Tagged: cache

Benchmarking Memcached

August 12th, 2013 in Performance 0 comments

We wanted a better understanding of how much time was being spent carrying out Memcached operations, and a deterministic method to benchmark operations isolated from the WordPress environment.

The result:

benchmark_memcached.php

What does it do?

The script allows you to specify one or more memcached servers to test (host:port), as well as provide fixture data in the form of a JSON file.

The script does the following:

  • Creates client Memcached instances using the PECL Memcache extension
  • Loads fixture data and converts to PHP arrays
  • Runs a simple benchmark — set, get and delete provided cache data in sequence, timing each method as it runs
  • Writes results to a CSV file

If you pass multiple memcached servers as input, the benchmarks will be run in two different modes:

  1. Pooled (one client sharing multiple servers)
  2. Individual (one client per server)

Gathering Test Data

We explored a few options for generating cache data to run through the benchmark script.

We leverged the built-in WP_Object_Cache class, specifically the one defined in the memcached object cache drop-in to extract actual cache data used during individual page requests.

function WP_Object_Cache() {
    . . .
    if ( defined( 'DUMP_CACHE_DATA' ) && DUMP_CACHE_DATA )
        register_shutdown_function( array( $this, 'cache_dump' ) );
    }
}

function cache_dump() {
    $filename = sprintf( '%s-%s.json', $_SERVER['HTTP_HOST'], rawurlencode( $_SERVER['REQUEST_URI'] ) );
    file_put_contents( "/tmp/cachedumps/$filename", json_encode( $this->cache ) );
    return true;
}

With this in place, cache dumps could be triggered by setting a constant in wp-config.php and pointing the browser at real pages in our install.

define( 'DUMP_CACHE_DATA', true );

The end result is a JSON file for each page, which we turned around and fed to the benchmark script.

Next Steps

Results coming soon…

Tagged , ,