Question

I own a community website of about 12.000 users (write heavy), 100 concurrent users max on a single VPS with 1Gb ram. The load rarely goes above 3 and response is quite good.

Currently a simple file cache is used to store DB query results to ease the load on the DB, but the website still can slow down over 220 concurrent users (load test).

How can I find out what the bottleneck is?

I assume that DB is fine as cache is working fine, however Disk IO could cause problem. Each pageload has about 10 includes and 10-20 querys from DB or from the file cache, plus lots of php processing.

I tried using memcache instead of the file cache, but to my suprise the load test seemed to like file cache more.

I plan to use Alternative PHP Cache, but I still don’t really understand how that cache is invalidated. I have a singe index.php that handles all requests. Will the cache store the result for each individual request? Will it clear the cache automatically if one of my includes (or query result from cache) change?

Any other suggestions for finding bottlenecks (tried xdebug)?

Thanks,
Hamlet

Answer 1

I plan to use Alternative PHP Cache,
but I still don’t really understand
how that cache is invalidated. I have
a singe index.php that handles all
requests. Will the cache store the
result for each individual request?
Will it clear the cache automatically
if one of my includes (or query result
from cache) change?

APC doesn’t cache output. It caches your compiled bytecode.

Essentially, a normal PHP request looks like this:

1. PHP files are parsed and compiled to bytecode
2. The PHP interpreter executes the bytecode

APC caches the result of the first step, so you aren’t reparsing/recompiling the same code over and over again. By default, it still stat()s your PHP files on every request, to see if the file has been modified since its cached copy was compiled — so any changes to your code will automatically invalidate the cached copy.

You can also use APC much like you’d use memcached, for storing arbitrary user data. Keep in mind, however:

1. A memcached server can serve data to multiple servers; data cached in APC can only really be used locally. Better to serve a gig of data from one memcached box to four servers, than to have 4 copies of that gig of data in APC on each individual server.
2. Memcached, in my experience, is better at handling large numbers of concurrent writes to a single cache key.
3. APC doesn’t seem to cope very well with its cache filling up. Fragmentation increases, and performance drops.

Also, beware: unless you’ve set up some sort of locking mechanism, your file-based cache is likely to become corrupt due to simultaneous writes. If you have implemented locking, that may become a bottleneck of its own. IMO, concurrency is tricky — let memcached/APC/the database deal with it.