From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [patch 1/3] mm: embed the memcg pointer directly into struct page Date: Mon, 03 Nov 2014 16:58:07 -0500 (EST) Message-ID: <20141103.165807.2039166055692354811.davem@davemloft.net> References: <20141103210607.GA24091@node.dhcp.inet.fi> <20141103213628.GA11428@phnom.home.cmpxchg.org> <20141103215206.GB24091@node.dhcp.inet.fi> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20141103215206.GB24091-nhfs4B5ZimeFUdmeq17FyvUpdFzICT1y@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: Text/Plain; charset="us-ascii" To: kirill-oKw7cIdHH8eLwutG50LtGA@public.gmane.org Cc: hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, mhocko-AlSwsSmVLrQ@public.gmane.org, vdavydov-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org, tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org From: "Kirill A. Shutemov" 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.