how to disable virtual memory of mac os

You do not want to disable VM. Use Activity Monitor to keep track of your computer's memory usage. Most likely you are misunderstanding what you see or think is going on. The amount of data swapped may have nothing to do with how much RAM you have.

About OS X Memory Management and Usage

Reading system memory usage in Activity Monitor
Memory Management in Mac OS X
Performance Guidelines- Memory Management in Mac OS X
A detailed look at memory usage in OS X

Understanding top output in the Terminal

The amount of available RAM for applications is the sum of Free RAM and Inactive RAM. This will change as applications are opened and closed or change from active to inactive status. The Swap figure represents an estimate of the total amount of swap space required for VM if used, but does not necessarily indicate the actual size of the existing swap file. If you are really in need of more RAM that would be indicated by how frequently the system uses VM. If you open the Terminal and run the top command at the prompt you will find information reported on Pageins () and Pageouts (). Pageouts () is the important figure. If the value in the parentheses is 0 (zero) then OS X is not making instantaneous use of VM which means you have adequate physical RAM for the system with the applications you have loaded. If the figure in parentheses is running positive and your hard drive is constantly being used (thrashing) then you need more physical RAM.

Firefox is just a slower browser, especially for launching & shutting down. In my experience, that's just the way the browser is.

Could you elaborate on what you mean by "cannot reboot or shutdown after one or two weeks"?

Does the computer hang during the shutdown, or is it just slow to shutdown? If the latter, how slow?

~Lyssa

But I'm pretty sure virtual memory is used. For example, after running firefox for a long time, it takes me a while to close it as some of its data is swapped to the disk and somehow firefox needs to access it.

You are confusing virtual memory with paging and swapping. Virtual memory is an essential part of the operating system. It gives the OS the ability to use any random 4K chunks (pages) of memory with any program. That is to say, VM means the OS can grab a bunch of available pages and create a range of virtual addresses that a program can use. Before VM, it was extremely difficult to manage physical memory.

Paging and swapping are a way of pretending you have more physical memory than is physically installed in the system. Before virtual memory there was just swapping of an entire program out of memory. With virtual memory it is possible to just page out parts of a program.

It is the paging and swapping that is annoying you, not virtual memory. An operating system without virtual memory is called DOS 🙂

Also I have a problem that my machine cannot be reboot and shutdown after running one or two weeks. I think it has something to do with virtual memory. So I want to try disabling virtual memory.

Neither virtual memory nor paging would affect your ability to shutdown or reboot.

You have something else going on.

Have you by any chance installed some intrusive 3rd party software, such as an anti-virus utility?

Can't be done, it is an integral part of the operating system.

As previously suggested, it is something in your system that is causing the problems.

What is the capacity of your hard drive and how much space is free?

As noted above, the page-swapping is an integral function of the Mac OS. But it in order to do it successfully, the OS needs free space on the hard drive. Keeping less than 15% space free at any time can make the OS run slow and potentially lead to corruption and damage in your disk directory.