From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail6.bemta7.messagelabs.com (mail6.bemta7.messagelabs.com [216.82.255.55]) by kanga.kvack.org (Postfix) with ESMTP id 7A5146B0139 for ; Tue, 21 Jun 2011 00:08:23 -0400 (EDT) Message-ID: <4E00192E.70901@redhat.com> Date: Tue, 21 Jun 2011 12:08:14 +0800 From: Cong Wang MIME-Version: 1.0 Subject: Re: [PATCH 1/3] mm: completely disable THP by transparent_hugepage=never References: <1308587683-2555-1-git-send-email-amwang@redhat.com> <20110620165844.GA9396@suse.de> <4DFF7E3B.1040404@redhat.com> <4DFF7F0A.8090604@redhat.com> <4DFF8106.8090702@redhat.com> <4DFF8327.1090203@redhat.com> <4DFF84BB.3050209@redhat.com> <4DFF8848.2060802@redhat.com> <20110620182558.GF4749@redhat.com> <20110620192117.GG20843@redhat.com> In-Reply-To: <20110620192117.GG20843@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrea Arcangeli Cc: Vivek Goyal , Rik van Riel , Mel Gorman , linux-kernel@vger.kernel.org, akpm@linux-foundation.org, Johannes Weiner , KAMEZAWA Hiroyuki , linux-mm@kvack.org ao? 2011a1'06ae??21ae?JPY 03:21, Andrea Arcangeli a??e??: > On Mon, Jun 20, 2011 at 02:25:58PM -0400, Vivek Goyal wrote: >> So I see some opprotunity there to save memory. But this 10kB >> definitely sounds trivial amount to me. > > Agree with you and Rik. Also I already avoided the big memory waste > (that for example isn't avoided in the ksmd and could be optimized > away without decreasing flexibility of KSM, and ksmd surely runs on > the kdump kernel too...) that is to make khugepaged exit and release > kernel stack when enabled=never (either done by sysfs or at boot with > transparent_hugepage=never) and all other structs associated with a > (temporarily) useless kernel thread. I agree to disable ksm in kdump kernel, thanks for pointing this out! I will look into later, and probably send a patch for this too. > > The khugepaged_slab_init and mm_slot_hash_init() maybe could be > deferred to when khugepaged starts, and be released when it shutdown > but it makes it more tricky/racey. If you really want to optimize > that, without preventing to ever enable THP again despite all .text > was compiled in and ready to run. You will likely save more if you > make ksmd exit when run=0 (which btw is a much more common config than > enabled=never with THP). And slots hashes are allocated by ksm too so > you could optimize those too if you want and allocate them only by the > time ksmd starts. The thing is that we can save ~10K by adding 3 lines of code as this patch showed, where else in kernel can you save 10K by 3 lines of code? (except some kfree() cases, of course) So, again, why not have it? ;) > > As long as it'd still possible to enable the feature again as it is > possible now without noticing an altered behavior from userland, I'm > not entirely against optimizing for saving ~8k of ram even if it > increases complexity a bit (more kernel code will increase .text a bit > though, hopefully not 8k more of .text ;). Why do we _force_ the feature to be tunable even when user completely don't want to disable it? Why not provide a way to let the user to decide which is better for him? When programming kernel, providing a mechanism rather than a policy is what I always keep in mind, I don't know why you violate this rule here, to be honest. :-/ Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org