Sergey,
Just tried with 50 PHP-FPM workers, and performance was even worse:
> This rush generated 5,004 successful hits in 1.0 min and we transferred 47.39 MB of data in and out of your app. The average hit rate of 80/second translates to about 6,982,940 > hits/day.
>
> The average response time was 276 ms.
>
> You've got bigger problems, though: 20.67% of the users during this rush experienced timeouts or errors!
Same problem logged in /var/log/nginx/error.log:
connect() to unix:/var/run/php-fpm/php.sock failed (11: Resource temporarily unavailable) while connecting to upstream