Top reports swap at 100%, free memory available

If you run out of physical memory, you use virtual memory, which stores the data in memory on disk. Reading from disk is several orders of magnitude slower than reading from memory, so this slows everything way down. (Exchanging data between real memory and virtual memory is "swapping". The space on disk is "swap space".)

If your app is "using swap", then you either need to use less memory or buy more RAM.

(Swap is useful because applications that aren't being used can be stored on disk until they are used. Then they can be "paged in" and run normally again. While it is not in memory, though, the OS can use that memory for something else, like disk cache. So it's a very useful feature, but if you don't have enough physical memory to run your program, you definitely need more memory. Fortunately, memory is really really cheap these days.)

Virtual memory isn't even necessarily memory. For example, if a process memory-maps a large file, the file is actually stored on disk, but it still takes up "address space" in the process.

Address space (ie. virtual memory in the process list) doesn't cost anything; it's not real. What's real is the RSS (RES) column, which is resident memory. That's how much of your actual memory a process is occupying.

But even that isn't the whole answer. If a process calls fork(), it splits into two parts, and both of them initially share all their RSS. So even if RSS was initially 1 GB, the result after forking would be two processes, each with an RSS of 1 GB, but you'd still only be using 1 GB of memory.

Confused yet? Here's what you really need to know: use the free command and check the results before and after starting your program (on the +/- buffers/cache line). That difference is how much new memory your newly-started program used.