On Fri, Oct 10, 2003 at 12:23:18AM +1000, Tim Starling wrote:
I wrote a little perl script to monitor the health of Wikipedia by regularly checking the file size of the main log. The idea was that it would email me if apache crashed. I soon discovered something rather interesting and unexpected.
Once every 15 minutes or so, Wikipedia shuts down for a minute or two. This is bad, needless to say. During these episodes, it shows few if any log entries. Load is depressed slightly (from ~20 to ~10), so CPU starvation didn't seem likely. I suspected database troubles.
Attached is a zip file containing the output from SHOW PROCESSLIST. There is one file showing "typical" output, between the episodes, and three showing output in the middle of an episode. One thing stands out like a sore thumb:
SELECT 1 FROM user_newtalk WHERE user_ip='167.206.112.85' SELECT 1 FROM user_newtalk WHERE user_ip='217.244.15.107' SELECT 1 FROM user_newtalk WHERE user_ip='80.55.166.58' SELECT 1 FROM user_newtalk WHERE user_ip='66.196.90.11' SELECT 1 FROM user_newtalk WHERE user_ip='172.176.254.188' SELECT 1 FROM user_newtalk WHERE user_ip='220.54.212.24' SELECT 1 FROM user_newtalk WHERE user_id=7580 SELECT 1 FROM user_newtalk WHERE user_ip='194.78.48.226' SELECT 1 FROM user_newtalk WHERE user_ip='80.225.14.29' SELECT 1 FROM user_newtalk WHERE user_ip='144.32.128.73' SELECT 1 FROM user_newtalk WHERE user_ip='62.216.15.127' SELECT 1 FROM user_newtalk WHERE user_ip='134.151.225.179' SELECT 1 FROM user_newtalk WHERE user_ip='213.81.145.123' SELECT 1 FROM user_newtalk WHERE user_ip='129.137.208.159' SELECT 1 FROM user_newtalk WHERE user_ip='195.93.72.17' SELECT 1 FROM user_newtalk WHERE user_ip='218.19.141.2' SELECT 1 FROM user_newtalk WHERE user_ip='68.11.187.242' SELECT 1 FROM user_newtalk WHERE user_ip='63.34.208.93' SELECT 1 FROM user_newtalk WHERE user_ip='81.196.21.16' SELECT 1 FROM user_newtalk WHERE user_ip='202.156.2.82'
Millions of user_newtalk requests, conspicuously absent from the typical dump. Many of them are doing "statistics", whatever that means, and the rest are locked. One side of me is curious as to what "statistics" means and why it produces this behaviour. The other side of me says
Could these hanging selects be just a part of the phenomenon? I mean, assume, the mysql has some reason not to respond, but still accepts new requests. Then all anonymous users reading a page will add a new process looking for a newtalk flag. They don't cause the slowdown, they are its victims.
So the question is: Why does it slow down every 15 minutes? Do we have some cron jobs running every quarter of an hour?
Regards,
JeLuf