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-rw-r--r--Documentation/cgroups/memory.txt70
1 files changed, 69 insertions, 1 deletions
diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index 8b8c28b..f336ede 100644
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -40,6 +40,7 @@ Features:
- soft limit
- moving (recharging) account at moving a task is selectable.
- usage threshold notifier
+ - memory pressure notifier
- oom-killer disable knob and oom-notifier
- Root cgroup has no limit controls.
@@ -65,6 +66,7 @@ Brief summary of control files.
memory.stat # show various statistics
memory.use_hierarchy # set/show hierarchical account enabled
memory.force_empty # trigger forced move charge to parent
+ memory.pressure_level # set memory pressure notifications
memory.swappiness # set/show swappiness parameter of vmscan
(See sysctl's vm.swappiness)
memory.move_charge_at_immigrate # set/show controls of moving charges
@@ -762,7 +764,73 @@ At reading, current status of OOM is shown.
under_oom 0 or 1 (if 1, the memory cgroup is under OOM, tasks may
be stopped.)
-11. TODO
+11. Memory Pressure
+
+The pressure level notifications can be used to monitor the memory
+allocation cost; based on the pressure, applications can implement
+different strategies of managing their memory resources. The pressure
+levels are defined as following:
+
+The "low" level means that the system is reclaiming memory for new
+allocations. Monitoring this reclaiming activity might be useful for
+maintaining cache level. Upon notification, the program (typically
+"Activity Manager") might analyze vmstat and act in advance (i.e.
+prematurely shutdown unimportant services).
+
+The "medium" level means that the system is experiencing medium memory
+pressure, the system might be making swap, paging out active file caches,
+etc. Upon this event applications may decide to further analyze
+vmstat/zoneinfo/memcg or internal memory usage statistics and free any
+resources that can be easily reconstructed or re-read from a disk.
+
+The "critical" level means that the system is actively thrashing, it is
+about to out of memory (OOM) or even the in-kernel OOM killer is on its
+way to trigger. Applications should do whatever they can to help the
+system. It might be too late to consult with vmstat or any other
+statistics, so it's advisable to take an immediate action.
+
+The events are propagated upward until the event is handled, i.e. the
+events are not pass-through. Here is what this means: for example you have
+three cgroups: A->B->C. Now you set up an event listener on cgroups A, B
+and C, and suppose group C experiences some pressure. In this situation,
+only group C will receive the notification, i.e. groups A and B will not
+receive it. This is done to avoid excessive "broadcasting" of messages,
+which disturbs the system and which is especially bad if we are low on
+memory or thrashing. So, organize the cgroups wisely, or propagate the
+events manually (or, ask us to implement the pass-through events,
+explaining why would you need them.)
+
+The file memory.pressure_level is only used to setup an eventfd. To
+register a notification, an application must:
+
+- create an eventfd using eventfd(2);
+- open memory.pressure_level;
+- write string like "<event_fd> <fd of memory.pressure_level> <level>"
+ to cgroup.event_control.
+
+Application will be notified through eventfd when memory pressure is at
+the specific level (or higher). Read/write operations to
+memory.pressure_level are no implemented.
+
+Test:
+
+ Here is a small script example that makes a new cgroup, sets up a
+ memory limit, sets up a notification in the cgroup and then makes child
+ cgroup experience a critical pressure:
+
+ # cd /sys/fs/cgroup/memory/
+ # mkdir foo
+ # cd foo
+ # cgroup_event_listener memory.pressure_level low &
+ # echo 8000000 > memory.limit_in_bytes
+ # echo 8000000 > memory.memsw.limit_in_bytes
+ # echo $$ > tasks
+ # dd if=/dev/zero | read x
+
+ (Expect a bunch of notifications, and eventually, the oom-killer will
+ trigger.)
+
+12. TODO
1. Add support for accounting huge pages (as a separate controller)
2. Make per-cgroup scanner reclaim not-shared pages first