Register window thrashing

[jermar]

The following snippet of userspace pseudo code demonstrates situation in which the kernel might repeatedly spill and fill several windows from the OTHERWIN area:

for (i = 0; i < 10000; i++) {

syscall();

}

If the syscall handler spills N userspace windows, all N windows will be restored from the userspace window buffer on return from the syscall in each iteration.

This sub-optimal behaviour was first pointed out by Mikulas Patocka.