This is not necessary for single server systems.
Nice idea to test the redis server directly first. In @rsumook case would be:
redis-cli -s /tmp/redis.sock ping
Having a look into the code, it seems that your webserver can’t reach the socket file: https://github.com/nextcloud/server/blob/master/lib/private/RedisFactory.php#L82 (by the way you see there, that timeout = 0.0 is set by default, thus could be left out 
)
The reason could be, that web server is somehow limited in having a look into the /tmp folder on certain systems. I got e.g. a problem when setting the tmp php upload folder to /var/tmp/php_upload_tmp: Missing upload-tmp or not writable - #6 by MichaIng
So I would really follow the default socket location:
nano /etc/redis.conf unixsocket /var/run/redis/redis.sock service redis-server restart nano /var/www/html/nextcloud/config/config.php 'host' => '/var/run/redis/redis.sock', service apache2 restart