HEADS UP! KSE needs more attention

Thomas Moestl tmm at freebsd.org
Mon Jun 7 14:48:09 GMT 2004

On Sun, 2004/06/06 at 14:59:21 -0700, Kris Kennaway wrote:
> On Sun, Jun 06, 2004 at 03:49:13PM -0600, Scott Long wrote:
> > amd64 is approaching critical mass for tier-1.  There are a number of
> > developers that own amd64 hardware now, and a number of users who are
> > asking about it on the mailing lists.  Peter is finishing up the last
> > blocking item for it (kld's) not including the observed KSE problems.
> > It's very close and I _will_ hold up the release for it to get done.
> > amd64 is the future of commodity computing and we aren't going to
> > ignore it for 5-STABLE.
> amd64 has a bug with swapping - when something begins to access swap,
> the entire system becomes almost entirely unresponsive (e.g. no mouse
> response for up to 10 seconds) until it stops.  Peter has some ideas
> about it, but it's a serious enough bug that it forced me to stop
> using amd64 as my desktop machine (hello, kde!).

Hmmm, I have encountered a similar problem on sparc64 once; the
reason was that vm_pageout_map_deactivate_pages() calls
pmap_remove() for the range from the start to the end of the
process's vm_map when a process is swapped out. Start and end
are VM_MIN_ADDRESS and VM_MAXUSER_ADDRESS respectively, and on
64-bit architectures, that range is very large (128TB on ia64
if I'm not mistaken), so the iteration in pmap_remove() must
be carefully designed to make as large steps as possible to
avoid long run times (or to not iterate over the range at all
if it becomes too large, which we did on sparc64).

It seems that the amd64 version of pmap_remove() will essentially
always iterate in 2MB (level 2 page table) steps, regardless of
whether there is mapping for the respective level 2 table in the
table levels above; that means that in the previously mentioned case,
the outer loop will usually run for about 67 million iterations (the
resident count guard may not be of much use here if a stack page is
left at the very end of the address space). Since there are a few
memory accesses needed in each iterations, that may already be the
cause of such a delay.

I have no hardware to test this, so all of the above is just a wild-
assed guess; but maybe it is of use (and sorry for the spam if it
is not).

	- Thomas

Thomas Moestl	<t.moestl at tu-bs.de>	http://www.tu-bs.de/~y0015675/
		<tmm at FreeBSD.org>	http://people.FreeBSD.org/~tmm/
"I realized that the purpose of writing is to inflate weak ideas,
 obscure poor reasoning, and inhibit clarity."	-- Calvin and Hobbes

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