How To Calculate an Idle PC in 0.8.6


Historically, and often for new users of GNS3 this can be frustrating. However, in the last couple of releases this has become much more stable, reliable and effective. We will be covering two methods to accomplish this.


Method 1


  1. Start GNS3 (optional but recommended; run as administrator) and drag a router or two to the workspace. Select the green start button to get it running.
  2. Start a performance monitoring tool as shown below.



Note the high cpu utilization. Selecting a good Idle PC value will remedy this.


  1. Right click on a router and select Idle PC from the drop down menu. This will issue a warning; click yes to calculate a new Idle PC value.



4. The GNS3 logo will come up on the screen while it is calculating the Idle PC value.




5. Once that is complete, you will be presented with drop down value choices, as below:




6. There should be a few choices for values in the drop down. If presented with a value that has an asterisk *, choose that one.

7. Your cpu utilization should drop significantly once an Idle PC value is selected. If not, repeat the process and select a different value.




Method 2 (Preferred)


  1. As in method 1, drag a router to the workspace, start it, then select Edit—IOS images and hypervisors.
  2. In the pop-up, Highlight the image you are calculating for and select auto calculation to the right of the empty Idle PC field.





3. Select Yes.

4. GNS3 will calculate the value. Select close.







5. Check performance with your performance monitor.


This How-To was performed on a notebook with a lower end Pentium B940 @ 2 GHz with only 4 G ram. Better results can be achieved with more ram and a stronger processor. However, it is safe to say with this latest release the folks at GNS3 have made substantial improvements. A third party program such as Process Lasso can also help with performance issues.


In the screenshot below there are 16 7200 routers operational.



Note that while the cpu is doing fine, ram is taking a big hit.