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

First of all, you need to handle If-Modified-Since requests and such appropriately, as James said. That error states that: "When I ask your server if that image is modified since the last time, it sends the whole image instead of a simple yes/no".

The time between the connection and the first byte is generally the time your PHP script takes to run. It is apparent that something is happening when that script starts to run.

1. Have you considered profiling it? It may have some issues.
2. Combined with the above issue, your script may be running many more times than needed. Ideally, it should generate thumbs only if the original image is modified and send cached thumbs for every other request. Have you checked that the script is generating the images unnecessarily (e.g. for each request)?

Generating proper headers through the application is a bit tricky, plus they may get overwritten by the server. And you are exposed to abuse as anyone sending some no-cache request headers will cause your thumbnail generator to run continuously (and raise loads). So, if possible, try to save those generated thumbs, call the saved images directly from your pages and manage headers from .htaccess. In this case, you wouldn't even need anything in your .htaccess if your server is configured properly.

Other than these, you can apply some of the bright optimization ideas from the performance parts of this overall nice SO question on how to do websites the right way, like splitting your resources into cookieless subdomains, etc. But at any rate, a 3k image shouldn't take a second to load, this is apparent when compared to other items in the graphs. You should try to spot the problem before optimizing.

Answer 2

I had a similar problem a few days ago & i found head.js. It's a Javascript Plugin which allows you to load all JS files paralell. Hope that helps.

Answer 3

Sorry to say, you provide to few data. And you already had some good suggestions.

How are you serving those images ? If you're streaming those via PHP you're doing a very bad thing, even if they are already generated.

NEVER STREAM IMAGES WITH PHP. It will slow down your server, no matter the way you use it.

Put them in a accessible folder, with a meaningful URI. Then call them directly with their real URI. If you need on the fly generation you should put an .htaccess in the images directory which redirects to a generator php-script only if the request image is missing. (this is called cache-on-request strategy).

Doing that will fix php session, browser-proxy, caching, ETAGS, whatever all at once.

WP-Supercache uses this strategy, if properly configured.

I wrote this some time ago ( http://code.google.com/p/cache-on-request/source/detail?r=8 ), last revisions are broken, but I guess 8 or less should work and you can grab the .htaccess as an example just to test things out (although there are better ways to configure the .htaccess than the way I used to).

I described that strategy in this blog post ( http://www.stefanoforenza.com/need-for-cache/ ). It is probably badly written but it may help clarifying things up.

Further reading: http://meta.wikimedia.org/wiki/404_handler_caching

Answer 4

The majority of the slow issue is your TTFB (Time to first byte) being too high. This is a hard one to tackle without getting intimate with your server config files, code and underlying hardware, but I can see it's rampant on every request. You got too much green bars (bad) and very little blue bars (good). You might want to stop optimizing the frontend for a bit, as I believe you've done much in that area. Despite the adage that "80%-90% of the end-user response time is spent on the frontend", I believe yours is occuring in the backend.

TTFB is backend stuff, server stuff, pre-processing prior to output and handshaking.

Time your code execution to find slow stuff like slow database queries, time entering and exiting functions/methods to find slow functions. If you use php, try Firephp. Sometimes it is one or two slow queries being run during startup or initializtion like pulling session info or checking authentication and what not. Optimizing queries can lead to some good perf gains. Sometimes code is run using php prepend or spl autoload so they run on everything. Other times it can be mal configured apache conf and tweaking that saves the day.

Look for inefficient loops. Look for slow fetching calls of caches or slow i/o operations caused by faulty disk drives or high disk space usage. Look for memory usages and what's being used and where. Run a webpagetest repeated test of 10 runs on a single image or file using only first view from different locations around the world and not the same location. And read your access and error logs, too many developers ignore them and rely only on outputted onscreen errors. If your web host has support, ask them for help, if they don't maybe politely ask them for help anyway, it won't hurt.

You can try DNS Prefetching to combat the many domains and resources, http://html5boilerplate.com/docs/DNS-Prefetching/

Is the server your own a good/decent server? Sometimes a better server can solve a lot of problems. I am a fan of the 'hardware is cheap, programmers are expensive' mentality, if you have the chance and the money upgrade a server. And/Or use a CDN like maxcdn or cloudflare or similar.

Good Luck!

(p.s. i don't work for any of these companies. Also the cloudflare link above will argue that TTFB is not that important, I threw that in there so you can get another take.)

Answer 5

If there isn't a really good reason (usually there isn't) your images shouldn't invoke the PHP interpreter.

Create a rewrite rule for your web server that servers the image directly if it is found on the file system. If it's not, redirect to your PHP script to generate the image. When you edit the image, change the images filename to force users that have a cached version to fetch the newly edited image.

If it doesn't work at least you will now it doesn't have anything to do with the way the images are created and checked.

Answer 6

As some browsers only download 2 parallels downloads per domain, could you not add additional domains to shard the requests over two to three different hostnames. e.g. 1.imagecdn.com 2.imagecdn.com