Re: [PATCH 6/6] hugetlb: update hugetlb documentation for mempolicy based management.
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On Tue, Sep 08, 2009 at 12:51:48PM -0700, David Rientjes wrote: > On Tue, 8 Sep 2009, Mel Gorman wrote: > > > > Yes, but the caveat I'm pointing out (and is really clearly described in > > > your documentation changes here) is that existing applications, shell > > > scripts, job schedulers, whatever, which currently free all system > > > hugepages (or do so at a consistent interval down to the surplus > > > value to reclaim memory) will now leak disjoint pages since the freeing is > > > now governed by its mempolicy. > > > > While this is a possibility, it makes little sense to assume that behaviour. To > > be really bitten by the change, the policy used to allocate huge pages needs > > to be different than the policy used to free them. This would be a bit > > screwy as it would imply the job scheduler allocated pages that would > > then be unusable by the job if policies were being obeyed which makes > > very little sense. > > > > Au contraire, the hugepages= kernel parameter is not restricted to any > mempolicy. > I'm not seeing how it would be considered symmetric to compare allocation at a boot-time parameter with freeing happening at run-time within a mempolicy. It's more plausible to me that such a scenario will having the freeing thread either with no policy or the ability to run with no policy applied. > > > If the benefits of doing this > > > significantly outweigh that potential for userspace breakage, I have no > > > objection to it. I just can't say for certain that it is. > > > > > > > An application depending on memory policies to be ignored is pretty broken > > to begin with. > > > > Theoretically, yes, but not in practice. /proc/sys/vm/nr_hugepages has > always allocated and freed with disregard to current's mempolicy prior to > this patchset and it wouldn't be "broken" for an application to assume > that it will continue to do so. I don't think we're going to agree on this one. I find it very unlikely that the process doing the allocation and freeing is going to have different memory policies. > More broken is assuming that such an > application should have been written to change its mempolicy to include > all nodes that have hugepages prior to freeing because someday the kernel > would change to do mempolicy-restricted hugepage freeing. > It wouldn't have to be rewritten. At very worst, rearranged at startup to have the same policy when allocating and freeing. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe from this list: send the line "unsubscribe linux-numa" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html