Why Varnish sess_timeout is so low by default?

Question

Varnish has parameter sess_timeout(docs here), by default it is set to 5 seconds. Which means that after 5 seconds the session will be closed, and next page load will require will require extra 100ms (in average) to connect to server (I've described this issue here).

• Why this parameter is so low by default?
• If I increase it to 60 seconds, will it cause any problems on the server?
• Does it matter what do I use behind the Varnish - nginx or apache? Or varnish optimizes the connections by itself?
• What's the recommended value for average website (e.g. Magento store with 500 active users at a time)?

Answer 1

sess_timeout is tuned to avoid keeping state around when it is not needed. Worker threads are (in high traffic situations) a precious resource, and having one waiting around doing nothing isn't productive.

For all HTTP clients I know, manual netcat/telnet excluded, it does not take 5s to push through the 100-150 byte long HTTP request.

You can safely increase this to 60s if you feel you need to. If you are using this for long-running connections, you should probably use return(pipe) instead; different timers apply there.