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runtime: write much more direct test for semaphore waiter scalability
This test originally existed as two tests in test/locklinear.go, but this checked against actual locks and was flaky. The test was checking a property of a deep part of the runtime but from a much higher level, and it's easy for nondeterminism due to scheduling to completely mess that up, especially on an oversubscribed system. That test was then moved to the sync package with a more rigorous testing methodology, but it could still flake pretty easily. Finally, this CL makes semtable more testable, exports it in export_test.go, then writes a very direct scalability test for exactly the situation the original test described. As far as I can tell, this is much, much more stable, because it's single-threaded and is just checking exactly the algorithm we need to check. Don't bother trying to bring in a test that checks for O(log n) behavior on the other kind of iteration. It'll be perpetually flaky because the underlying data structure is a treap, so it's only _expected_ to be O(log n), but it's very easy for it to get unlucky without a large number of iterations that's too much for a simple test. Fixes #53381. Change-Id: Ia1cd2d2b0e36d552d5a8ae137077260a16016602 Reviewed-on: https://go-review.googlesource.com/c/go/+/412875 Reviewed-by: Michael Pratt <mpratt@google.com>
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@ -1163,10 +1163,38 @@ var Semacquire = semacquire
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var Semrelease1 = semrelease1
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func SemNwait(addr *uint32) uint32 {
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root := semroot(addr)
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root := semtable.rootFor(addr)
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return atomic.Load(&root.nwait)
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}
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const SemTableSize = semTabSize
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// SemTable is a wrapper around semTable exported for testing.
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type SemTable struct {
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semTable
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}
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// Enqueue simulates enqueuing a waiter for a semaphore (or lock) at addr.
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func (t *SemTable) Enqueue(addr *uint32) {
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s := acquireSudog()
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s.releasetime = 0
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s.acquiretime = 0
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s.ticket = 0
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t.semTable.rootFor(addr).queue(addr, s, false)
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}
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// Dequeue simulates dequeuing a waiter for a semaphore (or lock) at addr.
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//
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// Returns true if there actually was a waiter to be dequeued.
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func (t *SemTable) Dequeue(addr *uint32) bool {
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s, _ := t.semTable.rootFor(addr).dequeue(addr)
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if s != nil {
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releaseSudog(s)
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return true
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}
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return false
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}
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// mspan wrapper for testing.
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//
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//go:notinheap
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@ -35,7 +35,7 @@ import (
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// where n is the number of distinct addresses with goroutines blocked
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// on them that hash to the given semaRoot.
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// See golang.org/issue/17953 for a program that worked badly
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// before we introduced the second level of list, and test/locklinear.go
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// before we introduced the second level of list, and TestSemTableOneAddrCollisionLinear
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// for a test that exercises this.
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type semaRoot struct {
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lock mutex
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@ -43,14 +43,20 @@ type semaRoot struct {
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nwait uint32 // Number of waiters. Read w/o the lock.
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}
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var semtable semTable
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// Prime to not correlate with any user patterns.
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const semTabSize = 251
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var semtable [semTabSize]struct {
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type semTable [semTabSize]struct {
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root semaRoot
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pad [cpu.CacheLinePadSize - unsafe.Sizeof(semaRoot{})]byte
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}
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func (t *semTable) rootFor(addr *uint32) *semaRoot {
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return &t[(uintptr(unsafe.Pointer(addr))>>3)%semTabSize].root
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}
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//go:linkname sync_runtime_Semacquire sync.runtime_Semacquire
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func sync_runtime_Semacquire(addr *uint32) {
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semacquire1(addr, false, semaBlockProfile, 0)
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@ -113,7 +119,7 @@ func semacquire1(addr *uint32, lifo bool, profile semaProfileFlags, skipframes i
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// sleep
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// (waiter descriptor is dequeued by signaler)
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s := acquireSudog()
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root := semroot(addr)
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root := semtable.rootFor(addr)
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t0 := int64(0)
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s.releasetime = 0
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s.acquiretime = 0
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@ -157,7 +163,7 @@ func semrelease(addr *uint32) {
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}
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func semrelease1(addr *uint32, handoff bool, skipframes int) {
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root := semroot(addr)
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root := semtable.rootFor(addr)
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atomic.Xadd(addr, 1)
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// Easy case: no waiters?
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@ -214,10 +220,6 @@ func semrelease1(addr *uint32, handoff bool, skipframes int) {
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}
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}
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func semroot(addr *uint32) *semaRoot {
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return &semtable[(uintptr(unsafe.Pointer(addr))>>3)%semTabSize].root
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}
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func cansemacquire(addr *uint32) bool {
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for {
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v := atomic.Load(addr)
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@ -5,6 +5,7 @@
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package runtime_test
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import (
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"internal/testenv"
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. "runtime"
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"sync"
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"sync/atomic"
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@ -101,3 +102,46 @@ func testSemaHandoff() bool {
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return res == 1 // did the waiter run first?
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}
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func TestSemTableOneAddrCollisionLinear(t *testing.T) {
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testenv.CheckLinear(t, func(scale float64) func(*testing.B) {
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n := int(1000 * scale)
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return func(b *testing.B) {
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tab := Escape(new(SemTable))
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u := make([]uint32, SemTableSize+1)
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b.ResetTimer()
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// Simulate two locks colliding on the same semaRoot.
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//
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// Specifically enqueue all the waiters for the first lock,
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// then all the waiters for the second lock.
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//
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// Then, dequeue all the waiters from the first lock, then
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// the second.
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//
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// Each enqueue/dequeue operation should be O(1), because
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// there are exactly 2 locks. This could be O(n) if all
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// the waiters for both locks are on the same list, as it
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// once was.
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for i := 0; i < n; i++ {
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if i < n/2 {
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tab.Enqueue(&u[0])
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} else {
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tab.Enqueue(&u[SemTableSize])
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}
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}
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for i := 0; i < n; i++ {
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var ok bool
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if i < n/2 {
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ok = tab.Dequeue(&u[0])
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} else {
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ok = tab.Dequeue(&u[SemTableSize])
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}
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if !ok {
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b.Fatal("failed to dequeue")
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}
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}
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}
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})
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}
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@ -333,115 +333,3 @@ func BenchmarkMutexSpin(b *testing.B) {
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}
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})
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}
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const runtimeSemaHashTableSize = 251 // known size of runtime hash table
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func TestMutexLinearOne(t *testing.T) {
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testenv.CheckLinear(t, func(scale float64) func(*testing.B) {
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n := int(1000 * scale)
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return func(b *testing.B) {
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ch := make(chan struct{})
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locks := make([]RWMutex, runtimeSemaHashTableSize+1)
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b.ResetTimer()
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var wgStart, wgFinish WaitGroup
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for i := 0; i < n; i++ {
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wgStart.Add(1)
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wgFinish.Add(1)
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go func() {
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wgStart.Done()
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locks[0].Lock()
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ch <- struct{}{}
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wgFinish.Done()
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}()
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}
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wgStart.Wait()
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wgFinish.Add(1)
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go func() {
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for j := 0; j < n; j++ {
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locks[1].Lock()
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locks[runtimeSemaHashTableSize].Lock()
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locks[1].Unlock()
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runtime.Gosched()
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locks[runtimeSemaHashTableSize].Unlock()
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}
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wgFinish.Done()
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}()
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for j := 0; j < n; j++ {
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locks[1].Lock()
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locks[runtimeSemaHashTableSize].Lock()
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locks[1].Unlock()
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runtime.Gosched()
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locks[runtimeSemaHashTableSize].Unlock()
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}
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b.StopTimer()
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for i := 0; i < n; i++ {
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<-ch
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locks[0].Unlock()
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}
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wgFinish.Wait()
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}
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})
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}
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func TestMutexLinearMany(t *testing.T) {
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if runtime.GOARCH == "arm" && os.Getenv("GOARM") == "5" {
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// stressLockMany reliably fails on the linux-arm-arm5spacemonkey
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// builder. See https://golang.org/issue/24221.
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return
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}
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testenv.CheckLinear(t, func(scale float64) func(*testing.B) {
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n := int(1000 * scale)
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return func(b *testing.B) {
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locks := make([]RWMutex, n*runtimeSemaHashTableSize+1)
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b.ResetTimer()
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var wgStart, wgFinish WaitGroup
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for i := 0; i < n; i++ {
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wgStart.Add(1)
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wgFinish.Add(1)
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go func(i int) {
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locks[(i+1)*runtimeSemaHashTableSize].Lock()
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wgStart.Done()
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locks[(i+1)*runtimeSemaHashTableSize].Lock()
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locks[(i+1)*runtimeSemaHashTableSize].Unlock()
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wgFinish.Done()
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}(i)
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}
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wgStart.Wait()
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go func() {
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for j := 0; j < n; j++ {
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locks[1].Lock()
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locks[0].Lock()
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locks[1].Unlock()
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runtime.Gosched()
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locks[0].Unlock()
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}
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}()
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for j := 0; j < n; j++ {
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locks[1].Lock()
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locks[0].Lock()
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locks[1].Unlock()
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runtime.Gosched()
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locks[0].Unlock()
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}
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b.StopTimer()
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for i := 0; i < n; i++ {
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locks[(i+1)*runtimeSemaHashTableSize].Unlock()
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}
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wgFinish.Wait()
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}
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})
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}
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