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

[Ingo Molnar <mingo@elte.hu>]

* Paul Jackson <pj@sgi.com> wrote:

> Linus wrote:
> > Maybe you'd be willing on compromising by using a few kernel boot-time 
> > command line options for your not-very-common load.
> 
> If we were only a few options away from running Andy's varying load 
> mix with something close to ideal performance, we'd be in fat city, 
> and Andy would never have been driven to write that rant.
> 
> There's more to it than that, but it is not as impossible as a battery 
> with the efficiencies you (and the rest of us) dream of.

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?

i have implemented precisely such a scheme for 'bigpages' years ago, and 
it worked reasonably well. (i was lazy and didnt implement it as a 
resizable zone, but as a list of large pages taken straight off the 
buddy allocator. This made dynamic resizing really easy and i didnt have 
to muck with the buddy and mem_map[] data structures that zone-resizing 
forces us to do. It had the disadvantage of those pages skewing the 
memory balance of the affected zone.)

my quick solution was good enough that on a test-system i could resize 
the pool across Oracle test-runs, when the box was otherwise quiet. I'd 
expect a well-controlled HPC system to be equally resizable.

what we cannot offer is a guarantee to be able to grow the pool. Hence 
the /proc mechanism would be called:

	/proc/sys/vm/try_to_grow_hugemem_pool

to clearly stress the 'might easily fail' restriction. But if userspace 
is well-behaved on Andy's systems (which it seems to be), then in 
practice it should be resizable. On a generic system, only the boot-time 
option is guaranteed to allocate as much RAM as possible. And once this 
functionality has been clearly communicated and separated, the 'try to 
alloc a large page' thing could become more agressive: it could attempt 
to construct large pages if it can.

i dont think we object to such a capability, as long as the restrictions 
are clearly communicated. (and no, that doesnt mean some obscure 
Documentation/ entry - the restrictions have to be obvious from the 
primary way of usage. I.e. no /proc/sys/vm/hugemem_pool_size thing where 
growing could fail.)

	Ingo

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