On Mon, Nov 03, 2014 at 04:58:07PM -0500, David Miller wrote: > From: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> > Date: Mon, 3 Nov 2014 23:52:06 +0200 > > > On Mon, Nov 03, 2014 at 04:36:28PM -0500, Johannes Weiner wrote: > >> On Mon, Nov 03, 2014 at 11:06:07PM +0200, Kirill A. Shutemov wrote: > >> > On Sat, Nov 01, 2014 at 11:15:54PM -0400, Johannes Weiner wrote: > >> > > Memory cgroups used to have 5 per-page pointers. To allow users to > >> > > disable that amount of overhead during runtime, those pointers were > >> > > allocated in a separate array, with a translation layer between them > >> > > and struct page. > >> > > > >> > > There is now only one page pointer remaining: the memcg pointer, that > >> > > indicates which cgroup the page is associated with when charged. The > >> > > complexity of runtime allocation and the runtime translation overhead > >> > > is no longer justified to save that *potential* 0.19% of memory. > >> > > >> > How much do you win by the change? > >> > >> Heh, that would have followed right after where you cut the quote: > >> with CONFIG_SLUB, that pointer actually sits in already existing > >> struct page padding, which means that I'm saving one pointer per page > >> (8 bytes per 4096 byte page, 0.19% of memory), plus the pointer and > >> padding in each memory section. I also save the (minor) translation > >> overhead going from page to page_cgroup and the maintenance burden > >> that stems from having these auxiliary arrays (see deleted code). > > > > I read the description. I want to know if runtime win (any benchmark data?) > > from moving mem_cgroup back to the struct page is measurable. > > > > If the win is not significant, I would prefer to not occupy the padding: > > I'm sure we will be able to find a better use for the space in struct page > > in the future. > > I think the simplification benefits completely trump any performan > metric. I agree. Also, nobody is using that space currently, and I can save memory by moving the pointer in there. Should we later add another pointer to struct page we are only back to the status quo - with the difference that booting with cgroup_disable=memory will no longer save the extra pointer per page, but again, if you care that much, you can disable memory cgroups at compile-time. So don't look at it as occpuying the padding, it is rather taking away the ability to allocate that single memcg pointer at runtime, while at the same time saving a bit of memory for common configurations until somebody else needs the struct page padding. -- 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>