memcached: which is faster, doing an add (and checking result), or doing a get (and set when returning false)

Question

The title of this question isn't so clear, but the code and question is straightforward.

Let's say I want to show my users an ad once per day. To accomplish this, every time they visit a page on my site, I check to see if a certain memcache key has any data stored on it. If so, don't show an ad. If not, store the value '1' in that key with an expiration of 86400.

I can do this 2 ways:

//version a
$key='OPD_'.date('Ymd').'_'.$type.'_'.$user;
if($memcache->get($key)===false){
 $memcache->set($key,'1',false,$expire);
 //show ad
}

//version b
$key='OPD_'.date('Ymd').'_'.$type.'_'.$user;
if($memcache->add($key,'1',false,$expire)){
 //show ad
}

Now, it might seem obvious that b is better, it always makes 1 memcache call. However, what is the overhead of "add" vs. "get"? These aren't the real comparisons... and I just made up these numbers, but let's say 1 add ~= 1 set ~= 5 get in terms of effort, and the average user views 5 pages a day:

a: (5 get * 1 effort) + (1 set * 5 effort) = 10 units of effort

b: (5 add * 5 effort) = 25 units of effort

Would it make sense to always do the add call? Is this an unnecessary micro-optimization?

Answer 1

Here's some quick and dirty code I whipped up to test this, if anyone is interested:

<?php
require('include.php');
$memcache = new Memcache();

foreach(Config::$CONFIG['memcache_server'] as $memcache_server){
    $memcache->addServer($memcache_server,11211,false);
}

$iterations = 300;
$max_pages_per_visit = 25;

$time_now = microtime(true);
for($pages_per_visit = 1; $pages_per_visit<=$max_pages_per_visit; $pages_per_visit++){
    foreach(array('gs','a') as $method){
        $start = microtime(true);
        for($x = 0; $x < $iterations; $x++){
            $key = 'testmc'.$time_now.'_'.$pages_per_visit.'_'.$method.'_'.$x;
            switch($method){
                case 'gs':
                    for($y = 0 ; $y < $pages_per_visit; $y++){
                        if($memcache->get($key)===false){
                            $memcache->set($key,'1',null,5);
                        }
                    }
                    break;
                case 'a':
                    for($y = 0 ; $y < $pages_per_visit; $y++){
                        $memcache->add($key,'1',null,5);
                    }
                    break;
            }
        }
        $end = microtime(true);
        $results[$pages_per_visit][$method] = $end - $start;
    }
}

//print results
print('<pre>');
foreach($results as $pages_per_visit => $data){
    $speed_diff = $data['gs'] - $data['a'];
    $speed_percentage = round($speed_diff / $data['gs'] * 100,2);
    echo($pages_per_visit.' pages : add is faster by :'.$speed_diff.' ('.$speed_percentage.')%'.PHP_EOL);
}