Eliminate wlocks when updating existing histograms, gauges (#116)

apcu_add always takes a wlock in the current version 5.1.22, which
leads to lots of lock contention in high concurrency scenarios. Since
the most common operation ought to be updating an already existing
key, wrap apcu_add in an apcu_exists that only takes a rlock.

Signed-off-by: Tobias Bengtsson <[email protected]>
Co-authored-by: Lukas Kämmerling <[email protected]>

Author: TobiasBengtsson

src/Prometheus/Storage/APC.php

@@ -56,11 +56,13 @@ public function updateHistogram(array $data): void
     {
         // Initialize the sum
         $sumKey = $this->histogramBucketValueKey($data, 'sum');
-        $new = apcu_add($sumKey, $this->toBinaryRepresentationAsInteger(0));
+        if (!apcu_exists($sumKey)) {
+            $new = apcu_add($sumKey, $this->toBinaryRepresentationAsInteger(0));
 
-        // If sum does not exist, assume a new histogram and store the metadata
-        if ($new) {
-            apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+            // If sum does not exist, assume a new histogram and store the metadata
+            if ($new) {
+                apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+            }
         }
 
         // Atomically increment the sum
@@ -83,8 +85,11 @@ public function updateHistogram(array $data): void
         }
 
         // Initialize and increment the bucket
-        apcu_add($this->histogramBucketValueKey($data, $bucketToIncrease), 0);
-        apcu_inc($this->histogramBucketValueKey($data, $bucketToIncrease));
+        $bucketKey = $this->histogramBucketValueKey($data, $bucketToIncrease);
+        if (!apcu_exists($bucketKey)) {
+            apcu_add($bucketKey, 0);
+        }
+        apcu_inc($bucketKey);
     }
 
     /**
@@ -94,11 +99,15 @@ public function updateSummary(array $data): void
     {
         // store meta
         $metaKey = $this->metaKey($data);
-        apcu_add($metaKey, $this->metaData($data));
+        if (!apcu_exists($metaKey)) {
+            apcu_add($metaKey, $this->metaData($data));
+        }
 
         // store value key
         $valueKey = $this->valueKey($data);
-        apcu_add($valueKey, $this->encodeLabelValues($data['labelValues']));
+        if (!apcu_exists($valueKey)) {
+            apcu_add($valueKey, $this->encodeLabelValues($data['labelValues']));
+        }
 
         // trick to handle uniqid collision
         $done = false;
@@ -118,9 +127,11 @@ public function updateGauge(array $data): void
             apcu_store($valueKey, $this->toBinaryRepresentationAsInteger($data['value']));
             apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
         } else {
-            $new = apcu_add($valueKey, $this->toBinaryRepresentationAsInteger(0));
-            if ($new) {
-                apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+            if (!apcu_exists($valueKey)) {
+                $new = apcu_add($valueKey, $this->toBinaryRepresentationAsInteger(0));
+                if ($new) {
+                    apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+                }
             }
             // Taken from https://github.com/prometheus/client_golang/blob/66058aac3a83021948e5fb12f1f408ff556b9037/prometheus/value.go#L91
             $done = false;

src/Prometheus/Storage/APCng.php

@@ -113,8 +113,11 @@ public function updateHistogram(array $data): void
         }
 
         // Initialize and increment the bucket
-        apcu_add($this->histogramBucketValueKey($data, $bucketToIncrease), 0);
-        apcu_inc($this->histogramBucketValueKey($data, $bucketToIncrease));
+        $bucketKey = $this->histogramBucketValueKey($data, $bucketToIncrease);
+        if (!apcu_exists($bucketKey)) {
+            apcu_add($bucketKey, 0);
+        }
+        apcu_inc($bucketKey);
     }
 
     /**