How many cores and how much RAM does your server have? Before you restart the server how many PHP processes are there and what is "top" saying?
Count php processes
ps -ef | grep php
Work out top (edit your question to copy and paste the text of top in, formatted so it displays correctly. The first 4 or 5 lines are the most important)
top
I suspect you're trying to use more resources than your server has. PHP can use a lot of memory - 100MB per thread (child) or more sometimes. If you've told it to use 600 children that could be 60GB of RAM so you're well into swap.
I have an EC2 t3.nano with 500MB of RAM and 500MB of swap, running MySQL, PHP, Nginx, and a few others things. I set pm.max_children = 4 . Here's my key settings for the dynamaic PHP pool for my very lightly loaded server
pm = dynamic
pm.max_children = 4
pm.start_servers = 2
pm.min_spare_servers = 1
pm.max_spare_servers = 2
; This next line restarts the PHP thread once it's serviced 100 threads - I have a buggy program
pm.max_requests = 100
I have another PHP pool which is only used for testing, which is rare, so it defaults to no spare PHP processes
pm = ondemand
pm.max_children = 2
pm.process_idle_timeout = 120s;
pm.max_requests = 50
In my Nginx configuration I use this command to send my test site to the PHP test pool
fastcgi_pass php56-fpm-testpool;
Suggestions
Use a configuration something like this, to reduce RAM usage. This really depends on your load and you should tweak the numbers.
pm = dynamic
pm.max_children = 75
pm.start_servers = 25
pm.min_spare_servers = 10
pm.max_spare_servers = 20
pm.max_requests = 500