Memcache questions

January 6th, 2009 by webmaster

I just had memcache installed on our server. Seems to work OK, but I had it running only on our private "test" forum. When I also tried to enable it for our "live" forum, we had database errors and even had one person mention they saw the contents of our testing forum appear instead of the main forum.

Do I need to be running more than one Memcache instance if I have two forums running? IOW, could I start a secon memcached instance listening on a different port, and point the main forum to it rather than the other one?

Also, does anyone know if a utility to monitor Memcache stats? I'm hoping to find something like "top" that would work, and I did find something I thought would work (mdbtop.py), but it is written for Memcachedb and not Memcache. Even if I could print out a simple status screen, I'd be able to use that. There is a way to set it up for Cacti, which I have running, but some times I'd rather see raw numbers than data on a graph.

Finally, what are some optimum settings for Memcache? We have about 400 users online during peak hours, 3 million posts, almost 15,000 users. etc. I'm thinking of submitting a server optimization request here since a lot has changed since the last time I ran it. Our server's running well, but we're using just a vanilla my.cnf file right now. (And for George, aka eva2000, I found a neat utility that shows recommendations for tuning MySQL... :D )

Thanks Andy. To play it safe, I set it up so they not only use different prefixes, I have two instances running, listening on different ports. I have enough memory so I don't have to worry about the usage.

If all it does is hold the datastore, I don't think it would speed up the forums all that much. I was using datastore in a file previously.

Rule of the thumb:
One server? Use query_cache, serving data through TCP is way slower then directly from local memory.
Two or more servers (ie. 1 Web and 1 DB)? Use a distributed cache. memcached is the best because you can expand and scale it as one entity, compared to XCache who builds several small caches... unique for each server. So, with memcached, you will serve the same data to all workers. You keep on adding workers and the distributed cache expands without need of a fancy setup. With XCache, each server has it's own law and the cache decides what to server from local memory.

If you're seeing wrong information, chances are, your datastore is mixed up in memcache. Please see this post on how to setup datastore prefix so it doesn't get mixed up in the forums: http://www.vbulletin.com/forum/showpost.php?p=1553189&postcount=2

One thing to note is that vBulletin 3.x only uses memcache for datastore... I don't think you'll need more than 8 or 16mb of memcache memory for one forum -- my forum (9 forums & categories, default + 2 usergroups, default + additonal 1 style) uses about 75kb of datastore, for example. I believe vBulletin 4.x will try to make more use of memcache, but for now, its doesn't require tons of space.

From what I've heard/read, memcache is a bit faster than file cache because of the overheads. But I'm no expert in this field, so I can't really comment :)

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Memcache questions