all: call atomic.Load* in front of atomic.CompareAndSwap* at places where the atomic.CompareAndSwap* returns false most of the time

This allows avoiding slow inter-CPU synchornization induced by atomic.CompareAndSwap*

app/vmauth/auth_config.go

@@ -271,8 +271,10 @@ func (up *URLPrefix) getLeastLoadedBackendURL() *backendURL {
 		if bu.isBroken() {
 			continue
 		}
-		if atomic.CompareAndSwapInt32(&bu.concurrentRequests, 0, 1) {
+		if atomic.LoadInt32(&bu.concurrentRequests) == 0 {
 			// Fast path - return the backend with zero concurrently executed requests.
+			// Do not use atomic.CompareAndSwapInt32(), since it is much slower on systems with many CPU cores.
+			atomic.AddInt32(&bu.concurrentRequests, 1)
 			return bu
 		}
 	}

app/vmselect/prometheus/prometheus.go

@@ -321,7 +321,9 @@ func exportHandler(qt *querytracer.Tracer, w http.ResponseWriter, cp *commonPara
 		firstLineSent := uint32(0)
 		writeLineFunc = func(xb *exportBlock, workerID uint) error {
 			bb := sw.getBuffer(workerID)
-			if atomic.CompareAndSwapUint32(&firstLineOnce, 0, 1) {
+			// Use atomic.LoadUint32() in front of atomic.CompareAndSwapUint32() in order to avoid slow inter-CPU synchronization
+			// in fast path after the first line has been already sent.
+			if atomic.LoadUint32(&firstLineOnce) == 0 && atomic.CompareAndSwapUint32(&firstLineOnce, 0, 1) {
 				// Send the first line to sw.bw
 				WriteExportPromAPILine(bb, xb)
 				_, err := sw.bw.Write(bb.B)

lib/bloomfilter/filter.go

@@ -79,6 +79,8 @@ func (f *filter) Add(h uint64) bool {
 		w := atomic.LoadUint64(&bits[i])
 		for (w & mask) == 0 {
 			wNew := w | mask
+			// The wNew != w most of the time, so there is no need in using atomic.LoadUint64
+			// in front of atomic.CompareAndSwapUint64 in order to try avoiding slow inter-CPU synchronization.
 			if atomic.CompareAndSwapUint64(&bits[i], w, wNew) {
 				isNew = true
 				break

lib/mergeset/table.go

@@ -738,7 +738,11 @@ func (tb *Table) flushBlocksToParts(ibs []*inmemoryBlock, isFinal bool) {
 		if isFinal {
 			tb.flushCallback()
 		} else {
-			atomic.CompareAndSwapUint32(&tb.needFlushCallbackCall, 0, 1)
+			// Use atomic.LoadUint32 in front of atomic.CompareAndSwapUint32 in order to avoid slow inter-CPU synchronization
+			// at fast path when needFlushCallbackCall is already set to 1.
+			if atomic.LoadUint32(&tb.needFlushCallbackCall) == 0 {
+				atomic.CompareAndSwapUint32(&tb.needFlushCallbackCall, 0, 1)
+			}
 		}
 	}
 }

lib/storage/storage.go

@@ -650,14 +650,19 @@ func (s *Storage) startFreeDiskSpaceWatcher() {
 		freeSpaceBytes := fs.MustGetFreeSpace(s.path)
 		if freeSpaceBytes < freeDiskSpaceLimitBytes {
 			// Switch the storage to readonly mode if there is no enough free space left at s.path
-			if atomic.CompareAndSwapUint32(&s.isReadOnly, 0, 1) {
+			//
+			// Use atomic.LoadUint32 in front of atomic.CompareAndSwapUint32 in order to avoid slow inter-CPU synchronization
+			// when the storage is already in read-only mode.
+			if atomic.LoadUint32(&s.isReadOnly) == 0 && atomic.CompareAndSwapUint32(&s.isReadOnly, 0, 1) {
 				// log notification only on state change
 				logger.Warnf("switching the storage at %s to read-only mode, since it has less than -storage.minFreeDiskSpaceBytes=%d of free space: %d bytes left",
 					s.path, freeDiskSpaceLimitBytes, freeSpaceBytes)
 			}
 			return
 		}
-		if atomic.CompareAndSwapUint32(&s.isReadOnly, 1, 0) {
+		// Use atomic.LoadUint32 in front of atomic.CompareAndSwapUint32 in order to avoid slow inter-CPU synchronization
+		// when the storage isn't in read-only mode.
+		if atomic.LoadUint32(&s.isReadOnly) == 1 && atomic.CompareAndSwapUint32(&s.isReadOnly, 1, 0) {
 			logger.Warnf("enabling writing to the storage at %s, since it has more than -storage.minFreeDiskSpaceBytes=%d of free space: %d bytes left",
 				s.path, freeDiskSpaceLimitBytes, freeSpaceBytes)
 		}