Messages in this thread | | | Date | Fri, 22 Feb 2013 12:26:08 +0800 | From | Ric Mason <> | Subject | Re: [PATCH 1/7] ksm: add some comments |
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On 02/21/2013 04:19 PM, Hugh Dickins wrote: > Added slightly more detail to the Documentation of merge_across_nodes, > a few comments in areas indicated by review, and renamed get_ksm_page()'s > argument from "locked" to "lock_it". No functional change. > > Signed-off-by: Hugh Dickins <hughd@google.com> > --- > Documentation/vm/ksm.txt | 16 ++++++++++++---- > mm/ksm.c | 18 ++++++++++++++---- > 2 files changed, 26 insertions(+), 8 deletions(-) > > --- mmotm.orig/Documentation/vm/ksm.txt 2013-02-20 22:28:09.456001057 -0800 > +++ mmotm/Documentation/vm/ksm.txt 2013-02-20 22:28:23.580001392 -0800 > @@ -60,10 +60,18 @@ sleep_millisecs - how many milliseconds > > merge_across_nodes - specifies if pages from different numa nodes can be merged. > When set to 0, ksm merges only pages which physically > - reside in the memory area of same NUMA node. It brings > - lower latency to access to shared page. Value can be > - changed only when there is no ksm shared pages in system. > - Default: 1 > + reside in the memory area of same NUMA node. That brings > + lower latency to access of shared pages. Systems with more > + nodes, at significant NUMA distances, are likely to benefit > + from the lower latency of setting 0. Smaller systems, which > + need to minimize memory usage, are likely to benefit from > + the greater sharing of setting 1 (default). You may wish to > + compare how your system performs under each setting, before > + deciding on which to use. merge_across_nodes setting can be > + changed only when there are no ksm shared pages in system: > + set run 2 to unmerge pages first, then to 1 after changing > + merge_across_nodes, to remerge according to the new setting.
What's the root reason merge_across_nodes setting just can be changed only when there are no ksm shared pages in system? Can they be unmerged and merged again during ksmd scan?
> + Default: 1 (merging across nodes as in earlier releases) > > run - set 0 to stop ksmd from running but keep merged pages, > set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run", > --- mmotm.orig/mm/ksm.c 2013-02-20 22:28:09.456001057 -0800 > +++ mmotm/mm/ksm.c 2013-02-20 22:28:23.584001392 -0800 > @@ -87,6 +87,9 @@ > * take 10 attempts to find a page in the unstable tree, once it is found, > * it is secured in the stable tree. (When we scan a new page, we first > * compare it against the stable tree, and then against the unstable tree.) > + * > + * If the merge_across_nodes tunable is unset, then KSM maintains multiple > + * stable trees and multiple unstable trees: one of each for each NUMA node. > */ > > /** > @@ -524,7 +527,7 @@ static void remove_node_from_stable_tree > * a page to put something that might look like our key in page->mapping. > * is on its way to being freed; but it is an anomaly to bear in mind. > */ > -static struct page *get_ksm_page(struct stable_node *stable_node, bool locked) > +static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it) > { > struct page *page; > void *expected_mapping; > @@ -573,7 +576,7 @@ again: > goto stale; > } > > - if (locked) { > + if (lock_it) { > lock_page(page); > if (ACCESS_ONCE(page->mapping) != expected_mapping) { > unlock_page(page); > @@ -703,10 +706,17 @@ static int remove_stable_node(struct sta > return 0; > } > > - if (WARN_ON_ONCE(page_mapped(page))) > + if (WARN_ON_ONCE(page_mapped(page))) { > + /* > + * This should not happen: but if it does, just refuse to let > + * merge_across_nodes be switched - there is no need to panic. > + */ > err = -EBUSY; > - else { > + } else { > /* > + * The stable node did not yet appear stale to get_ksm_page(), > + * since that allows for an unmapped ksm page to be recognized > + * right up until it is freed; but the node is safe to remove. > * This page might be in a pagevec waiting to be freed, > * or it might be PageSwapCache (perhaps under writeback), > * or it might have been removed from swapcache a moment ago. > > -- > 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/ . > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
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