Re: [kvm-devel] performance with guests running 2.4 kernels (specifically RHEL3)

[Andrea Arcangeli <andrea@qumranet.com>]

On Wed, May 28, 2008 at 05:57:21PM +0300, Avi Kivity wrote:
> What about CONFIG_HIGHPTE?

Ah yes sorry! Official 2.4 has no highpte capability but surely RH
backported highpte to 2.4 so that would explain the cpu time spent in
kswapd _guest_ context.

If highpte is the problem and you've troubles reproducing, I recommend
running some dozen of those in background on the 2.4 VM that has the
ZERO_PAGE support immediately after boot. This will ensure there will
be tons of pagetables in highmemory. This should allocate purely
pagetables and allow for a worst case of highpte.  Check with
/proc/meminfo that the pagetable number goes up of a few megabytes for
each one of those tasks. Then just try to allocate some real ram (not
zeropage) and if there's a problem with highptes it should be possible
to reproduce it with so many highptes allocated in the system. Guest
VM size should be 2G, you don't really need more than 2G to reproduce
by using the below ZERO_PAGE trick.

#include <unistd.h>
#include <stdlib.h>
#include <string.h>

int main()
{
	char *p1, *p2;
	p1 = malloc(512*1024*1024);
	p2 = malloc(512*1024*1024);
	if (memcmp(p1, p2, 512*1024*1024))
		*(char *)0 = 0;
	pause();

	return 0;
}