perf: tail latency with goSched (#2033)

Alternate implementation to #2016 that doesn't reduce RPS with lower
amounts of threads

threadregular.go

@@ -2,7 +2,9 @@ package frankenphp
 
 import (
 	"context"
+	"runtime"
 	"sync"
+	"sync/atomic"
 )
 
 // representation of a non-worker PHP thread
@@ -16,9 +18,10 @@ type regularThread struct {
 }
 
 var (
-	regularThreads     []*phpThread
-	regularThreadMu    = &sync.RWMutex{}
-	regularRequestChan chan contextHolder
+	regularThreads       []*phpThread
+	regularThreadMu      = &sync.RWMutex{}
+	regularRequestChan   chan contextHolder
+	queuedRegularThreads = atomic.Int32{}
 )
 
 func convertToRegularThread(thread *phpThread) {
@@ -81,6 +84,7 @@ func (handler *regularThread) waitForRequest() string {
 		// go back to beforeScriptExecution
 		return handler.beforeScriptExecution()
 	case ch = <-regularRequestChan:
+	case ch = <-handler.thread.requestChan:
 	}
 
 	handler.ctx = ch.ctx
@@ -100,23 +104,35 @@ func (handler *regularThread) afterRequest() {
 func handleRequestWithRegularPHPThreads(ch contextHolder) error {
 	metrics.StartRequest()
 
-	select {
-	case regularRequestChan <- ch:
-		// a thread was available to handle the request immediately
-		<-ch.frankenPHPContext.done
-		metrics.StopRequest()
+	runtime.Gosched()
 
-		return nil
-	default:
-		// no thread was available
+	if queuedRegularThreads.Load() == 0 {
+		regularThreadMu.RLock()
+		for _, thread := range regularThreads {
+			select {
+			case thread.requestChan <- ch:
+				regularThreadMu.RUnlock()
+				<-ch.frankenPHPContext.done
+				metrics.StopRequest()
+
+				return nil
+			default:
+				// thread was not available
+			}
+		}
+		regularThreadMu.RUnlock()
 	}
 
 	// if no thread was available, mark the request as queued and fan it out to all threads
+	queuedRegularThreads.Add(1)
 	metrics.QueuedRequest()
+
 	for {
 		select {
 		case regularRequestChan <- ch:
+			queuedRegularThreads.Add(-1)
 			metrics.DequeuedRequest()
+
 			<-ch.frankenPHPContext.done
 			metrics.StopRequest()
 
@@ -125,7 +141,9 @@ func handleRequestWithRegularPHPThreads(ch contextHolder) error {
 			// the request has triggered scaling, continue to wait for a thread
 		case <-timeoutChan(maxWaitTime):
 			// the request has timed out stalling
+			queuedRegularThreads.Add(-1)
 			metrics.DequeuedRequest()
+			metrics.StopRequest()
 
 			ch.frankenPHPContext.reject(ErrMaxWaitTimeExceeded)