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Re: low memory problem - pool calls vmem calls uvm calls pool etc.

On 04/12/13 01:07, Lars Heidieker wrote:
On 12/03/2013 12:03 PM, David Sainty wrote:
On 03/12/13 23:30, Lars Heidieker wrote:
On 12/03/2013 11:09 AM, David Sainty wrote:
I believe I'm seeing pretty much the same problem, on a stock NetBSD 6.1
amd64 Xen DOMU.  Usually happens during /etc/daily for some reason, but
sadly not reliably, nor when I run /etc/daily in a tight loop.  But it
does seem to happen every few days, and I can't drop to ddb when it

pool_cache_put_slow() has to allocate some administrative storage, via
vmem_alloc(), then uvm_km_kmem_alloc().  In uvm_km_kmem_alloc() it calls
first vmem_alloc() and then uvm_pagealloc().  If uvm_pagealloc() fails,
it calls vmem_free() on the allocation from vmem_alloc() and returns

The problem is that vmem_free() attempts to re-pool the allocation (if
QCACHE is defined), which starts the whole process again.

The patch is not right, it leads to inconsistencies.
It's fixed on the netbsd-6 branch already.

vmem_free(kmem_va_arena, va, size);
should be
vmem_free(vm, va, size);

Hmmm, yes, true.  But do you think that alone will solve the low memory
triggered problem?

At a glance they seem like independent problems to me...

The recursion happened due to freeing to the wrong arena.
Take a look at the pool_allocator_meta and how vmem recurses. It's one
vmem arena (kmem_va_arena) using quantum caches and one (actually two
kmem_meta_arena(s) stacked on top of kmem_meta_arena) for meta
allocations eg pool_allocator_meta those arenas are stacked up on one
arena without quatum caching (kmem_arena).
So "normal" allocations utilized this cache, but no pool_caches are
involved for "meta" allocations.

So it might recurse once while switching to allocate via the
kmem_meta_arena and from there on no pool_caches are involved.

I see, yes, because qcache_max is zero for those arenas, so vmem_free() is safe. Excellent! Thanks for the explanation.

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