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SubjectRe: [PATCH 3/3] mm/memcg: Avoid reclaiming below hard protection
On 2018/12/3 PM 7:57, Michal Hocko wrote:
> On Mon 03-12-18 16:01:19, Xunlei Pang wrote:
>> When memcgs get reclaimed after its usage exceeds min, some
>> usages below the min may also be reclaimed in the current
>> implementation, the amount is considerably large during kswapd
>> reclaim according to my ftrace results.
> And here again. Describe the setup and the behavior please?

step 1
mkdir -p /sys/fs/cgroup/memory/online
cd /sys/fs/cgroup/memory/online
echo 512M > memory.max
echo 409600000 > memory.min
echo $$ > tasks
dd if=/dev/sda of=/dev/null

while true; do sleep 1; cat memory.current ; cat memory.min; done

step 2
create global memory pressure by allocating annoymous and cached
pages to constantly trigger kswap: dd if=/dev/sdb of=/dev/null

step 3
Then observe "online" groups, hundreds of kbytes a little over
memory.min can cause tens of MiB to be reclaimed by kswapd.

Here is one of test results I got:
cat memory.current; cat memory.min; echo;
409485312 // current
409600000 // min

385052672 // See current got over reclaimed for 23MB
409600000 // min

Its corresponding ftrace output I monitored:
kswapd_0-281 [000] .... 304.706632: shrink_node_memcg:
min_excess=24, nr_reclaimed=6013, sc->nr_to_reclaim=1499997, exceeds

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