PHP-FPM WARNING: [pool www] server reached max_children setting (50), consider raising it

Yep, with no change in payments for people with existing servers. Even though my spec has changed (3 x 120GB SSD now), it's £5 a month less

 

Actually, seems like their front page shows 3 x SSD, but they are only giving 2

SSD-3 Help

 

Hey George--question about this. In theory what you say makes complete sense, but question about how it works out in practice.

I've got a VPS with 2 cores, load average between 0.1-0.2, so it's got plenty of CPU to spare. As long as CPU isn't the bottleneck, will php response times still get slower as max_children increases?

I'm guessing my bottlenecks are RAM/Disk IO, so would increasing the children allow the CPU to handle more users concurrently in almost the same speed because the extra processes let the CPU start working on new requests while waiting on ram/disk?

I'm playing around with the settings, but trying to also make sure my mental model of how this is working is correct.

Other reason I ask is I've got a few medium-traffic sites on this box, and a number of low-traffic sites as well. I've been setting `pm.type = ondemand` for the low-traffic sites (every php app has its own separate php-fpm pool), but if it just takes up a little extra CPU cycles, and I've got extra to burn, then I might as well change that to dynamic with start_servers=1 and min_spare_servers=1. These sites will never be high-traffic, so latency rather than concurrency is the thing to optimize for them and having a pre-warmed php process would cut down on latency. I've been avoiding that because I didn't want the low-traffic sites to starve resources from the medium-traffic ones... your statement above about more cumulative php processes going will slow things down makes sense in theory. But if it's just CPU cycles, then in practice those aren't the bottleneck on this box, so despite the theory, I should still set them to dynamic. Thoughts?

And yeah, I know it won't make a huge difference either way in real life, but as you know, I like to understand how things actually work under the covers.

 

What does those parameters do?