Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19

[Martin J. Bligh]

>> just to make sure i didnt get it wrong, wouldnt we get most of the 
>> benefits Andy is seeking by having a: boot-time option which sets aside 
>> a "hugetlb zone", with an additional sysctl to grow (or shrink) the pool 
>> - with the growing happening on a best-effort basis, without guarantees?
> 
> Boot-time option to set the hugetlb zone, yes.
> 
> Grow-or-shrink, probably not. Not in practice after bootup on any machine 
> that is less than idle.
> 
> The zones have to be pretty big to make any sense. You don't just grow 
> them or shrink them - they'd be on the order of tens of megabytes to 
> gigabytes. In other words, sized big enough that you will _not_ be able to 
> create them on demand, except perhaps right after boot.
> 
> Growing these things later simply isn't reasonable. I can pretty much 
> guarantee that any kernel I maintain will never have dynamic kernel 
> pointers: when some memory has been allocated with kmalloc() (or 
> equivalent routines - pretty much _any_ kernel allocation), it stays put. 
> Which means that if there is a _single_ kernel alloc in such a zone, it 
> won't ever be then usable for hugetlb stuff.
> 
> And I don't want excessive complexity. We can have things like "turn off 
> kernel allocations from this zone", and then wait a day or two, and hope 
> that there aren't long-term allocs. It might even work occasionally. But 
> the fact is, a number of kernel allocations _are_ long-term (superblocks, 
> root dentries, "struct thread_struct" for long-running user daemons), and 
> it's simply not going to work well in practice unless you have set aside 
> the "no kernel alloc" zone pretty early on.

Exactly. But that's what all the anti-fragmentation stuff was about - trying
to pack unfreeable stuff together. 

I don't think anyone is proposing dynamic kernel pointers inside Linux,
except in that we could possibly change the P-V mapping underneath from
the hypervisor, so that the phys address would change, but you wouldn't
see it. Trouble is, that's mostly done on a larger-than-page size
granularity, so we need SOME larger chunk to switch out (preferably at
least a large-paged size, so we can continue to use large TLB entries for
the kernel mapping).

However, the statically sized option is hugely problematic too.

M.
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