No memory reclaim while reaching MemoryHigh

[Stefan Priebe - Profihost AG <s.priebe@profihost.ag>]

Hello all,

i hope i added the right list and people - if i missed someone i would
be happy to know.

While using kernel 4.19.55 and cgroupv2 i set a MemoryHigh value for a
varnish service.

It happens that the varnish.service cgroup reaches it's MemoryHigh value
and stops working due to throttling.

But i don't understand is that the process itself only consumes 40% of
it's cgroup usage.

So the other 60% is dirty dentries and inode cache. If i issue an
echo 3 > /proc/sys/vm/drop_caches

the varnish cgroup memory usage drops to the 50% of the pure process.

I thought that the kernel would trigger automatic memory reclaim if a
cgroup reaches is memory high value to drop caches.

Isn't it? does it needs a special flag or tuning? Is this expected?

Before drop caches:
   Memory: 13.1G (high: 13.0G)

After drop caches:
   Memory: 5.8G (high: 13.0G)

Greets,
Stefan