Cron Run and Cache

Question

I'm running Drupal 7.25 on my Ubuntu server. I have a process that is kicked off by user interaction (form) that needs to run in the background. I currently have cron.php being run every minute via crontab. I read this article:

http://www.metaltoad.com/blog/how-drupals-cron-killing-you-your-sleep-simple-cache-warmer

Does running cron.php still clear cache in D7?

Answer 1

Yes, running cron.php still clear cache, as running the cron will call system_cron which calls "cache_clear_all" method:

$cache_tables = array_merge(module_invoke_all('flush_caches'), $core);
foreach ($cache_tables as $table) {
  cache_clear_all(NULL, $table);
}

Answer 2

If you take a look at system_cron(), you will see

$cache_tables = array_merge(module_invoke_all('flush_caches'), $core);
foreach ($cache_tables as $table) {
  cache_clear_all(NULL, $table);
}

Reading the docs for cache_clear_all($cid, $bin):

$cid: If set, the cache ID or an array of cache IDs. Otherwise, all cache entries that can expire are deleted. The $wildcard argument will be ignored if set to NULL.

This means that a full cache clear won't be done, but entries that are older than their TTL are removed from the cache.

However, {cache_page} uses CACHE_TEMPORARY for the TTL (see drupal_page_set_cache(), so these will be removed each cron run. Though the code support it, I have never seen anything other that a -1 in the exipres column in {cache_page}, no matter what the TTLs on the performance settings are.