How swap space works in this example?

Question

Say I have total RAM memory of 1GB , HD of 10 GB with 250 MB swap space

Say I have two files(f1 and f2) each of 500 MB opened on my my laptop. Now both files are in memory and have consumed 1 GB of RAM. Currently f2 is in front of me. Now I start writing in f2 with 100 MB of characters.

Now will OS will swap space here and write 100 MB from f1 to hard disk . When I will switch f1 then OS will bring back those 100 MB of space and put 100 MB of f2 in hard disk.

I there will be lot of other intricacies but just wanted to understand high level fundamentals of how swap space works. Is my understanding correct here ?

Answer 1

Forget about having large files. Just say you have an object of 500 MB, another object of 500 MB, and you want to add 100 MB to the second object. And we ignore the fact that OS, code, and other applications also need space.

The OS will not care about the objects. It will just pick 100 MB of RAM that haven't been used for a while, write them to the swap space, and have 100 MB available for your new additions.

If you examine both objects, since they are 1100 MB and only 1000 MB are in RAM, you will always run into situations where you try to read memory not in RAM. At that point, some other memory is written to the swap file, and the missing memory loaded.

Depending on your algorithms, you may run into a situation where RAM is always written to the swap file just before it is accessed and is read again, which would be the worst case. If that happens, you either suffer from very inefficient execution, or you have to change your algorithms.