CPU spikes when concurrently using an XP VM and its complete clone on Debian

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Host Linux sandy 3.2.0-4-amd64 #1 SMP Debian 3.2.65-1+deb7u2 x86_64 GNU/Linux Virtualbox 4.3.26-98988 (happened also with 4.2.28-97679)

I have an XP virtual machine with a test installation of oracle rdbms. To do some speed optimizations on some plsql code and compare results, I cloned it (complete) and changed name, mac and ip.

When one vm is running and idling and I launch the other one, both the machines go to 200% cpu consumption (quad core host). After a (long) while the second machine boot completes, and they both go back to idling.

When I log in on the second vm, the first starts spinning again. After a while the login is completed and both go back to idle.

Whatever I try to do on one of the two, the other one immediately starts using all the available cpu, 50/50 with the first one.

Both the original vm and the clone run perfectly on their own, if the other one is shut down or even paused. If I run another unrelated XP guest with one of the two, the issue does not appear.

I'm using vboxheadless and connecting with Remote Display, but this can be triggered also when not connected to the desktop, e.g. launching a plsql cpu consuming loop via sqldeveloper (not i/o bound operations).

Typical output of top when the issue is occurring

top - 12:59:39 up 7 days, 15:02, 1 user, load average: 3,34, 5,32, 5,05 Tasks: 192 total, 2 running, 190 sleeping, 0 stopped, 0 zombie %Cpu(s): 1,5 us, 98,4 sy, 0,0 ni, 0,1 id, 0,0 wa, 0,0 hi, 0,0 si, 0,0 st KiB Mem: 12280376 total, 12063944 used, 216432 free, 68776 buffers KiB Swap: 7771064 total, 0 used, 7771064 free, 2834160 cache

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND

14613 vbox 20 0 4003m 3,6g 3,6g S 198,0 30,9 36:11.00 VBoxHeadless

9930 vbox 20 0 3981m 3,6g 3,6g S 196,0 30,9 40:37.10 VBoxHeadless