Messages in this thread | | | Date | Fri, 4 Nov 2005 07:38:20 +0100 | From | Ingo Molnar <> | Subject | Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19 |
| |
* 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |