Memcache Slowing Down Drupal

Question

Running seige (load tester) without memcache enabled I see stats similar to this:

Transactions:                    560 hits
Availability:                 100.00 %
Elapsed time:                  59.42 secs
Data transferred:               6.25 MB
Response time:                  1.56 secs
Transaction rate:               9.42 trans/sec
Throughput:                     0.11 MB/sec
Concurrency:                   14.71
Successful transactions:         543
Failed transactions:               0
Longest transaction:           10.57
Shortest transaction:           0.00

Notice that I'm getting about 560 transactions in 60 seconds, 9.42 trans/sec.

When I enable memcache (using the PECL memcache module) and re-run siege I see much worse performance:

Transactions:                    133 hits
Availability:                 100.00 %
Elapsed time:                  59.94 secs
Data transferred:               1.34 MB
Response time:                  6.43 secs
Transaction rate:               2.22 trans/sec
Throughput:                     0.02 MB/sec
Concurrency:                   14.27
Successful transactions:         129
Failed transactions:               0
Longest transaction:           12.22
Shortest transaction:           0.00

Notice that I've decreased my performance significantly. I was only able to process 133 hits in 60 seconds and the throughput is 2.22 transactions per second.

I have memcache setup on its own dedicated system with 4GB of RAM and here are the parameters: memcached -d -p 11211 -u memcached -m 2560 -c 2048 -P /var/run/memcached/memcached.pid

I noticed that if I go to /admin/reports/memcache I get the following PHP errors:

Notice: Undefined variable: stats in dmemcache_stats() (line 244 of /www/sites/all/modules/memcache/dmemcache.inc).
Warning: Invalid argument supplied for foreach() in dmemcache_stats() (line 244 of /www/sites/all/modules/memcache/dmemcache.inc).
Notice: Undefined variable: stats in dmemcache_stats() (line 263 of /www/sites/all/modules/memcache/dmemcache.inc).
There are no statistics being reported for this bin.

I'm not sure if that's related to the issue or not?

This is what I have in settings.php:

/**************************************************
 * MEMCACHE
 **************************************************/
// the path to the core cache file
include_once('./includes/cache.inc');
// the path to the memcache cache file
include_once('./sites/all/modules/memcache/memcache.inc');

$conf['cache_default_class'] = 'MemCacheDrupal';
$conf['cache_class_cache_form'] = 'DrupalDatabaseCache';
$conf['memcache_servers'] = array(
        'memcache.domain.local:11211' => 'cluster'
);

$conf['memcache_bins'] = array(
        'bin1' => 'cluster'
);

$conf['memcache_key_prefix'] = 's-drupal';

Why is memcache slowing down my site instead of speeding it up? Would switching to PECL memcached help?

Answer 1

The errors on the stats page are due to an unrelated issue with memcache reporting. I think the problem you have is with the conf setup. I believe it should be like this for a default set up where all tables are mapped to the same bin:

$conf['cache_default_class'] = 'MemCacheDrupal';
$conf['cache_class_cache_form'] = 'DrupalDatabaseCache';
$conf['memcache_servers'] = array(
    'memcache.domain.local:11211' => 'default'
);

$conf['memcache_key_prefix'] = 's-drupal';

If that doesn't do the trick you can explicitly code the mappings for each table:

$conf['memcache_bins'] = array(
  'cache' => 'default',
  'cache_filter' => 'default',
  'cache_menu' => 'default'
);

etc.