Having max/min spare servers as the same amount is a bad idea. This is incurs a substantial amount of process swapping, as every single request php-fpm is going to try and ensure there are precisely 256 idle servers.
Ok, help us out: Given that we have Wrk set to max out at 256 concurrent requests, what would the ideal tuning for php-fpm? A pull request would be ideal, but you can also just tell us. :)
I'd set minimum idle to something like 16 or 32. php-fpm will not create more than 32 workers/sec.
What happens now is 256 workers running and 256 simultaneous requests occur. So php-fpm sees 256 workers busy, 0 idle. The minimum idle is 256, so it attempts to start 256 additional processes.