[SOLVED] Apart from hardware, what affects the performance in terms of number of concurrent users?

Apart from hardware, what affects the performance in terms of number of concurrent users?

I'm developing an API and want to (of course) optimize performance in terms of number of concurrent users.

I have run some tests using Blitz (my app is on Appfog, PHP, 512MB, 1 instance) according to those tests my API can handle 11 concurrent users before response times get too high (>1000 ms).

For me it is surprisingly low. I can add more RAM and instances to improve the results but I suspect that my code could be smarter.

I did some tests, always with same hardware config. Result is number of concurrent users before exceeding 1000 ms in response time.

Using my actual API (with db-queries) --> 11 users
Using script that just outputs text (minimum processing) --> 40 users
Using script with sleep(2) function to simulate long response time --> 52 users (before exceeding (2000 + 1000 ms)
Using a memory intensive script (building data with for-loop): 95 users

I really don't see any correlation in the results (each test has been run many times with similar results). The more processing for the script - the more concurrent users?

What is that affects the number concurrent users (apart from hardware config)?

Solution

Generally there are two aspects you should think of:

bottlenecks like database or external APIs. You are as slow as the slowest component
look for locks that turn your concurrent code into sequential. See: Amdahl's law

The second point is related to the first one. Database or whatever you use in your code might be internally synchronized or might not cope well with concurrency.