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 <han...@cmpxchg.org> > > > 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? -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
