Simplified getMaxLag() to use getLagTimes()

* This method now benefits from more cache sharing and de-duplicated
  lag time querying to reduce connection stampedes.

Change-Id: I2f3b9a22e4adabea703fbae1f96e65fb65125e2b

diff --git a/includes/db/LoadBalancer.php b/includes/db/LoadBalancer.php
index 0ccbd93..9797f29 100644 (file)
--- a/includes/db/LoadBalancer.php
+++ b/includes/db/LoadBalancer.php
@@ -58,8 +58,8 @@ class LoadBalancer {
        private $mLaggedSlaveMode;
        /** @var string The last DB selection or connection error */
        private $mLastError = 'Unknown error';
-       /** @var array Process cache of LoadMonitor::getLagTimes() */
-       private $mLagTimes;
+       /** @var ProcessCacheLRU */
+       private $mProcCache;
 
        /**
         * @param array $params Array with keys:
@@ -108,6 +108,8 @@ class LoadBalancer {
                                }
                        }
                }
+
+               $this->mProcCache = new ProcessCacheLRU( 30 );
        }
 
        /**
@@ -1080,7 +1082,8 @@ class LoadBalancer {
        }
 
        /**
-        * Get the hostname and lag time of the most-lagged slave.
+        * Get the hostname and lag time of the most-lagged slave
+        *
         * This is useful for maintenance scripts that need to throttle their updates.
         * May attempt to open connections to slaves on the default DB. If there is
         * no lag, the maximum lag will be reported as -1.
@@ -1093,67 +1096,45 @@ class LoadBalancer {
                $host = '';
                $maxIndex = 0;
 
-               if ( $this->getServerCount() <= 1 ) { // no replication = no lag
-                       return array( $host, $maxLag, $maxIndex );
+               if ( $this->getServerCount() <= 1 ) {
+                       return array( $host, $maxLag, $maxIndex ); // no replication = no lag
                }
 
-               // Try to get the max lag info from the server cache
-               $key = 'loadbalancer:maxlag:cluster:' . $this->mServers[0]['host'];
-               $cache = ObjectCache::newAccelerator( array(), 'hash' );
-               $maxLagInfo = $cache->get( $key ); // (host, lag, index)
-
-               // Fallback to connecting to each slave and getting the lag
-               if ( !$maxLagInfo ) {
-                       foreach ( $this->mServers as $i => $conn ) {
-                               if ( $i == $this->getWriterIndex() ) {
-                                       continue; // nothing to check
-                               }
-                               $conn = false;
-                               if ( $wiki === false ) {
-                                       $conn = $this->getAnyOpenConnection( $i );
-                               }
-                               if ( !$conn ) {
-                                       $conn = $this->openConnection( $i, $wiki );
-                               }
-                               if ( !$conn ) {
-                                       continue;
-                               }
-                               $lag = $conn->getLag();
-                               if ( $lag > $maxLag ) {
-                                       $maxLag = $lag;
-                                       $host = $this->mServers[$i]['host'];
-                                       $maxIndex = $i;
-                               }
+               $lagTimes = $this->getLagTimes( $wiki );
+               foreach ( $lagTimes as $i => $lag ) {
+                       if ( $lag > $maxLag ) {
+                               $maxLag = $lag;
+                               $host = $this->mServers[$i]['host'];
+                               $maxIndex = $i;
                        }
-                       $maxLagInfo = array( $host, $maxLag, $maxIndex );
-                       $cache->set( $key, $maxLagInfo, 5 );
                }
 
-               return $maxLagInfo;
+               return array( $host, $maxLag, $maxIndex );
        }
 
        /**
         * Get lag time for each server
-        * Results are cached for a short time in memcached, and indefinitely in the process cache
+        *
+        * Results are cached for a short time in memcached/process cache
         *
         * @param string|bool $wiki
         * @return array Map of (server index => seconds)
         */
        function getLagTimes( $wiki = false ) {
-               # Try process cache
-               if ( isset( $this->mLagTimes ) ) {
-                       return $this->mLagTimes;
+               if ( $this->getServerCount() <= 1 ) {
+                       return array( 0 => 0 ); // no replication = no lag
                }
-               if ( $this->getServerCount() == 1 ) {
-                       # No replication
-                       $this->mLagTimes = array( 0 => 0 );
-               } else {
-                       # Send the request to the load monitor
-                       $this->mLagTimes = $this->getLoadMonitor()->getLagTimes(
-                               array_keys( $this->mServers ), $wiki );
+
+               if ( $this->mProcCache->has( 'slave_lag', 'times', 1 ) ) {
+                       return $this->mProcCache->get( 'slave_lag', 'times' );
                }
 
-               return $this->mLagTimes;
+               # Send the request to the load monitor
+               $times = $this->getLoadMonitor()->getLagTimes( array_keys( $this->mServers ), $wiki );
+
+               $this->mProcCache->set( 'slave_lag', 'times', $times );
+
+               return $times;
        }
 
        /**
@@ -1179,10 +1160,10 @@ class LoadBalancer {
        }
 
        /**
-        * Clear the cache for getLagTimes
+        * Clear the cache for slag lag delay times
         */
        function clearLagTimeCache() {
-               $this->mLagTimes = null;
+               $this->mProcCache->clear( 'slave_lag' );
        }
 }
 

diff --git a/includes/jobqueue/JobRunner.php b/includes/jobqueue/JobRunner.php
index e38aa5f..4d2c618 100644 (file)
--- a/includes/jobqueue/JobRunner.php
+++ b/includes/jobqueue/JobRunner.php
@@ -77,8 +77,7 @@ class JobRunner {
                }
 
                // Bail out if there is too much DB lag
-               // @note: getLagTimes() has better caching than getMaxLag()
-               $maxLag = max( wfGetLBFactory()->getMainLB( wfWikiID() )->getLagTimes() );
+               list( , $maxLag ) = wfGetLBFactory()->getMainLB( wfWikiID() )->getMaxLag();
                if ( $maxLag >= 5 ) {
                        $response['reached'] = 'slave-lag-limit';
                        return $response;