Re: No memory reclaim while reaching MemoryHigh

[Chris Down <chris@chrisdown.name>]

Hi Stefan,

Stefan Priebe - Profihost AG writes:
>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.

In that kernel version, the only throttling we have is reclaim-based throttling 
(I also have a patch out to do schedule-based throttling, but it's not in 
mainline yet). If the application is slowing down, it likely means that we are 
struggling to reclaim pages.

>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.

As a caching server, doesn't Varnish have a lot of hot inodes/dentries in 
memory? If they are hot, it's possible it's hard for us to evict them.

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

It does, that's the throttling you're seeing :-) I think more information is 
needed to work out what's going on here. For example: what do your kswapd 
counters look like? What does "stops working due to throttling" mean -- are you 
stuck in reclaim?

Thanks,

Chris