• David Rientjes's avatar
    oom: badness heuristic rewrite · a63d83f4
    David Rientjes authored
    This a complete rewrite of the oom killer's badness() heuristic which is
    used to determine which task to kill in oom conditions.  The goal is to
    make it as simple and predictable as possible so the results are better
    understood and we end up killing the task which will lead to the most
    memory freeing while still respecting the fine-tuning from userspace.
    
    Instead of basing the heuristic on mm->total_vm for each task, the task's
    rss and swap space is used instead.  This is a better indication of the
    amount of memory that will be freeable if the oom killed task is chosen
    and subsequently exits.  This helps specifically in cases where KDE or
    GNOME is chosen for oom kill on desktop systems instead of a memory
    hogging task.
    
    The baseline for the heuristic is a proportion of memory that each task is
    currently using in memory plus swap compared to the amount of "allowable"
    memory.  "Allowable," in this sense, means the system-wide resources for
    unconstrained oom conditions, the set of mempolicy nodes, the mems
    attached to current's cpuset, or a memory controller's limit.  The
    proportion is given on a scale of 0 (never kill) to 1000 (always kill),
    roughly meaning that if a task has a badness() score of 500 that the task
    consumes approximately 50% of allowable memory resident in RAM or in swap
    space.
    
    The proportion is always relative to the amount of "allowable" memory and
    not the total amount of RAM systemwide so that mempolicies and cpusets may
    operate in isolation; they shall not need to know the true size of the
    machine on which they are running if they are bound to a specific set of
    nodes or mems, respectively.
    
    Root tasks are given 3% extra memory just like __vm_enough_memory()
    provides in LSMs.  In the event of two tasks consuming similar amounts of
    memory, it is generally better to save root's task.
    
    Because of the change in the badness() heuristic's baseline, it is also
    necessary to introduce a new user interface to tune it.  It's not possible
    to redefine the meaning of /proc/pid/oom_adj with a new scale since the
    ABI cannot be changed for backward compatability.  Instead, a new tunable,
    /proc/pid/oom_score_adj, is added that ranges from -1000 to +1000.  It may
    be used to polarize the heuristic such that certain tasks are never
    considered for oom kill while others may always be considered.  The value
    is added directly into the badness() score so a value of -500, for
    example, means to discount 50% of its memory consumption in comparison to
    other tasks either on the system, bound to the mempolicy, in the cpuset,
    or sharing the same memory controller.
    
    /proc/pid/oom_adj is changed so that its meaning is rescaled into the
    units used by /proc/pid/oom_score_adj, and vice versa.  Changing one of
    these per-task tunables will rescale the value of the other to an
    equivalent meaning.  Although /proc/pid/oom_adj was originally defined as
    a bitshift on the badness score, it now shares the same linear growth as
    /proc/pid/oom_score_adj but with different granularity.  This is required
    so the ABI is not broken with userspace applications and allows oom_adj to
    be deprecated for future removal.
    Signed-off-by: default avatarDavid Rientjes <rientjes@google.com>
    Cc: Nick Piggin <npiggin@suse.de>
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Balbir Singh <balbir@in.ibm.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    a63d83f4
memcontrol.c 118 KB