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    SubjectRe: [PATCH 1/1] Memory usage limit notification addition to memcg
    * menage@google.com <menage@google.com> [2009-07-13 15:15:45]:

    > On Tue, Jul 7, 2009 at 5:56 PM, KAMEZAWA
    > Hiroyuki<kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > >
    > > I know people likes to wait for file descriptor to get notification in these days.
    > > Can't we have "event" file descriptor in cgroup layer and make it reusable for
    > > other purposes ?
    >
    > I agree - rather than having to add a separate "wait for value to
    > cross X threshold" file for each numeric usage value that people might
    > be concerned about, it would be better to have a generic way to do it
    > for any file. Given that this is a userspace API, it would be better
    > to work out at least the generic API first, even if the initial
    > implementation isn't generic.
    >
    > Properties that it should support include:
    >
    > - notification when a value crosses above or below a given threshold
    > (which would include binary cases such as OOM notification where the
    > value cross from "not-OOM" to "OOM"
    >
    > - independent thresholds for different waiters
    >
    > - epoll support (by using eventfd?)
    >
    > - automatic wakeup when a cgroup is removed
    >
    > - maybe optional wakeup when a thread attach occurs?
    >

    Please don't forget netlink, we've already got a genetlink socket for
    cgroups in the form og cgroupstats.

    > - not require more than read permissions on the file containing the
    > value being monitored
    >
    > I guess there are a few possible ways this could be exposed to userspace:
    >
    > 1) new ioctl on cgroups files. simple but probably not popular
    >
    > 2) new system call. maybe the cleanest, but involves changing every
    > arch and is hard to script
    >
    > 3) new per-cgroup file to control these e.g:
    > - create an eventfd
    > - open the control file to be monitored
    > - write the "<event_fd>, <control_fd> <threshold> to
    > cgroup.event_control to link them together
    > flexible and scriptable but maybe a clumsy interface in general
    >
    > Paul

    --
    Balbir


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