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Date: Tue, 21 Jun 2011 12:08:14 +0800 From: Cong Wang <amwang@...hat.com> To: Andrea Arcangeli <aarcange@...hat.com> CC: Vivek Goyal <vgoyal@...hat.com>, Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org, akpm@...ux-foundation.org, Johannes Weiner <jweiner@...hat.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>, linux-mm@...ck.org Subject: Re: [PATCH 1/3] mm: completely disable THP by transparent_hugepage=never 于 2011年06月21日 03:21, Andrea Arcangeli 写道: > 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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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