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    SubjectRe: [ckrm-tech] [RFC][PATCH 5/8] RSS controller task migration support
    Patrick.Le-Dot wrote:
    >> ...
    >> For implementing guarantees, we can use limits. Please see
    > Nack.
    > This seems to be correct for resources like cpu, disk or network
    > bandwidth but not for the memory just because nobody in this wiki
    > speaks about the kswapd and page reclaim (but it's true that a such
    > demon does not exist for cpu, disk or... then the problem is more
    > simple).
    > For a customer the main reason to use guarantee is to be sure that
    > some pages of a job remain in memory when the system is low on free
    > memory. This should be true even for a job in group/container A with
    > a smooth activity compared to a group/container B with a set of jobs
    > using memory more agressively...

    I am not against guarantees, but

    Consider the following scenario, let's say we implement guarantees

    1. If we account for kernel resources, how do you provide guarantees
    when you have non-reclaimable resources?
    2. If a customer runs a system with swap turned off (which is quite
    common), then anonymous memory becomes irreclaimable. If a group
    takes more than it's fair share (exceeds its guarantee), you
    have scenario similar to 1 above.

    > What happens if we use limits to implement guarantees ?
    >>> ...
    >>> The idea of getting a guarantee is simple:
    >>> if any group gi requires a Gi units of resource from R units available
    >>> then limiting all the rest groups with R - Gi units provides a desired
    >>> guarantee
    > If the limit is a "hard limit" then we have implemented reservation and
    > this is too strict.
    > If the limit is a "soft limit" then group/container B is autorized to
    > use more than the limit and nothing is guaranteed for group/container A...
    > Patrick

    Yes, but it is better than failing to meet a guarantee (if guarantees are
    desired :))


    Balbir Singh,
    Linux Technology Center,
    IBM Software Labs
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