[Qemu-devel] Guest bootup time varies with simulated memory size

[Adnan Khaleel]

In trying out various simulated memory sized in Qemu, there is a noticeable increase in guest bootup times with more memory. My metric of measuring how long linux takes to boot is probably not the most consistent but here is what I've observed.

In Qemu 0.12.5

512m ~ 4 mins

2048m ~ 7 mins

In the latest version of Qemu 0.14.0-rc0, things have improved

512m ~ 3 mins 30 secs

2048m ~ 4 mins 45 secs

As you can see, boot up time increases with more memory. My host system itself has 16G of memory so swapping is not the issue. 

Its quite likely that the guest linux kernel maybe doing something with this extra memory but I haven't looked into possible causes. I just wanted to see if the developers would know of an explanation as to whats happening as it would seem that adding more memory to the guest should improve performance. 

I will run some measurements after boot to see if there is any variation in overall simulation speed or if its just restricted to bootup.

- AK