Reduce delay caused by load balancer

Question

I currently am using mod_proxy_balance in Apache for load balancing. I have a client emulator which creates 100 threads and emulates user activities like browse the website. I tried making the client talk to the webserver directly and also through the load balancer. Here is what I found.

When client talks directly to webserver:

• delay = 28ms

When client talks to webserver through a loadbalancer (only one backend webserver):

• delay = 53ms

When client talks to webserver through a loadbalancer with two backend webservers:

• delay = 113ms

The delay keeps increasing, is there a way to get this delay down? It is a very important aspect to provide agreeable SLO.

Answer 1

You should switch to a load balancer like ha_proxy which is much better suited and performant than mod_proxy_balancer.

mod_proxy_balancer is inefficient and has a number of bugs.

Answer 2

I don't think mod_proxy_balancer is epoll-based, but it's better to check. If it's not based on epoll system call the cause of this delay is obvious, select system call and Apache synchronous processing model are not proper tools for balancing. You should try HAProxy or nginx instead.

Answer 3

Use a better load balancer, really. Your webserver is very good at serving requests, rather than proxy them hither and yon. A good load balancer has packet delays in the sub-millisecond range for existing connections, and shouldn't increase latency just because there's more than one backend.

I use IPVS for load balancing wherever I can, because it is fast and super-efficient. If it's a bit much of a stretch for you, though, you can at least use haproxy to provide good-quality load balancing to your backends.