_test.BenchmarkMutexWork-4                     1441.00       308.00  -78.63%
sync_test.BenchmarkMutexWork-8                     8532.00       847.00  -90.07%
sync_test.BenchmarkMutexWork-16                    8225.00      2760.00  -66.44%

sync_test.BenchmarkMutexWorkSlack                   793.00       793.00   +0.00%
sync_test.BenchmarkMutexWorkSlack-2                 418.00       414.00   -0.96%
sync_test.BenchmarkMutexWorkSlack-4                4481.00       480.00  -89.29%
sync_test.BenchmarkMutexWorkSlack-8                6317.00      1598.00  -74.70%
sync_test.BenchmarkMutexWorkSlack-16               9111.00      3038.00  -66.66%

R=rsc
CC=golang-dev
https://golang.org/cl/4631059

src/pkg/runtime/386/atomic.c

+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "runtime.h"
+
+#pragma textflag 7
+uint32
+runtime·atomicload(uint32 volatile* addr)
+{
+	return *addr;
+}

src/pkg/runtime/Makefile

@@ -47,6 +47,7 @@ OFILES_arm=\
 
 OFILES=\
 	asm.$O\
+	atomic.$O\
 	cgocall.$O\
 	chan.$O\
 	closure.$O\

src/pkg/runtime/amd64/atomic.c

+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "runtime.h"
+
+#pragma textflag 7
+uint32
+runtime·atomicload(uint32 volatile* addr)
+{
+	return *addr;
+}

src/pkg/runtime/arm/atomic.c

+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "runtime.h"
+
+#pragma textflag 7
+uint32
+runtime·atomicload(uint32 volatile* addr)
+{
+	return runtime·xadd(addr, 0);
+}

src/pkg/runtime/runtime.h

@@ -416,7 +416,10 @@ int32	runtime·write(int32, void*, int32);
 int32	runtime·mincore(void*, uintptr, byte*);
 bool	runtime·cas(uint32*, uint32, uint32);
 bool	runtime·casp(void**, void*, void*);
+// Don't confuse with XADD x86 instruction,
+// this one is actually 'addx', that is, add-and-fetch.
 uint32	runtime·xadd(uint32 volatile*, int32);
+uint32  runtime·atomicload(uint32 volatile*);
 void	runtime·jmpdefer(byte*, void*);
 void	runtime·exit1(int32);
 void	runtime·ready(G*);

src/pkg/runtime/sema.goc

@@ -23,104 +23,68 @@ package runtime
 typedef struct Sema Sema;
 struct Sema
 {
-	uint32 *addr;
+	uint32 volatile *addr;
 	G *g;
 	Sema *prev;
 	Sema *next;
 };
 
-// TODO: For now, a linked list; maybe a hash table of linked lists later.
-static Sema *semfirst, *semlast;
-static Lock semlock;
+typedef struct SemaRoot SemaRoot;
+struct SemaRoot
+{
+        Lock;
+	Sema *head;
+	Sema *tail;
+	// Number of waiters. Read w/o the lock.
+	uint32 volatile nwait;
+};
+
+// Prime to not correlate with any user patterns.
+#define SEMTABLESZ 251
+
+static union
+{
+	SemaRoot;
+	// Modern processors tend to have 64-byte cache lines,
+	// potentially with 128-byte effective cache line size for reading.
+	// While there are hypothetical architectures
+	// with 16-4096 byte cache lines, 128 looks like a good compromise.
+	uint8 pad[128];
+} semtable[SEMTABLESZ];
+
+static SemaRoot*
+semroot(uint32 *addr)
+{
+	return &semtable[((uintptr)addr >> 3) % SEMTABLESZ];
+}
 
 static void
-semqueue(uint32 *addr, Sema *s)
+semqueue(SemaRoot *root, uint32 volatile *addr, Sema *s)
 {
+	s->g = g;
 	s->addr = addr;
-	s->g = nil;
-
-	runtime·lock(&semlock);
-	s->prev = semlast;
 	s->next = nil;
-	if(semlast)
-		semlast->next = s;
+	s->prev = root->tail;
+	if(root->tail)
+		root->tail->next = s;
 	else
-		semfirst = s;
-	semlast = s;
-	runtime·unlock(&semlock);
+		root->head = s;
+	root->tail = s;
 }
 
 static void
-semdequeue(Sema *s)
+semdequeue(SemaRoot *root, Sema *s)
 {
-	runtime·lock(&semlock);
 	if(s->next)
 		s->next->prev = s->prev;
 	else
-		semlast = s->prev;
+		root->tail = s->prev;
 	if(s->prev)
 		s->prev->next = s->next;
 	else
-		semfirst = s->next;
+		root->head = s->next;
 	s->prev = nil;
 	s->next = nil;
-	runtime·unlock(&semlock);
-}
-
-static void
-semwakeup(uint32 *addr)
-{
-	Sema *s;
-
-	runtime·lock(&semlock);
-	for(s=semfirst; s; s=s->next) {
-		if(s->addr == addr && s->g) {
-			runtime·ready(s->g);
-			s->g = nil;
-			break;
-		}
-	}
-	runtime·unlock(&semlock);
-}
-
-// Step 1 of sleep: make ourselves available for wakeup.
-// TODO(rsc): Maybe we can write a version without
-// locks by using cas on s->g.  Maybe not: I need to
-// think more about whether it would be correct.
-static void
-semsleep1(Sema *s)
-{
-	runtime·lock(&semlock);
-	s->g = g;
-	runtime·unlock(&semlock);
-}
-
-// Decided not to go through with it: undo step 1.
-static void
-semsleepundo1(Sema *s)
-{
-	runtime·lock(&semlock);
-	if(s->g != nil) {
-		s->g = nil;	// back ourselves out
-	} else {
-		// If s->g == nil already, semwakeup
-		// already readied us.  Since we never stopped
-		// running, readying us just set g->readyonstop.
-		// Clear it.
-		if(g->readyonstop == 0)
-			*(int32*)0x555 = 555;
-		g->readyonstop = 0;
-	}
-	runtime·unlock(&semlock);
-}
-
-// Step 2: wait for the wakeup.
-static void
-semsleep2(Sema *s)
-{
-	USED(s);
-	g->status = Gwaiting;
-	runtime·gosched();
 }
 
 static int32
@@ -128,52 +92,83 @@ cansemacquire(uint32 *addr)
 {
 	uint32 v;
 
-	while((v = *addr) > 0)
+	while((v = runtime·atomicload(addr)) > 0)
 		if(runtime·cas(addr, v, v-1))
 			return 1;
 	return 0;
 }
 
-// For now has no return value.
-// Might return an ok (not interrupted) bool in the future?
 void
-runtime·semacquire(uint32 *addr)
+runtime·semacquire(uint32 volatile *addr)
 {
 	Sema s;
+	SemaRoot *root;
 
 	// Easy case.
 	if(cansemacquire(addr))
 		return;
 
 	// Harder case:
-	//	queue
-	//	try semacquire one more time, sleep if failed
-	//	dequeue
-	//	wake up one more guy to avoid races (TODO(rsc): maybe unnecessary?)
-	semqueue(addr, &s);
+	//	increment waiter count
+	//	try cansemacquire one more time, return if succeeded
+	//	enqueue itself as a waiter
+	//	sleep
+	//	(waiter descriptor is dequeued by signaler)
+	root = semroot(addr);
 	for(;;) {
-		semsleep1(&s);
+		runtime·lock(root);
+		// Add ourselves to nwait to disable "easy case" in semrelease.
+		runtime·xadd(&root->nwait, 1);
+		// Check cansemacquire to avoid missed wakeup.
 		if(cansemacquire(addr)) {
-			semsleepundo1(&s);
-			break;
+			runtime·xadd(&root->nwait, -1);
+			runtime·unlock(root);
+			return;
 		}
-		semsleep2(&s);
+		// Any semrelease after the cansemacquire knows we're waiting
+		// (we set nwait above), so go to sleep.
+		semqueue(root, addr, &s);
+		g->status = Gwaiting;
+		runtime·unlock(root);
+		runtime·gosched();
+		if(cansemacquire(addr))
+			return;
 	}
-	semdequeue(&s);
-	semwakeup(addr);
 }
 
 void
-runtime·semrelease(uint32 *addr)
+runtime·semrelease(uint32 volatile *addr)
 {
-	uint32 v;
+	Sema *s;
+	SemaRoot *root;
 
-	for(;;) {
-		v = *addr;
-		if(runtime·cas(addr, v, v+1))
+	root = semroot(addr);
+	runtime·xadd(addr, 1);
+
+	// Easy case: no waiters?
+	// This check must happen after the xadd, to avoid a missed wakeup
+	// (see loop in semacquire).
+	if(runtime·atomicload(&root->nwait) == 0)
+		return;
+
+	// Harder case: search for a waiter and wake it.
+	runtime·lock(root);
+	if(runtime·atomicload(&root->nwait) == 0) {
+		// The count is already consumed by another goroutine,
+		// so no need to wake up another goroutine.
+		runtime·unlock(root);
+		return;
+	}
+	for(s = root->head; s; s = s->next) {
+		if(s->addr == addr) {
+			runtime·xadd(&root->nwait, -1);
+			semdequeue(root, s);
 			break;
 		}
-	semwakeup(addr);
+	}
+	runtime·unlock(root);
+	if(s)
+		runtime·ready(s->g);
 }
 
 func Semacquire(addr *uint32) {