Re: [PATCH 5/8] mm: memcontrol: account socket memory on unified hierarchy

From: Michal Hocko
Date: Thu Oct 29 2015 - 11:25:52 EST

On Tue 27-10-15 09:42:27, Johannes Weiner wrote:
> On Tue, Oct 27, 2015 at 05:15:54PM +0100, Michal Hocko wrote:
> > On Tue 27-10-15 11:41:38, Johannes Weiner wrote:
> Or it could be exactly the other way around when you have a workload
> that is heavy on filesystem metadata. I don't see why any scenario
> would be more important than the other.

Yes I definitely agree. No scenario is more important. We can only
come up with a default that makes more sense for the majority and
allow the minority to override. That was what I wanted to say basically.

> I'm not saying that distinguishing between consumers is wrong, just
> that "user memory vs kernel memory" is a false classification. Why do
> you call page cache user memory but dentry cache kernel memory? It
> doesn't make any sense.

We are not talking about dcache vs. page cache alone here, though. We
are talking about _all_ slab allocations vs. only user accessed memory.
The slab consumption is directly under kernel control. A great pile of
this logic is completly hidden from userspace. While user can estimate
the user memory it is hard (if possible) to do that for the kernel
memory footprint - not even mentioning this is variable and dependent on
the particular kernel version.

> > Also kmem accounting will make the load more non-deterministic because
> > many of the resources are shared between tasks in separate cgroups
> > unless they are explicitly configured. E.g. [id]cache will be shared
> > and first to touch gets charged so you would end up with more false
> > sharing.
> Exactly like page cache. This differentiation isn't based on reality.

Yes false sharing is an existing and long term problem already. I just
wanted to point out that the false sharing would be even a bigger
problem because some kernel tracked resources are shared more naturally
than file sharing.

> > > IMO that's an implementation detail and a historical artifact that
> > > should not be exposed to the user. And that's the thing I hate about
> > > the current opt-out knob.
> You carefully skipped over this part. We can ignore it for socket
> memory but it's something we need to figure out when it comes to slab
> accounting and tracking.

I am sorry, I didn't mean to skip this part, I though it would be clear
from the previous text. I think kmem accounting falls into the same
category. Have a sane default and a global boottime knob to override it
for those that think differently - for whatever reason they might have.


> Having page cache accounting built in while presenting dentry+inode
> cache as a configurable extension is completely random and doesn't
> make sense. They are both first class memory consumers. They're not
> separate categories. One isn't more "core" than the other.

Again we are talking about all slab allocations not just the dcache.

> > > For now, something like this as a boot commandline?
> > >
> > > cgroup.memory=nosocket
> >
> > That would work for me.
> Okay, then I'll go that route for the socket stuff.


Michal Hocko
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