Merge pull request #2047 from Automattic/update/alloptions-limit

ALLOPTIONS limit alert/block - account for the fact that pecl-memcache client automagically compresses the values before sending it to the server

alloptions-limit.php

@@ -17,16 +17,34 @@ function wpcom_vip_sanity_check_alloptions() {
 	}
 
 	// Warn should *always* be =< die
-	$alloptions_size_warn  =  800000;
-	$alloptions_size_die   = 1000000; // 1000000 ~ 1MB, too big for memcache
+	$alloptions_size_warn = 800000;
+	// 950000 is slightly less than ~ 1MB, which is the Memcached entry limit.
+	// The purpose of this limit is to safe-guard against a barrage of requests with cache sets for values that are too large
+	// Because WP would keep trying to set the data to Memcached, potentially resulting in Memcached (and site's) performance degradation.
+	$alloptions_size_die  = 950000;
 
 	$alloptions_size = wp_cache_get( 'alloptions_size' );
 
 	// Cache miss
 	if ( false === $alloptions_size ) {
-		$alloptions = wp_load_alloptions();
-
-		$alloptions_size = strlen( serialize( $alloptions ) );
+		$alloptions = serialize( wp_load_alloptions() );
+		$alloptions_size_uncompressed = strlen( $alloptions );
+		// We're using gzdeflate here because pecl-memcache uses Zlib compression for large values.
+		// See https://github.com/websupport-sk/pecl-memcache/blob/e014963c1360d764e3678e91fb73d03fc64458f7/src/memcache_pool.c#L303-L354
+		$alloptions_size_compressed = strlen( gzdeflate( $alloptions ) );
+
+		// We only compress the value if it's bigger than 20kb and the savings for the compressed value are more than 20%;
+		// See https://github.com/Automattic/wp-memcached/blob/811243804a892a4609a4581d9479fa80c4e4ac8d/object-cache.php#L782
+		// Make sure to use the correct size.
+		// This is an additional safe-guard, in reality the bigger the value the better the compression,
+		// So it's hard to get into a situation where you have a large value that not going to have at least 20% savings.
+		$diff = $alloptions_size_uncompressed - $alloptions_size_compressed;
+
+		if ( $alloptions_size_uncompressed > 20000 && $diff > $alloptions_size_uncompressed * 0.2 ) {
+			$alloptions_size = $alloptions_size_compressed;
+		} else {
+			$alloptions_size = $alloptions_size_uncompressed;
+		}
 
 		wp_cache_add( 'alloptions_size', $alloptions_size, '', 60 );
 	}