[Edit: Some useful background info can be found at topic6040.html (summary) or at http://rednectar.net/2013/02/24/dynamip ... d-finally/ (full Article)]

One of the most difficult problems new users have to come to grips with when starting out with GNS3 is the concept of an Idle-PC.

Get it right, and you will have a great GNS3 experience. Try to ignore it, and you will be forever miserable.

Here is my tedious and time consuming method of finding a good idle-pc value.

REPEAT FOR EVERY DIFFERENT IOS IMAGE YOU WISH TO RUN ON YOUR SYSTEM:

Step 1:
Windows: Open the windows task manager and sort by %CPU
Linux: Open a console window and enter the command top
Mac OS X: Open a terminal window and enter the command top -o cpu

Keep this window visible for the entire process

Step 2:
In GNS3, start a new topology with 1 router ONLY
Start the router
[edit: 2012.02.01 After reading instant000's detailed response on p2, the following step may not be a good idea, or at least it may be better putting theses lines into startup.txt instead, then move to step 3 at this point]
Open the console. When when the router is fully up, configure the following:


NOTE: While writing this post, I observed this step alone dropped the CPU usage from 98% to 1% on a Windows 7 install (running in a VM on OS X)

Step 3:
Back at your task manager or console window:
Take note of the amount of CPU being chewed by dynamips

Step 4:
In GNS3, right-click on the router and choose idle-pc

If NO values appear marked with *, try again

When you find a value marked with a *, WRITE IT DOWN
If MULTIPLE values appear with *, WRITE THEM ALL DOWN (in a column)
... then choose one of them

Step 5:

Check the CPU utilisation for dynamips in the task manager or console window.
Estimate the average CPU consumption for dynamips over say 15-20 seconds
WRITE IT DOWN next to the Idle-pc value you wrote down in step 4

If you have an idle-pc vlaue that shows less than 10-15% CPU, you may want to go to step 6,
Else, go back to step 4

Step 6:

Now that you have a good idle-pc, you need to know how to use it well.

Firstly, check that GNS3 has recorded you best value against the image you are using

That's in:
Edit->IOS Images and Hypervisors
Select the image you are using and check the IDLE PCs value

Now GNS3 will automatically use that value in any NEW topologies you create.

Step 7:

If you have any saved topologies (ie .net files) that have used this IOS, open the .net file and replace the idle-pc value found there with your new "good" idle-pc value.


Step 8:

Record the results you found (IOS & idle-pc values) in a spreadsheet and keep it!

Now go back to Step 1 and repeat for the next IOS image you use

General Tips for keeping CPU under control
Always use the same image for ALL routers in your topology if possible.
This means using the same router model as well. If this is not possible, use the same image for all routers of the same model.
ALWAYS set the exec-timeout 0 under line con 0
In GNS3 0.7.3 and later, you can set a base config for each IOS image under Edit->IOS Images and Hypervisors. Make sure the base config has the exec-timeout 0 under line con 0

UPDATE 2011-10-22
I've added a second article that you should read after this: Investigating Idlepc idlemax to minimise CPU usage

_________________
RedNectar
http://rednectar.net
@rednectarchris
GNS3 WorkBench-a VMware image of Ubuntu with GNS3 and VPCS installed and a collection of exercises/labs

About that, it will be great to have a apply button for idle-pc, like on "Change Symbol"

It's a bit late for the next release but it is on the TODO list ;)

_________________
Jeremy, GNS3 Programmer & Benevolent Dictator for Life.

You saw that I insist over and over and all over I find a new opportunity to remind you about the benefit of that "Apply" button :))

You really want this button ;) hehe

_________________
Jeremy, GNS3 Programmer & Benevolent Dictator for Life.

No (just kidding), I'm just the voice of the community ;))

you do in this process

in the Topology area place a router and set the types of the slots as you wish and start the router and wait boot process.If you want to increase the system performance use the cisco image unpacker 0.1 binary for windows.

once you placed the router in the topology area and the router will be started right click one the router & click the Idle pc option and wait for some time you will get the '*' marked idle pc value.
Then you select that value.

once you will get the value of the particular model of the router. Then you go to menu of the Edit-->IOS images and hypervisiors you see the idle pc value of particular model of the rouer,select that copy and paste another router's idle pc value of the same model of the router.


Example:

7200 router XXXXXX value

this model router idle pc's values use many routes in the same topology

Thanks nectar...this really helped me out to get through my 100% full util PC each time i run GNS3.
appreciate it very much.

I've tried multiple times to acquire an IdlePC value which has an asterix attached to it, but I keep getting just one value. (0xa000 [1000])

Do I have to depend on GNS3 to calculate the value for me each time, or is there any provision to enter this value manually?

After a couple of tries I am able to find a good Idlepc value and reduce cpu utilization to around 5%. The problem is that if I then stop/restart the image the utilization will again go up to 100% and only settle to around 50%. It never goes back down to 5%.

I can rerun the process of finding/selecting a good idlepc value, but again each time it only works for 1 session.

Anyone else have this issue?