Pushing the Limits: Virtual Memory

[vballin]

I was wrong. It makes even less sense to me now. The system says it is using C for virtual memory and there are roughly 900GB free on C.

 

[jimbo45]

Hi there
Paging and Swapping are totally different things

Swapping occurs when you have multiple users on a multi user system (usually) such as a server -- when a user is "idle" (or not actively consuming resources) an "image" of the users "address space" is then written out partially or totally to disk while another inactive user who has just become active is "swapped in".

Swapping involves a lot more I/O and on a server can really kill response times if too many users are logged on for the capaability of the server to supply them.

Things like "Virtual Machines" also can count as "multi-users" on a single user system such as XP or Windows 7-- having too many of these for the amount of RAM will also cause an excessive amount of "Swapping" too.

Paging is somewhat different -- often only a small part of RAM actually needs to be permanently in memory -- the OS maximises its use of memory by writing out parts of memory that haven't been used for a while to its paging file.

In OS design the algorithms are complex but often follow the "LRU" model -- the LEAST RECENTLY USED - which keeps track of page usage over a period of time so pages that are required will be in storage when needed.

Instruction "pre-fetching" also ensures data is available when needed. The I/O that occurs with paging on a not over loaded system is done when the system is doing other things like user I/O (writing user data to disk etc.) so isn't normally perceptable to the user.

However if the system gets too overloaded -- doing too many tasks, running too many apps etc then a condition known as "Thrashing" occurs -- this is when the system has to keep reading and writing paging data. In conjunction with excessive swapping the machine will appear to be STONE DEAD-- the disk light will usually be solid on.

There is LITTLE if any advantage to be gained in switching off the page mechanism. If at any time your system needs access to a page file and you haven't got one it will just stop immediately without any diagnostics whatesoever.

If you are that short of RAM --buy some more it's cheap enough now.

If you can also place the page file(s) on FAST not too often used disks as well. An SSD would be ideal for this BTW.

Cheers
jimbo