Cached, PHP generated Thumbnails load slowly

Question

Question Part A ▉ (100 bountys, awarded)
Main question was how to make this site, load faster. First we needed to read these waterfalls. Thanks all fo

Answer 1

I've found the URL of your website and checked an individual jpg file from the homepage. While the loading time is reasonable now (161ms), it's waiting for 126ms, which is far too much.

Your last-modified headers are all set to Sat, 01 Jan 2011 12:00:00 GMT, which looks too "round" to be the real date of generation ;-)

Since Cache-control is "public, max-age=14515200", arbitrary last-modified headers will could cause problem after 168 days.

Anyway, this is not the real reason for delays.

You have to check what your thumbnail generator do when the thumbnail already exists and what could consume so much time checking and delivering the picture.

You could install xdebug to profile the script and see where the bottlenecks are.

Maybe the whole thing uses a framework or connects to some database for nothing. I've seen very slow mysql_connect() on some servers, mostly because they were connecting using TCP and not socket, sometimes with some DNS issues.

I understand you can't post your paid generator here but I'm afraid there are too many possible issues...

Answer 2

I am far from an expert but...

In regards to this: "An If-Modified-Since conditional request returned the full content unchanged." and my comments.

The code used to generate the Thumbnails should be checking for the following:

1. Is there a cached version of the thumbnail.
2. Is the cached version newer than the original image.

If either of these are false the thumbnail should be generated and returned no matter what. If they are both true then the following check should be made:

1. Is there a HTTP_IF_MODIFIED_SINCE header
2. Is the cached version's last modified time the same as the HTTP_IF_MODIFIED_SINCE

If either of these are false the cached thumbnail should be returned.

If both of these are true then a 304 http status should be returned. I'm not sure if its required but I also personally return the Cache-Control, Expires and Last-Modified headers along with the 304.

In regards to GZipping, I've been informed that there is no need to GZip images so ignore that part of my comment.

Edit: I didn't notice your addition to your post.

session_cache_limiter('public');
header("Content-type: " . $this->_mime);
header("Expires: " . gmdate("D, d M Y H:i:s", time() + 2419200) . " GMT");
// I'm sure Last-Modified should be a static value. not dynamic as you have it here.
header("Last-Modified: " . gmdate("D, d M Y H:i:s",time() - 404800000) . " GMT");

I'm also sure that your code needs to check for the HTTP_IF_MODIFIED_SINCE header and react to it. Just setting these headers and your .htaccess file won't provide the required result.

I think you need something like this:

$date = 'D, d M Y H:i:s T'; // DATE_RFC850
$modified = filemtime($filename);
$expires = strtotime('1 year'); // 1 Year

header(sprintf('Cache-Control: %s, max-age=%s', 'public', $expires - time()));
header(sprintf('Expires: %s', date($date, $expires)));
header(sprintf('Last-Modified: %s', date($date, $modified)));
header(sprintf('Content-Type: %s', $mime));

if(isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
    if(strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) === $modified) {
        header('HTTP/1.1 304 Not Modified', true, 304);
        // Should have been an exit not a return. After sending the not modified http
        // code, the script should end and return no content.
        exit();
    }
}
// Render image data

Answer 3

Have you tried replacing the php generated thumnails by regular images to see if there is any difference ? The problem could be around - a bug in your php code leading to a regeneration of the thumbnail upon each server invocation - a delay in your code ( sleep()?) associated with a clock problem - a hardrive issue causing a very bad race condition since all the thumbnails get loaded/generated at the same time.

Answer 4

Have you tried to set up several subdomains under NGINX webserver specially for serving static data like images and stylesheets? Something helpful could be already found in this topic.

Answer 5

Wow, it's hard to explain things using that image.. But here, some tries:

• files 33-36 load that late, because they are dynamically loaded within the swf, and the swf (25) is loaded first completely before it loads any additional content
• files 20 & 21 are maybe (I don't know, because I don't know your code) libraries that are loaded by all.js (11), but for 11 to execute, it waits for the whole page (and assets) to load (you should change that to domready)
• files 22-32 are loaded by those two libraries, again after those are completely loaded

Answer 6

So your PHP script is generating the thumbnails on every page load? First off, if the images that are being thumbnailed are not changing that often, could you set up a cache such that they don't have to be parsed each time the page loads? Secondly, is your PHP script using something like imagecopyresampled() to create the thumbnails? That's a non-trivial downsample and the PHP script won't return anything until its done shrinking things down. Using imagecopymerged() instead will reduce the quality of the image, but speed up the process. And how much of a reduction are you doing? Are these thumbnails 5% the size of the original image or 50%? A greater size of the original image likely is leading to a slowdown since the PHP script has to get the original image in memory before it can shrink it and output a smaller thumbnail.