php5-fpm children and requests

Question

I have a question. I own a 128mb vps with a simple blog that gets just a hundred hits per day. I have nginx + php5-fpm installed. Considering the low visits and the ram I decided to set fpm to static with 1 server running. While I was doing my random tests like running php scripts through http that last over 30 minutes I tried to open the blog in the same machine and noticed that the site was basically unreachable. So I went to the configuration and read this:

     The number of child processes to be created when pm is set to 'static' and the
; maximum number of child processes to be created when pm is set to 'dynamic'.
; **This value sets the limit on the number of simultaneous requests that will be
; served**

What shocked me the most was that I didn't know because I always assumed that a php children would handle hundreds of requests at the same time like a http server would do! Did it get it right? If for example I launch 2 php-fpm children and launch 2 "long scripts" at the same time all the sites using the same php backend will be unreachable?? How is this usable? You may think: -duh! a php script (web page) is usually processed in 100ms- ... no doubt about that but what happens if you have pages that could run for about 10 secs each and I have 10 visitors with php-fpm with 5 servers so accepting only 5 requests per time at the same time? They'll all be queued or will experience timeouts?

I'm honestly used to run sites in Windows with Apache and mod_php I never experienced these issues because apparently those limits don't apply being a different way of using PHP.

This also raises another question. If I have file_1.php with sleep(20) and file_2.php with just an echo, if I run file_1 and then file_2 with the fastcgi machine the second file will request the creation of another server to handle the php request using 4MB RAM more. If I do the same with apache/mod_php the second file will only use 30KB more of RAM (in the apache server). Considering this why is mod_php is considering the "bad guy" if the ram used is actually less...I know I'm missing the big picture here.

Answer 1

You've basically got it right. You configured a static number of workers (and that number was "one") -- so that's exactly what you got.

But you don't understand quite how things typically work, since you say:

I always assumed that a php children would handle hundreds of requests at the same time like a http server would do!

I'm not really familiar with nginx, but consider the typical mod_php setup in apache. If you're using mod_php, then you're using the prefork mpm for apache. So every concurrent http requests is handled by a distinct httpd process (no threads). If you're tuning your apache/mod_php server for low-memory, you're going to have to tweak apache settings to limit the number of processes it will spawn (in particular, MaxClients).

Failing to tune this stuff means that when you get a large traffic spike, apache starts spawning a huge number of heavy processes (remember, it's mod_php, so you have the whole PHP interpreter embedded in each httpd process), and you run out of memory, and then everything starts swapping, and your server starts emitting smoke.

Tuned properly (meaning: tuned so that you ignore requests instead of allocating memory you don't have for more processes), clients will time out, but when traffic subsides, things go back to normal.

Compare that with fpm, and a smarter web server architecture like apache-worker, or nginx. Now you have some, much larger, pool of threads (still configurable!) to handle http requests, and a separate pool of php-fpm processes to handle just the requests that require PHP. It's basically the same thing, if you don't set limits on how many processes/threads can be created, you are asking for trouble. But if you do tune, you come out ahead, since only a fraction of your requests use PHP. So essentially, the average amount of memory needed per http requests is lower -- thus you can handle more requests with the same amount of memory.

But setting the number to "1" is too extreme. At "1", it doesn't even matter if you choose static or dynamic, since either way you'll just have one php-fpm process.

So, to try to give explicit answers to particular questions:

You may think: -duh! a php script (web page) is usually processed in 100ms- ... no doubt about that but what happens if you have pages that could run for about 10 secs each and I have 10 visitors with php-fpm with 5 servers so accepting only 5 requests per time at the same time? They'll all be queued or will experience timeouts?

Yes, they'll all queue, and eventually timeout. The fact that you regularly have scripts that take 10 seconds to run is the real culprit here, though. There are lots of ways to architect around that (caching, work queues, etc), but the right solution depends entirely on what you're trying to do.

I'm honestly used to run sites in Windows with Apache and mod_php I never experienced these issues because apparently those limits don't apply being a different way of using PHP.

They do apply. You can set up an apache/mod_php server the same way as you have with nginx/php-fpm -- just set apache's MaxClients to 1!

This also raises another question. If I have file_1.php with sleep(20) and file_2.php with just an echo, if I run file_1 and then file_2 with the fastcgi machine the second file will request the creation of another server to handle the php request using 4MB RAM more. If I do the same with apache/mod_php the second file will only use 30KB more of RAM (in the apache server). Considering this why is mod_php is considering the "bad guy" if the ram used is actually less...I know I'm missing the big picture here.

Especially on linux, lots of things that report memory usage can be very misleading. But think about it this way: that 30kb is negligible. That's because most of PHP's memory was already allocated when some httpd process got started.

128MB VPS is pretty tight, but should be able to handle more than one php-process.

If you want to optimize, do something like this:

For PHP:

pm = static
pm.max_children=4

for nginx, figure out how to control processes and thread count (whatever the equivalent to apache's MaxClients, StartServers, MinSpareServers, MaxSpareServers)

Then figure out how to generate some realistic load (apachebench, siege, jmeter, etc). use vmstat, free, and top to watch your memory usage. Adjust pm.max_children and the nginx stuff to be as high as possible without causing any significant swap (according to vmstat)