Using memcached for url rewrite ~ evaluation

[motorcity]

Hello,

I started using memcached about 2 1/2 months ago. It's being used to do url rewrite and cache the results.

I'm using http://pecl.php.net/package/memcache to monitor what's going on.

These are pretty typical results;

hits = 40.6%

misses = 59.4%

free = 32.5%

used = 67.5%

I'm on a pretty good vpn with a huge database and moderate to low traffic.

Any thoughts on this?

 

[gizmola]

I"m not sure what type of feedback you're looking for.  Usually people use memcache to cache database query results in memory.  Url's sound like a small set of data, so it's hard to understand why the cache hit rate wouldn't be higher over time.

[motorcity]

My thinking right along has been expecting much better hit to miss ratio.

I'm currently reporting

request rate (hits. misses) = .21 cache requests/second

hit rate = .09 cache requests/second

miss rate= .13 cache requests/second

set rate = .13 cache requests/second

 

Cache used = 43.2MBytes

Cache size = 64MBytes

 

Do I increase the cache size? Or something else?

[gizmola]

What is the eviction rate?

[motorcity]

Using the program noted above clicking on "variables" it lists by slab;

Item count, Age, Evicted

I find 21 slabs which vary from item counts of 1 to as many as 12000.

Also quite random is the age, anywhere from a day to multiple weeks. (current uptime is 2 weeks).

Every slab has "No" for evicted.

 

Shorter answer, I don't find a rate for eviction.

 

 

[gizmola]

If items aren't being evicted, then the cache is not filled.  So the answer seems to be that whatever routines are adding items to the cache, and checking the cache are not finding a match in the cache 60% of the time.

[motorcity]

That sounds so logical that it almost hurts.

Thanks. ~for the confirmation.

I'm going back to the author of this code. Been meaning to for awhile.

I'll post back here if anyone is interested.

Obviously memcache is a fairly rare bird, and issues with it are even more rare.

[gizmola]

The rank and file php programmer who only does small sites might not use memcache, but for larger sites, it's heavily used.  It's also extremely simple, consisting aside from connections of literally two functions... set() and get().  Set is pretty much set('key', 'value') where typically a key might be something like:  '/foo/bar/something/'.  In other words the structure of the key can be whatever you want, but it's vital that the routine that tries the get() has a matching key, or you may end up looking for cache entries that are there, but never get found because of some problem in the way the key was generated.  You could probably do some analysis yourself and look for the memcache routines in the code you have.  Good luck.