MySQL update/write query analysis (query profiling)

Do you have a slow MySQL update/insert/delete query?

Obviously, for ‘SELECT’ queries you can prepend the query with “EXPLAIN ” – however that doesn’t work for the other query types (UPDATE/INSERT/DELETE).

So, one solution which may explain why the query is slow is to turn on MySQL’s profiling functionality, like in the following example :
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Zend_Cache – automatic cache cleaning can be bad, mmkay?

$customer uses Zend_Cache in their codebase – and I noticed that every so often a page request would take ~5 seeconds (for no apparent reason), while normally they take < 1 second …

Some rummaging and profiling with xdebug showed that some requests looked like :

xdebug profiling output - note lots of zend_cache stuff
xdebug profiling output - note lots of zend_cache stuff

Note how there are 25,000 or so calls for various Zend_Cache_Backend_File thingys (fetch meta data, load contents, flock etc etc).

This alternative rendering might make it more clear – especially when compared with the image afterwards :

zend cache dominating the call map
zend cache dominating the call map

while a normal request should look more like :

a "normal" request - note Zend_Cache is not dominating
a "normal" request - note Zend_Cache is not dominating

Zend_Cache has a ‘automatic_cleaning_mode’ frontend parameter – which is by default set to 10 (i.e. 10% of all write requests to the cache result in it checking if there is anything to garbage collect/clean). Since we’re nearly always writing something to the cache, this results in 10% of requests triggering the cleaning logic.

See http://framework.zend.com/manual/en/zend.cache.frontends.html.

 

The cleaning is now run via a cron job something like :

$cache_instance->clean(Zend_Cache::CLEANING_MODE_OLD);

 

Checking varnish configuration syntax

If you’ve updated your varnish server’s configuration, there doesn’t seem to be an equivalent of ‘apachectl configtest’ for it, but you can do :

varnishd -C -f /etc/varnish/default.vcl

If everything is correct, varnish will then dump out the generated configuration. Otherwise you’ll get an error message pointing you to a specific line number.