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authorChris Down <chris@chrisdown.name>2019-10-06 17:58:32 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2019-10-07 15:47:20 -0700
commit9783aa9917f8ae24759e67bf882f1aba32fe4ea1 (patch)
tree2ddbf2490acba84a3f96442b65a830758a079430 /Documentation/admin-guide
parent518a86713078168acd67cf50bc0b45d54b4cce6c (diff)
downloadlinux-9783aa9917f8ae24759e67bf882f1aba32fe4ea1.tar.bz2
mm, memcg: proportional memory.{low,min} reclaim
cgroup v2 introduces two memory protection thresholds: memory.low (best-effort) and memory.min (hard protection). While they generally do what they say on the tin, there is a limitation in their implementation that makes them difficult to use effectively: that cliff behaviour often manifests when they become eligible for reclaim. This patch implements more intuitive and usable behaviour, where we gradually mount more reclaim pressure as cgroups further and further exceed their protection thresholds. This cliff edge behaviour happens because we only choose whether or not to reclaim based on whether the memcg is within its protection limits (see the use of mem_cgroup_protected in shrink_node), but we don't vary our reclaim behaviour based on this information. Imagine the following timeline, with the numbers the lruvec size in this zone: 1. memory.low=1000000, memory.current=999999. 0 pages may be scanned. 2. memory.low=1000000, memory.current=1000000. 0 pages may be scanned. 3. memory.low=1000000, memory.current=1000001. 1000001* pages may be scanned. (?!) * Of course, we won't usually scan all available pages in the zone even without this patch because of scan control priority, over-reclaim protection, etc. However, as shown by the tests at the end, these techniques don't sufficiently throttle such an extreme change in input, so cliff-like behaviour isn't really averted by their existence alone. Here's an example of how this plays out in practice. At Facebook, we are trying to protect various workloads from "system" software, like configuration management tools, metric collectors, etc (see this[0] case study). In order to find a suitable memory.low value, we start by determining the expected memory range within which the workload will be comfortable operating. This isn't an exact science -- memory usage deemed "comfortable" will vary over time due to user behaviour, differences in composition of work, etc, etc. As such we need to ballpark memory.low, but doing this is currently problematic: 1. If we end up setting it too low for the workload, it won't have *any* effect (see discussion above). The group will receive the full weight of reclaim and won't have any priority while competing with the less important system software, as if we had no memory.low configured at all. 2. Because of this behaviour, we end up erring on the side of setting it too high, such that the comfort range is reliably covered. However, protected memory is completely unavailable to the rest of the system, so we might cause undue memory and IO pressure there when we *know* we have some elasticity in the workload. 3. Even if we get the value totally right, smack in the middle of the comfort zone, we get extreme jumps between no pressure and full pressure that cause unpredictable pressure spikes in the workload due to the current binary reclaim behaviour. With this patch, we can set it to our ballpark estimation without too much worry. Any undesirable behaviour, such as too much or too little reclaim pressure on the workload or system will be proportional to how far our estimation is off. This means we can set memory.low much more conservatively and thus waste less resources *without* the risk of the workload falling off a cliff if we overshoot. As a more abstract technical description, this unintuitive behaviour results in having to give high-priority workloads a large protection buffer on top of their expected usage to function reliably, as otherwise we have abrupt periods of dramatically increased memory pressure which hamper performance. Having to set these thresholds so high wastes resources and generally works against the principle of work conservation. In addition, having proportional memory reclaim behaviour has other benefits. Most notably, before this patch it's basically mandatory to set memory.low to a higher than desirable value because otherwise as soon as you exceed memory.low, all protection is lost, and all pages are eligible to scan again. By contrast, having a gradual ramp in reclaim pressure means that you now still get some protection when thresholds are exceeded, which means that one can now be more comfortable setting memory.low to lower values without worrying that all protection will be lost. This is important because workingset size is really hard to know exactly, especially with variable workloads, so at least getting *some* protection if your workingset size grows larger than you expect increases user confidence in setting memory.low without a huge buffer on top being needed. Thanks a lot to Johannes Weiner and Tejun Heo for their advice and assistance in thinking about how to make this work better. In testing these changes, I intended to verify that: 1. Changes in page scanning become gradual and proportional instead of binary. To test this, I experimented stepping further and further down memory.low protection on a workload that floats around 19G workingset when under memory.low protection, watching page scan rates for the workload cgroup: +------------+-----------------+--------------------+--------------+ | memory.low | test (pgscan/s) | control (pgscan/s) | % of control | +------------+-----------------+--------------------+--------------+ | 21G | 0 | 0 | N/A | | 17G | 867 | 3799 | 23% | | 12G | 1203 | 3543 | 34% | | 8G | 2534 | 3979 | 64% | | 4G | 3980 | 4147 | 96% | | 0 | 3799 | 3980 | 95% | +------------+-----------------+--------------------+--------------+ As you can see, the test kernel (with a kernel containing this patch) ramps up page scanning significantly more gradually than the control kernel (without this patch). 2. More gradual ramp up in reclaim aggression doesn't result in premature OOMs. To test this, I wrote a script that slowly increments the number of pages held by stress(1)'s --vm-keep mode until a production system entered severe overall memory contention. This script runs in a highly protected slice taking up the majority of available system memory. Watching vmstat revealed that page scanning continued essentially nominally between test and control, without causing forward reclaim progress to become arrested. [0]: https://facebookmicrosites.github.io/cgroup2/docs/overview.html#case-study-the-fbtax2-project [akpm@linux-foundation.org: reflow block comments to fit in 80 cols] [chris@chrisdown.name: handle cgroup_disable=memory when getting memcg protection] Link: http://lkml.kernel.org/r/20190201045711.GA18302@chrisdown.name Link: http://lkml.kernel.org/r/20190124014455.GA6396@chrisdown.name Signed-off-by: Chris Down <chris@chrisdown.name> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Reviewed-by: Roman Gushchin <guro@fb.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Tejun Heo <tj@kernel.org> Cc: Dennis Zhou <dennis@kernel.org> Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/admin-guide')
-rw-r--r--Documentation/admin-guide/cgroup-v2.rst20
1 files changed, 14 insertions, 6 deletions
diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 0fa8c0e615c2..5361ebec3361 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -615,8 +615,8 @@ on an IO device and is an example of this type.
Protections
-----------
-A cgroup is protected to be allocated upto the configured amount of
-the resource if the usages of all its ancestors are under their
+A cgroup is protected upto the configured amount of the resource
+as long as the usages of all its ancestors are under their
protected levels. Protections can be hard guarantees or best effort
soft boundaries. Protections can also be over-committed in which case
only upto the amount available to the parent is protected among
@@ -1096,7 +1096,10 @@ PAGE_SIZE multiple when read back.
is within its effective min boundary, the cgroup's memory
won't be reclaimed under any conditions. If there is no
unprotected reclaimable memory available, OOM killer
- is invoked.
+ is invoked. Above the effective min boundary (or
+ effective low boundary if it is higher), pages are reclaimed
+ proportionally to the overage, reducing reclaim pressure for
+ smaller overages.
Effective min boundary is limited by memory.min values of
all ancestor cgroups. If there is memory.min overcommitment
@@ -1118,7 +1121,10 @@ PAGE_SIZE multiple when read back.
Best-effort memory protection. If the memory usage of a
cgroup is within its effective low boundary, the cgroup's
memory won't be reclaimed unless memory can be reclaimed
- from unprotected cgroups.
+ from unprotected cgroups. Above the effective low boundary (or
+ effective min boundary if it is higher), pages are reclaimed
+ proportionally to the overage, reducing reclaim pressure for
+ smaller overages.
Effective low boundary is limited by memory.low values of
all ancestor cgroups. If there is memory.low overcommitment
@@ -2482,8 +2488,10 @@ system performance due to overreclaim, to the point where the feature
becomes self-defeating.
The memory.low boundary on the other hand is a top-down allocated
-reserve. A cgroup enjoys reclaim protection when it's within its low,
-which makes delegation of subtrees possible.
+reserve. A cgroup enjoys reclaim protection when it's within its
+effective low, which makes delegation of subtrees possible. It also
+enjoys having reclaim pressure proportional to its overage when
+above its effective low.
The original high boundary, the hard limit, is defined as a strict
limit that can not budge, even if the OOM killer has to be called.