High performance

Possible improvement with caches

markus_petrux — Mon, 07 Jul 2008 01:17:55 +0000

Hi,

I would like to share some thoughts about caching in Drupal, and then see what you people think about it. Not sure if this is new though.

Sometimes, cached objects are created on demand (while a page request occurs) and use expiration times. For exemple, cache_filter table usage in check_markup. In these cases, the logic is more or less, like this:

if ($cached = cache_get($cache_id, 'cache_filter')) {
  return $cached->data;
}
// object is not cached, so here we do a lot of stuff to
// build the thing, then cache it with an expiration time
cache_set($cache_id, $text, 'cache_filter', time() + (60 * 60 * 24));

Well, the problem here is that if several page requests come at the same time, there will be several processes doing the same job and caching the same object, concurrently. For hi traffic sites this might be a problem.

There's a little change that can be done to minimize this effect. Using a variation of the example above, code would look like this:

// We here make sure cache object is NOT expired.
if ($cached = cache_get($cache_id, 'cache_filter') && $cached->expire > time()) {
  return $cached->data;
}
// If we got an expired object, make sure a minimal set of concurrent requests
// do the same job that we're about to do the process and build data.
if (!empty($cached->data)) {
  cache_set($cache_id, $cached->data, 'cache_filter', time() + 30);
}

// Ok, so here we do a lot of stuff to // build the thing, then cache it with an expiration time cache_set($cache_id, $text, 'cache_filter', time() + (60 * 60 * 24));

I believe comments show what I'm trying to mean. If we had something cached that is expired, we store the object again giving it a few more seconds, so a minimal set of possible concurrent requests do the same job at the same time.

If we're using InnoDB, then the cached record is in the buffer pool for sure, so we the overhead in updating the expiration time is minimal versus the cost of doing the same job X times concurrently. Here "the job" is output filtering, where the node could be complex or long. The same login could be applied for cached pages.

I'm using this approach on a site that currently has around 20000 page views a day. Not much, but enough no have notined the benefits of this method. I'm using a particular way to cache pages. It works for any combination of user roles. Cached pages are cleared when related content is updated, which is something particular of the site implementation. Also, to save space for unused cached objects, an expiration time of 5 minutes is used, so cron can keep the cache page table with reasonable size. Since this uses expiration times, here's when the method explained above is doing a great job. I'm using the same method to cache blocks and certain queries. The site can be found here if you want to take a look.

Well, not sure if this can be of any use, but I thought it would be interesting to share. Maybe the method outlined above could be applied in some places od Drupal core, that use cache with expiration times. I haven't found any report in the Drupal issues queue.

Second thing I would like to mention is that maybe cache_set could be improved by using REPLACE INTO rather that the UPDATE/no-affected-rows/INSERT approach. Well, only for MySQL enabled sites, since this is a MySQL extension, but when performance is a point, I think something like this worths.

This might be a noticable benefit specially when caching pages or big chuncks of data. With REPLACE, there's just one statement transmitted over the network, to where the MySQL lives, so the MySQL server can deal with this kind of statements faster.

Here's a post in the MySQL Performance Blog about REPLACE INTO:

http://www.mysqlperformanceblog.com/2007/01/18/insert-on-duplicate-key-u...

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