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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>