In Plesk CP under the Server > System Statistics this is what I found:

MEMORY USAGE
Total: 1003.91 MB
Used: 914.05 MB
Free: 89.86 MB
Shared: 0 B
Buffer: 47.26 MB
Cached: 618.86 MB
Usage: 91.05%

SWAP USAGE
Total: 1.95 GB
Used: 50.55 MB
Free: 1.90 GB
Usage: 2.54%

HARD DISK USAGE
Total: 113 784.95 MB
Used: 6 551.02 MB
Available: 101 453.91 MB
Capacity: 5.76%

I'm running RHE on a dual Xeon 2.8 MHz with a Gig RAM, with Plesk 7.0.4
I have about a dozen websites running, only one worth mentioning that runs vBulletin with about 2000 vistors per day. But that's about it.

Is this a real problem? If so, where should I begin to look?

I wouldn't quite worry at this point of time. The bulk of your RAM if I am not wrong is allocated to Cached and most importantly your SWAP usage is very low. Start worrying when your server feels sluggish and the SWAP climbing to a subsequent number.

bamaster - I had this same problem awhile back. And i think it may have been mr eddy that answered mine too. It was a very educational post, but basically it said that you can't think about unix/linux memory management the same way you think about windows memory management. In linux it is a good think to be using a lot of your memory - it actually increases performance. I'll see if I can find the post and post a link to it here.

I cannot claim to be an expert on Linux memory management, but I do know that eddy is absolutely right. When Linux no longer needs a block of memory, it does not simply release that memory; Instead, it keeps hold of the memory for the application that used it in case that particular block needs to be used again.

However, as soon as any other applications need more memory, Linux will immediately give up that memory that has been cached to the other application. It does increase performance because it doesn't have to constantly re-allocate memory to an application, as it keeps some aside for the future.

The best way to find out how much memory you *actually* have available is using the "free" command. On the top row is the free and used memory, but this includes all cached and buffered memory (also indicated on that row). The second row (+/- buffers/cache) shows how much memory is really being used and is available, and this is simply the figures from the top row with all buffered and cached memory subtracted from it. So:

total used free shared buffers cached
Mem: 1036048 850920 185128 0 40356 702972
-/+ buffers/cache: 107592 928456

shows that 850,920KB of memory is being used, but 40,356KB of it is buffered and 702,972KB is being cached. That means the actual memory used is used - buffers - cached, which is 850920 - 40356 - 702972, which is of course 107,592KB.

So Linux would gladly give up that extra 702MB of memory for any application that requests it, and would simply discard the buffers and/or cache that it had.

The only time to get worried is when the buffers and cached figures are very low, and used is very high, meaning almost all of the memory is actually in use and active.

Sorry if this post seems a bit long-winded