Re: netbsd-6: pagedaemon freeze when low on memory

[Richard Hansen <rhansen%bbn.com@localhost>]

On 2013-03-04 16:10, Lars Heidieker wrote:
> On 2013-03-04 20:43, Richard Hansen wrote:
>> I believe I have found a bug in the pagedaemon (uvm_pageout() in
>> src/sys/uvm/uvm_pdaemon.c) that causes the system to freeze when
>> the kmem_arena runs low (<10% free):
[snip]
> 
> if you increase kern.maxvnodes a lot you need to increase the kmem
> allowed in order to accommodate all the vnodes.
> Try recompiling a kernel with NKMEMPAGES_MAX option set higher then
> the default (360mb on i386) eg 450mb.

Ah, that's good to know, thanks!  I'll turn up NKMEMPAGES_MAX.

But, isn't this a bug regardless of the values of kern.maxvnodes and
NKMEMPAGES_MAX?  Shouldn't the pagedaemon not freeze under memory pressure?

Some background:  Turning up kern.maxvnodes is just a convenient way to
demonstrate the freeze on an unmodified NetBSD kernel.  I first
encountered this freeze in a custom kernel that kmem_alloc()s a lot more
memory than an unmodified NetBSD kernel.  I'll turn up NKMEMPAGES_MAX in
the custom kernel, but it'd still be desirable if the kernel handled
out-of-memory conditions more gracefully.

If an iteration in the pagedaemon doesn't free anything, perhaps
kmem_alloc(size, KM_SLEEP) should return NULL?  Or maybe the kernel
should panic()?  Maybe the pagedaemon should ask other subsystems (e.g.,
vnode cache) to free some memory?

Thanks,
Richard