nginx high active connections

Reinis Rozitis r at
Tue Jul 3 15:09:50 UTC 2012

> Active connections in the range of 400-450 is okay
> but sometimes it goes to like 4,000. at this point in time. This is abnormal

> What does this mean? Is it that nginx is not able to take the load?

It is rarely the webserver itself which can't handle the load/connections (having even 10k connections is no big deal) but it is 
hard to get to any conclusions and/or give any solutions knowing only 2 numbers  - 400 / 4000. One needs at least some application 
specifics (is there a dynamic language involved (php/python/...) or only static files are being served / are there any db backends 
involved / does the undelaying filesystem can handle the load (no wa?) etc etc?).

Just out of experience 2  cases come into mind - it's either slowdown of the application - single request starts to take up too much 
server time and then the rest just pile up.
Other possibility is some sort of DDOS (but I would put this as less likely case).

First thing to do is to check the logs:
- check the nginx error log if there are notification (of file descriptors running out of workers being too few / upstream servers 
not responding)

- if you use php (with fpm) check the error_log too -  lines like '[pool www] seems busy (you may need to increase pm.start_servers, 
or pm.min/max_spare_servers)' might indicate that there are sudden changes in request patterns or execution speed and all worker 
childs are busy. It's also useful to enable set request_slowlog_timeout so php logs any request taking too much time to execute.

- if you use any DBs check those logs too (like MySQL .err log or best if you have a slow query log enabled ( log_slow_queries / 
long_query_time = 2 ))

There is no magical quick fix or answer to your question :)


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