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SubjectRe: [PATCH -V6 07/14] memcg: Add HugeTLB extension
On Thu, 24 May 2012 14:52:26 -0700 (PDT)
David Rientjes <rientjes@google.com> wrote:

> On Mon, 16 Apr 2012, Aneesh Kumar K.V wrote:
>
> > This patch implements a memcg extension that allows us to control HugeTLB
> > allocations via memory controller. The extension allows to limit the
> > HugeTLB usage per control group and enforces the controller limit during
> > page fault. Since HugeTLB doesn't support page reclaim, enforcing the limit
> > at page fault time implies that, the application will get SIGBUS signal if it
> > tries to access HugeTLB pages beyond its limit. This requires the application
> > to know beforehand how much HugeTLB pages it would require for its use.
> >
> > The charge/uncharge calls will be added to HugeTLB code in later patch.
> > Support for memcg removal will be added in later patches.
> >
>
> Again, I disagree with this approach because it's adding the functionality
> to memcg when it's unnecessary; it would be a complete legitimate usecase
> to want to limit the number of globally available hugepages to a set of
> tasks without incurring the per-page tracking from memcg.
>
> This can be implemented as a seperate cgroup and as we move to a single
> hierarchy, you lose no functionality if you mount both cgroups from what
> is done here.
>
> It would be much cleaner in terms of
>
> - build: not requiring ifdefs and dependencies on CONFIG_HUGETLB_PAGE,
> which is a prerequisite for this functionality and is not for
> CONFIG_CGROUP_MEM_RES_CTLR,
>
> - code: seperating hugetlb bits out from memcg bits to avoid growing
> mm/memcontrol.c beyond its current 5650 lines, and
>
> - performance: not incurring any overhead of enabling memcg for per-
> page tracking that is unnecessary if users only want to limit hugetlb
> pages.
>
> Kmem accounting and swap accounting is really a seperate topic and makes
> sense to be incorporated directly into memcg because their usage is a
> single number, the same is not true for hugetlb pages where charging one
> 1GB page is not the same as charging 512 2M pages. And we have no
> usecases for wanting to track kmem or swap only without user page
> tracking, what would be the point?
>
> There's a reason we don't enable CONFIG_CGROUP_MEM_RES_CTLR in the
> defconfig, we don't want the extra 1% metadata overhead of enabling it and
> the potential performance regression from doing per-page tracking if we
> only want to limit a global resource (hugetlb pages) to a set of tasks.
>
> So please consider seperating this functionality out into its own cgroup,
> there's no reason not to do it and it would benefit hugetlb users who
> don't want to incur the disadvantages of enabling memcg entirely.

These arguments look pretty strong to me. But poorly timed :(


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