We've recently set up a Redis cluster to handle our PHP sessions since we use a load balancer to direct traffic into different craft instances.
We're seeing issues with redis when making back to back PHP session calls (using craft wrapper) very quickly. If we update a variable in the httpSession and then try to update it/get it very quickly it isn't updating. If we wait to access it then it is updated correctly.
How it's set up: We modified the Apache2 php.ini to update the session handler to redis and the save path to our redis cluster (bypassing doing this via Craft). So Craft still uses its' default session handling but when it makes the request to save the session it gets redirected by apache to our redis.
Is it more beneficial to tie this to Craft by updating the General.php with the session override options? Will we see faster/better session handling? Is there a session lock happening in craft? Is Craft opening/closing the session after each httpSessionService call?