Question

Swap space is some space in virtual memory. but why it called as swap space?

Answer 1

swap space is not part of virtual memory. First of all virtual memory is "virtual"- it is not physically existing. It is like a view through which CPU sees actual physical memory. 

Now virtual to physical memory mapping can be done in many ways. Swap space is not a strict necessity. If it is there, it works as an extension to RAM- but still page access is slower whenever a page in swap is accessed. 

https://www.centos.org/docs/5/html/5.2/Deployment_Guide/s1-swap-what-is.html

Answer 2

Swap space is the space on the disk reserved for the full virtual memory space of a process.

When Operating System creates a process, it also creates (or reserves) a space on disk for all the pages of that process.

Each process contains its own swap space in the disk.

It is called swap space because it is the space(or place) in the disk from where the pages are exchanged(swapped) with main memory, for any process.

Comments

Are you sure of this? This is from my system now.

$ free -m
             total       used       free     shared    buffers     cached
Mem:          3934       2662       1271         79        107        736
-/+ buffers/cache:       1819       2114
Swap:        19071          0      19071

---

I guess, since 1271 MB physical memory is still available thus there is no need for swapping , so it is showing 0MB usage. I read it from Hennessy & Patterson sir.

---

Well yes, it might be mentioning "assuming physical memory space is not large enough". And virtual address space would be meaning those which are assigned to the process.

---

Here is the transcript of that paragraph from the book sir :

"Because we do not know ahead of time when a page in memory will be chosen
to be replaced, the operating system usually creates the space on disk for all the
pages of a process when it creates the process. This disk space is called the swap
space. At that time, it also creates a data structure to record where each virtual
page is stored on disk. This data structure may be part of the page table or may be
an auxiliary data structure indexed in the same way as the page table.

The operating system also creates a data structure that tracks which processes
and which virtual addresses use each physical page. When a page fault occurs, if all
the pages in main memory are in use, the operating system must choose a page to
replace......."