mm, memcg: make scan aggression always exclude protection
This patch is an incremental improvement on the existing memory.{low,min}
relative reclaim work to base its scan pressure calculations on how much
protection is available compared to the current usage, rather than how
much the current usage is over some protection threshold.
This change doesn't change the experience for the user in the normal
case too much. One benefit is that it replaces the (somewhat
arbitrary) 100% cutoff with an indefinite slope, which makes it easier
to ballpark a memory.low value.
As well as this, the old methodology doesn't quite apply generically to
machines with varying amounts of physical memory. Let's say we have a
top level cgroup, workload.slice, and another top level cgroup,
system-management.slice. We want to roughly give 12G to
system-management.slice, so on a 32GB machine we set memory.low to 20GB
in workload.slice, and on a 64GB machine we set memory.low to 52GB.
However, because these are relative amounts to the total machine size,
while the amount of memory we want to generally be willing to yield to
system.slice is absolute (12G), we end up putting more pressure on
system.slice just because we have a larger machine and a larger
workload to fill it, which seems fairly unintuitive. With this new
behaviour, we don't end up with this unintended side effect.
Previously the way that memory.low protection works is that if you are 50%
over a certain baseline, you get 50% of your normal scan pressure. This
is certainly better than the previous cliff-edge behaviour, but it can be
improved even further by always considering memory under the currently
enforced protection threshold to be out of bounds. This means that we can
set relatively low memory.low thresholds for variable or bursty workloads
while still getting a reasonable level of protection, whereas with the
previous version we may still trivially hit the 100% clamp. The previous
100% clamp is also somewhat arbitrary, whereas this one is more concretely
based on the currently enforced protection threshold, which is likely
easier to reason about.
There is also a subtle issue with the way that proportional reclaim worked
previously -- it promotes having no memory.low, since it makes pressure
higher during low reclaim. This happens because we base our scan pressure
modulation on how far memory.current is between memory.min and memory.low,
but if memory.low is unset, we only use the overage method. In most
cromulent configurations, this then means that we end up with *more*
pressure than with no memory.low at all when we're in low reclaim, which
is not really very usable or expected.
With this patch, memory.low and memory.min affect reclaim pressure in a
more understandable and composable way. For example, from a user
standpoint, "protected" memory now remains untouchable from a reclaim
aggression standpoint, and users can also have more confidence that bursty
workloads will still receive some amount of guaranteed protection.
Link: http://lkml.kernel.org/r/20190322160307.GA3316@chrisdown.name
Signed-off-by: Chris Down <chris@chrisdown.name>
Reviewed-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2 files changed
