Re: [patch 1/3] mm: embed the memcg pointer directly into struct page

[Johannes Weiner]

On Tue, Nov 04, 2014 at 04:00:39PM +0100, Michal Hocko wrote:
> On Tue 04-11-14 09:09:37, Johannes Weiner wrote:
> > On Tue, Nov 04, 2014 at 02:41:10PM +0100, Michal Hocko wrote:
> > > On Tue 04-11-14 08:27:01, Johannes Weiner wrote:
> > > > From: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > > Subject: [patch] mm: move page->mem_cgroup bad page handling into generic code fix
> > > > 
> > > > Remove obsolete memory saving recommendations from the MEMCG Kconfig
> > > > help text.
> > > 
> > > The memory overhead is still there. So I do not think it is good to
> > > remove the message altogether. The current overhead might be 4 or 8B
> > > depending on the configuration. What about
> > > "
> > > 	Note that setting this option might increase fixed memory
> > > 	overhead associated with each page descriptor in the system.
> > > 	The memory overhead depends on the architecture and other
> > > 	configuration options which have influence on the size and
> > > 	alignment on the page descriptor (struct page). Namely
> > > 	CONFIG_SLUB has a requirement for page alignment to two words
> > > 	which in turn means that 64b systems might not see any memory
> > > 	overhead as the additional data fits into alignment. On the
> > > 	other hand 32b systems might see 8B memory overhead.
> > > "
> > 
> > What difference does it make whether this feature maybe costs an extra
> > pointer per page or not?  These texts are supposed to help decide with
> > the selection, but this is not a "good to have, if affordable" type of
> > runtime debugging option.  You either need cgroup memory accounting
> > and limiting or not.  There is no possible trade-off to be had.
> 
> If you are compiling the kernel for your specific usecase then it
> is clear. You enable only what you really need/want. But if you are
> providing a pre-built kernel and considering which features to enable
> then an information about overhead might be useful. You can simply
> disable the feature for memory restricted kernel flavors.
> 
> > Slub and numa balancing don't mention this, either, simply because
> > this cost is negligible or irrelevant when it comes to these knobs.
> 
> I agree that the overhead seems negligible but does it hurt us to
> mention it though?

Yes, it's fairly misleading.  What about the instructions it adds to
the fault hotpaths?  The additional memory footprint of each cgroup
created?  You're adding 9 lines to point out one specific cost aspect,
when the entire feature is otherwise summed up in two lines.  The
per-page overhead of memcg is not exceptional or unexpected if you
know what it does - which you should when you enable it, even as a
distributor - and such a gross overrepresentation in the help text is
more confusing than helpful.
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