From: Nai Xia <nai.xia@gmail.com> To: Andrea Arcangeli <aarcange@redhat.com> Cc: Rik van Riel <riel@redhat.com>, Izik Eidus <izik.eidus@ravellosystems.com>, Avi Kivity <avi@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Hugh Dickins <hughd@google.com>, Chris Wright <chrisw@sous-sol.org>, linux-mm <linux-mm@kvack.org>, Johannes Weiner <hannes@cmpxchg.org>, linux-kernel <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org> Subject: Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking Date: Thu, 23 Jun 2011 09:36:37 +0800 [thread overview] Message-ID: <BANLkTik4+A0owJhKKb27KO0HtQ0v-KzU9g@mail.gmail.com> (raw) In-Reply-To: <20110623004404.GE20843@redhat.com> On Thu, Jun 23, 2011 at 8:44 AM, Andrea Arcangeli <aarcange@redhat.com> wrote: > On Thu, Jun 23, 2011 at 08:31:56AM +0800, Nai Xia wrote: >> On Thu, Jun 23, 2011 at 7:59 AM, Andrea Arcangeli <aarcange@redhat.com> wrote: >> > On Thu, Jun 23, 2011 at 07:37:47AM +0800, Nai Xia wrote: >> >> On 2MB pages, I'd like to remind you and Rik that ksmd currently splits >> >> huge pages before their sub pages gets really merged to stable tree. >> >> So when there are many 2MB pages each having a 4kB subpage >> >> changed for all time, this is already a concern for ksmd to judge >> >> if it's worthwhile to split 2MB page and get its sub-pages merged. >> > >> > Hmm not sure to follow. KSM memory density with THP on and off should >> > be identical. The cksum is computed on subpages so the fact the 4k >> > subpage is actually mapped by a hugepmd is invisible to KSM up to the >> > point we get a unstable_tree_search_insert/stable_tree_search lookup >> > succeeding. >> >> I agree on your points. >> >> But, I mean splitting the huge page into normal pages when some subpages >> need to be merged may increase the TLB lookside timing of CPU and >> _might_ hurt the workload ksmd is scanning. If only a small portion of false >> negative 2MB pages are really get merged eventually, maybe it's not worthwhile, >> right? > > Yes, there's not threshold to say "only split if we could merge more > than N subpages", 1 subpage match in two different hugepages is enough > to split both and save just 4k but then memory accesses will be slower > for both 2m ranges that have been splitted. But the point is that it > won't be slower than if THP was off in the first place. So in the end > all we gain is 4k saved but we still run faster than THP off, in the > other hugepages that haven't been splitted yet. Yes, so ksmd is still doing good compared to THP off. Thanks for making my mind clearer :) > >> But, well, just like Rik said below, yes, ksmd should be more aggressive to >> avoid much more time consuming cost for swapping. > > Correct the above logic also follows the idea to always maximize > memory merging in KSM, which is why we've no threshold to wait N > subpages to be mergeable before we split the hugepage. > > I'm unsure if admins in real life would then start to use those > thresholds even if we'd implement them. The current way of enabling > KSM a per-VM (not per-host) basis is pretty simple: the performance > critical VM has KSM off, non-performance critical VM has KSM on and it > prioritizes on memory merging. > Hmm, yes, you are right. Thanks, Nai
WARNING: multiple messages have this Message-ID (diff)
From: Nai Xia <nai.xia@gmail.com> To: Andrea Arcangeli <aarcange@redhat.com> Cc: Rik van Riel <riel@redhat.com>, Izik Eidus <izik.eidus@ravellosystems.com>, Avi Kivity <avi@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Hugh Dickins <hughd@google.com>, Chris Wright <chrisw@sous-sol.org>, linux-mm <linux-mm@kvack.org>, Johannes Weiner <hannes@cmpxchg.org>, linux-kernel <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org> Subject: Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking Date: Thu, 23 Jun 2011 09:36:37 +0800 [thread overview] Message-ID: <BANLkTik4+A0owJhKKb27KO0HtQ0v-KzU9g@mail.gmail.com> (raw) In-Reply-To: <20110623004404.GE20843@redhat.com> On Thu, Jun 23, 2011 at 8:44 AM, Andrea Arcangeli <aarcange@redhat.com> wrote: > On Thu, Jun 23, 2011 at 08:31:56AM +0800, Nai Xia wrote: >> On Thu, Jun 23, 2011 at 7:59 AM, Andrea Arcangeli <aarcange@redhat.com> wrote: >> > On Thu, Jun 23, 2011 at 07:37:47AM +0800, Nai Xia wrote: >> >> On 2MB pages, I'd like to remind you and Rik that ksmd currently splits >> >> huge pages before their sub pages gets really merged to stable tree. >> >> So when there are many 2MB pages each having a 4kB subpage >> >> changed for all time, this is already a concern for ksmd to judge >> >> if it's worthwhile to split 2MB page and get its sub-pages merged. >> > >> > Hmm not sure to follow. KSM memory density with THP on and off should >> > be identical. The cksum is computed on subpages so the fact the 4k >> > subpage is actually mapped by a hugepmd is invisible to KSM up to the >> > point we get a unstable_tree_search_insert/stable_tree_search lookup >> > succeeding. >> >> I agree on your points. >> >> But, I mean splitting the huge page into normal pages when some subpages >> need to be merged may increase the TLB lookside timing of CPU and >> _might_ hurt the workload ksmd is scanning. If only a small portion of false >> negative 2MB pages are really get merged eventually, maybe it's not worthwhile, >> right? > > Yes, there's not threshold to say "only split if we could merge more > than N subpages", 1 subpage match in two different hugepages is enough > to split both and save just 4k but then memory accesses will be slower > for both 2m ranges that have been splitted. But the point is that it > won't be slower than if THP was off in the first place. So in the end > all we gain is 4k saved but we still run faster than THP off, in the > other hugepages that haven't been splitted yet. Yes, so ksmd is still doing good compared to THP off. Thanks for making my mind clearer :) > >> But, well, just like Rik said below, yes, ksmd should be more aggressive to >> avoid much more time consuming cost for swapping. > > Correct the above logic also follows the idea to always maximize > memory merging in KSM, which is why we've no threshold to wait N > subpages to be mergeable before we split the hugepage. > > I'm unsure if admins in real life would then start to use those > thresholds even if we'd implement them. The current way of enabling > KSM a per-VM (not per-host) basis is pretty simple: the performance > critical VM has KSM off, non-performance critical VM has KSM on and it > prioritizes on memory merging. > Hmm, yes, you are right. Thanks, Nai -- 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: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2011-06-23 1:36 UTC|newest] Thread overview: 96+ messages / expand[flat|nested] mbox.gz Atom feed top 2011-06-21 12:55 [PATCH 0/2 V2] ksm: take dirty bit as reference to avoid volatile pages scanning Nai Xia 2011-06-21 12:55 ` Nai Xia 2011-06-21 13:26 ` [PATCH 1/2 " Nai Xia 2011-06-21 13:26 ` Nai Xia 2011-06-21 21:42 ` Chris Wright 2011-06-21 21:42 ` Chris Wright 2011-06-22 0:02 ` Nai Xia 2011-06-22 0:02 ` Nai Xia 2011-06-22 0:42 ` Chris Wright 2011-06-22 0:42 ` Chris Wright 2011-06-21 13:32 ` [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking Nai Xia 2011-06-21 13:32 ` Nai Xia 2011-06-22 0:21 ` Chris Wright 2011-06-22 0:21 ` Chris Wright 2011-06-22 4:43 ` Nai Xia 2011-06-22 4:43 ` Nai Xia 2011-06-22 6:15 ` Izik Eidus 2011-06-22 6:15 ` Izik Eidus 2011-06-22 6:38 ` Nai Xia 2011-06-22 6:38 ` Nai Xia 2011-06-22 15:46 ` Chris Wright 2011-06-22 15:46 ` Chris Wright 2011-06-22 10:43 ` Avi Kivity 2011-06-22 10:43 ` Avi Kivity 2011-06-22 11:05 ` Izik Eidus 2011-06-22 11:05 ` Izik Eidus 2011-06-22 11:10 ` Avi Kivity 2011-06-22 11:10 ` Avi Kivity 2011-06-22 11:19 ` Izik Eidus 2011-06-22 11:19 ` Izik Eidus 2011-06-22 11:24 ` Avi Kivity 2011-06-22 11:24 ` Avi Kivity 2011-06-22 11:28 ` Avi Kivity 2011-06-22 11:28 ` Avi Kivity 2011-06-22 11:31 ` Avi Kivity 2011-06-22 11:31 ` Avi Kivity 2011-06-22 11:33 ` Nai Xia 2011-06-22 11:33 ` Nai Xia 2011-06-22 11:39 ` Izik Eidus 2011-06-22 11:39 ` Izik Eidus 2011-06-22 15:39 ` Rik van Riel 2011-06-22 15:39 ` Rik van Riel 2011-06-22 16:55 ` Andrea Arcangeli 2011-06-22 16:55 ` Andrea Arcangeli 2011-06-22 23:37 ` Nai Xia 2011-06-22 23:37 ` Nai Xia 2011-06-22 23:59 ` Andrea Arcangeli 2011-06-22 23:59 ` Andrea Arcangeli 2011-06-23 0:31 ` Nai Xia 2011-06-23 0:31 ` Nai Xia 2011-06-23 0:44 ` Andrea Arcangeli 2011-06-23 0:44 ` Andrea Arcangeli 2011-06-23 1:36 ` Nai Xia [this message] 2011-06-23 1:36 ` Nai Xia 2011-06-23 0:00 ` Rik van Riel 2011-06-23 0:00 ` Rik van Riel 2011-06-23 0:42 ` Nai Xia 2011-06-23 0:42 ` Nai Xia 2011-06-22 23:13 ` Nai Xia 2011-06-22 23:13 ` Nai Xia 2011-06-22 23:25 ` Andrea Arcangeli 2011-06-22 23:25 ` Andrea Arcangeli 2011-06-23 1:30 ` Nai Xia 2011-06-23 1:30 ` Nai Xia 2011-06-22 23:28 ` Rik van Riel 2011-06-22 23:28 ` Rik van Riel 2011-06-23 0:52 ` Nai Xia 2011-06-23 0:52 ` Nai Xia 2011-06-22 11:24 ` Nai Xia 2011-06-22 15:03 ` Andrea Arcangeli 2011-06-22 15:03 ` Andrea Arcangeli 2011-06-22 15:19 ` Izik Eidus 2011-06-22 15:19 ` Izik Eidus 2011-06-22 23:19 ` Nai Xia 2011-06-22 23:19 ` Nai Xia 2011-06-22 23:44 ` Andrea Arcangeli 2011-06-22 23:44 ` Andrea Arcangeli 2011-06-23 0:14 ` Nai Xia 2011-06-23 0:14 ` Nai Xia 2011-06-22 23:42 ` Nai Xia 2011-06-22 23:42 ` Nai Xia 2011-06-21 13:36 ` [PATCH 2/2 V2] ksm: take dirty bit as reference to avoid volatile pages scanning Nai Xia 2011-06-21 13:36 ` Nai Xia 2011-06-21 22:38 ` Chris Wright 2011-06-21 22:38 ` Chris Wright 2011-06-22 0:04 ` Nai Xia 2011-06-22 0:04 ` Nai Xia 2011-06-22 0:35 ` Chris Wright 2011-06-22 0:35 ` Chris Wright 2011-06-22 4:47 ` Nai Xia 2011-06-22 4:47 ` Nai Xia 2011-06-22 10:55 ` Nai Xia 2011-06-22 10:55 ` Nai Xia 2011-06-22 0:46 ` [PATCH 0/2 " Chris Wright 2011-06-22 0:46 ` Chris Wright 2011-06-22 4:15 ` Nai Xia
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