Ignore cancelled

- Stop treating cancelled as timeouts (for gRPC) as poor client behavior can adversely affect the server concurrency limiter
- For `gradient` make the backoff ratio for drops configurable and change the default to 0.9 (from 0.5) since the current default can result in overly aggressive load shedding.

concurrency-limits-core/src/main/java/com/netflix/concurrency/limits/limit/GradientLimit.java

@@ -52,6 +52,7 @@ public static class Builder {
         private MetricRegistry registry = EmptyMetricRegistry.INSTANCE;
         private double rttTolerance = 2.0;
         private int probeInterval = 1000;
+        private double backoffRatio = 0.9;
 
         /**
          * Minimum threshold for accepting a new rtt sample.  Any RTT lower than this threshold
@@ -153,6 +154,18 @@ public Builder metricRegistry(MetricRegistry registry) {
             this.registry = registry;
             return this;
         }
+
+        /**
+         * Ratio applied to the limit when a timeout was identified within the sampling window.  The default value is
+         * 0.9.  A value of 1.0 means no backoff.
+         * @param backoffRatio
+         * @return
+         */
+        public Builder backoffRatio(double backoffRatio) {
+            Preconditions.checkArgument(backoffRatio >= 0.5 && backoffRatio <= 1.0, "backoffRatio must be in the range [0.5, 1.0]");
+            this.backoffRatio = backoffRatio;
+            return this;
+        }
 
         @Deprecated
         public Builder probeMultiplier(int probeMultiplier) {
@@ -205,14 +218,16 @@ public static GradientLimit newDefault() {
 
     private final double rttTolerance;
 
+    private final double backoffRatio;
+
     private final SampleListener minRttSampleListener;
 
     private final SampleListener minWindowRttSampleListener;
 
     private final SampleListener queueSizeSampleListener;
 
     private final int probeInterval;
-    
+
     private int resetRttCounter;
 
     private GradientLimit(Builder builder) {
@@ -223,6 +238,7 @@ private GradientLimit(Builder builder) {
         this.queueSize = builder.queueSize;
         this.smoothing = builder.smoothing;
         this.rttTolerance = builder.rttTolerance;
+        this.backoffRatio = builder.backoffRatio;
         this.probeInterval = builder.probeInterval;
         this.resetRttCounter = nextProbeCountdown();
         this.rttNoLoadMeasurement = new MinimumMeasurement();
@@ -272,7 +288,7 @@ public int _update(final long startTime, final long rtt, final int inflight, fin
         double newLimit;
         // Reduce the limit aggressively if there was a drop
         if (didDrop) {
-            newLimit = estimatedLimit/2;
+            newLimit = estimatedLimit * backoffRatio;
         // Don't grow the limit if we are app limited
         } else if (inflight < estimatedLimit / 2) {
             return (int)estimatedLimit;

concurrency-limits-grpc/build.gradle

@@ -7,7 +7,7 @@ sourceCompatibility = JavaVersion.VERSION_1_8
 dependencies {
     compile project(":concurrency-limits-core")
 
-    compile "io.grpc:grpc-core:1.9.0"
+    compileOnly "io.grpc:grpc-core:1.9.0"
 
     testCompile project(":concurrency-limits-spectator")
 

concurrency-limits-grpc/src/main/java/com/netflix/concurrency/limits/grpc/server/ConcurrencyLimitServerInterceptor.java

@@ -160,7 +160,6 @@ public void close(Status status, Metadata trailers) {
                                         } finally {
                                             safeComplete(() -> {
                                                 switch (status.getCode()) {
-                                                    case CANCELLED:
                                                     case DEADLINE_EXCEEDED:
                                                         listener.onDropped();
                                                         break;