I'm on a shared server, and the server load is usally around 0.5 - 3. but lately I've seen spikes as high as 25, which I understand is really high.. so my question is: On a shared host, is that number the load of the whole server, or just my site? If the whole server, is there any way I can see if it is one of my sites that is causing the load? thanks.
Probably the whole server unless it's set up as a virtual private server somehow. Easiest to tell is to wipe all your files and see if it drops to 0. But I understand that may be a bit impractical.
How are you getting this value for the load? If the server is using data mounted via NFS, it's not uncommon to see quite high loads without any real effect on performance. Do pages actually load slowly?
Load Average is something that's difficult for some people to understand. The load average is just the number of processes in the run queue, meaning processes which are ready to run. Understanding *why* there are a number of processes that are ready to run is key to knowing if it's a problem. If they're waiting to run because there are a bunch of CPU-heavy processes that are using up all of their allocated time, a high load average might be indicative that something is wrong. Ideally, a load average equal to the number of CPUs in a system means that you're pretty much fully utilizing the CPUs without any contention. So for a single CPU system a load average of 1, a quad CPU system a load average of 4... temporary spikes up to 25 on a quad CPU box might happen when stats are running or something, but if the load is consistently at that level, it may very well be an issue. Depending how the server's configured, you may be able to see the processes you own with the top command... and see what percentage of CPU you're using.