how to tell when your cache is full?

[Ian]

Probably not the answer you want to hear, but memory is so cheap right now I
don't think I have ever seen someone worry about the cache being full.

We have a 20GB cache just from the spare memory on all of our servers and
our database is only 6GB in size.



On 5/24/07, Ben Hartshorne <memcache at green.hartshorne.net> wrote:
>
> Hi,
>
> I am trying to evaluate the schedule for expanding our cache.  We have
> several different types of data all getting thrown into one large
> central cache (made up of 1 instance on each of 4 machines).  Some of it
> is transient (caching 'popular' data for 5 minutes) and some of it is
> more permanent (1wk expiration time).  All data is backed by permanent
> storage for cache misses.  On cache miss, the data is repopulated into
> memcache from permanent storage.
>
> My problem - the 'stats' command has a metric 'curr_items' that reports
> the current number of items stored.  However, when a piece of data
> expires, that counter is not decremented until you issue a 'get' on the
> data and fail.
>
> In order to cache 'popular' data, all data of that type is cached,
> assuming that the popular ones will be hit and updated within the
> expiration time, and so remain in the cache, while the unpopular data
> will just expire and nobody cares.
>
> The problem with the curr_items stat is that if I ever get a cache miss
> on the transient data, I immediately fetch it from the database and
> stick it back in the cache, causing the curr_items to decrement and then
> increment again.  Data that is unpopular is stored (causing an
> increment) but never retrieved, so the curr_items never decrements.  The
> effect is a monotomically increasing number in curr_items until it tops
> out (at 2352883, though I'm not sure what's special about that number).
>
> Because of the different types of data and the changing popularity of
> data, cache hit percentage is not a good proxy for telling me when my
> cache has filled up.
>
> At the moment, I am pretty sure the cache is not full because the more
> persistent data (1wk expiry time) usually sticks around, though I don't
> have a good metric to prove that, it seems to be the case.
>
> How do I tell when my cache is full and I need to add the 5th server?  I
> tried watching the memory utilization as reported by the OS but it is
> also monotomically increasing until it tops out at the limit given to
> memcache (7GB, in this case).
>
>
> Thanks for any advice you might have,
>
> -ben
>
>
> --
> Ben Hartshorne
> email: ben at hartshorne.net
> http://ben.hartshorne.net
>
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