Site down after lots of user logins.

[salmiak]

I have a site with user login using the LoginRegisterPro module. Where I host an embedded youtube livestream (weekly). On that page I track the users activity using AJAX storing the time they spend on the page and what they vote for in a live poll(also using AJAX).

Often it works without issue (mostly around 150 users)

But when we go over about 200-300 concurrent users things started to slow down and I got hundereds of these errors: 

Error:  Exception: Unable to obtain lock for session (retry in 30s) (in /path/to/wire/modules/Session/SessionHandlerDB/SessionHandlerDB.module line 96)

After a while the site  stopped responding.

What can I do to solve this?

[dragan]

Check if the sessions table is really innoDB (it should be). Also, maybe try to decrease this value:

$this->set('lockSeconds', 50); // max number of seconds to wait to obtain DB row lock

But most likely, the DB engine doesn't have enough horse-power to handle the load. Are you on a shared hosting environment?

You might also try to install SessionLoginThrottle module (core), to prevent too many DB-hits from users who are trying to login without success (repeatedly).

[salmiak]

I tried setting it to:

$this->set('lockSeconds', 1); // max number of seconds to wait to obtain DB row lock
$this->set('retrySeconds', 1); // seconds after which to retry on a lock fail

And after a load test with https://github.com/rakyll/hey

It seems to improve things from handeling max 10 requests per second (continiously) to about 35. 

A few questions:.

What would be the downside of this?
Why is it this high in the first place?