Messages in this thread Patch in this message | | | Subject | Re: Re: [PATCH] ksm : use checksum and memcmp for rb_tree | From | Kyeongdon Kim <> | Date | Tue, 7 Nov 2017 10:39:21 +0900 |
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Sorry, re-send this email because of the Delivery failed message (to linux-kernel)
On 2017-10-30 오후 10:22, Timofey Titovets wrote: > 2017-10-30 15:03 GMT+03:00 Kyeongdon Kim <kyeongdon.kim@lge.com>: > > The current ksm is using memcmp to insert and search 'rb_tree'. > > It does cause very expensive computation cost. > > In order to reduce the time of this operation, > > we have added a checksum to traverse before memcmp operation. > > > > Nearly all 'rb_node' in stable_tree_insert() function > > can be inserted as a checksum, most of it is possible > > in unstable_tree_search_insert() function. > > In stable_tree_search() function, the checksum may be an additional. > > But, checksum check duration is extremely small. > > Considering the time of the whole cmp_and_merge_page() function, > > it requires very little cost on average. > > > > Using this patch, we compared the time of ksm_do_scan() function > > by adding kernel trace at the start-end position of operation. > > (ARM 32bit target android device, > > over 1000 sample time gap stamps average) > > > > On original KSM scan avg duration = 0.0166893 sec > > 24991.975619 : ksm_do_scan_start: START: ksm_do_scan > > 24991.990975 : ksm_do_scan_end: END: ksm_do_scan > > 24992.008989 : ksm_do_scan_start: START: ksm_do_scan > > 24992.016839 : ksm_do_scan_end: END: ksm_do_scan > > ... > > > > On patch KSM scan avg duration = 0.0041157 sec > > 41081.461312 : ksm_do_scan_start: START: ksm_do_scan > > 41081.466364 : ksm_do_scan_end: END: ksm_do_scan > > 41081.484767 : ksm_do_scan_start: START: ksm_do_scan > > 41081.487951 : ksm_do_scan_end: END: ksm_do_scan > > ... > > > > We have tested randomly so many times for the stability > > and couldn't see any abnormal issue until now. > > Also, we found out this patch can make some good advantage > > for the power consumption than KSM default enable. > > > > Signed-off-by: Kyeongdon Kim <kyeongdon.kim@lge.com> > > --- > > mm/ksm.c | 49 +++++++++++++++++++++++++++++++++++++++++++++---- > > 1 file changed, 45 insertions(+), 4 deletions(-) > > > > diff --git a/mm/ksm.c b/mm/ksm.c > > index be8f457..66ab4f4 100644 > > --- a/mm/ksm.c > > +++ b/mm/ksm.c > > @@ -150,6 +150,7 @@ struct stable_node { > > struct hlist_head hlist; > > union { > > unsigned long kpfn; > > + u32 oldchecksum; > > unsigned long chain_prune_time; > > }; > > /* > > May be just checksum? i.e. that's can be "old", where checksum can > change, > in stable tree, checksum also stable. > > Also, as checksum are stable, may be that make a sense to move it out > of union? (I'm afraid of clashes) > > Also, you miss update comment above struct stable_node, about checksum > var. > Thanks for your comment, and we may change those lines like below :
+ * @oldchecksum: previous checksum of the page about a stable_node * @nid: NUMA node id of stable tree in which linked (may not match kpfn) */ struct stable_node { @@ -159,6 +160,7 @@ struct stable_node { */ #define STABLE_NODE_CHAIN -1024 int rmap_hlist_len; + u32 oldchecksum; #ifdef CONFIG_NUMA
And I think if checksum are matched, then we can use original memcmp logic in stable tree. the worst case that I imagine is no page merging(just in that moment). But, in my humble opinion, there will be no critical memory issue. but just return. (as I said, we tested a lot to check some abnormal memory operation, but so far, so good - only performance improvement) > > @@ -1522,7 +1523,7 @@ static __always_inline struct page > *chain(struct stable_node **s_n_d, > > * This function returns the stable tree node of identical content if > found, > > * NULL otherwise. > > */ > > -static struct page *stable_tree_search(struct page *page) > > +static struct page *stable_tree_search(struct page *page, u32 > checksum) > > { > > int nid; > > struct rb_root *root; > > @@ -1540,6 +1541,8 @@ static struct page *stable_tree_search(struct > page *page) > > > > nid = get_kpfn_nid(page_to_pfn(page)); > > root = root_stable_tree + nid; > > + if (!checksum) > > + return NULL; > > That's not a pointer, and 0x0 - is a valid checksum. > Also, jhash2 not so collision free, i.e.: > jhash2((uint32_t *) &num, 2, 17); > > Example of collisions, where hash = 0x0: > hash: 0x0 - num: 610041898 > hash: 0x0 - num: 4893164379 > hash: 0x0 - num: 16423540221 > hash: 0x0 - num: 29036382188 > > You also compare values, so hash = 0, is a acceptable checksum. > well, if then, I can remove this check line. > > Thanks, > anyway in general idea looks good. > > Reviewed-by: Timofey Titovets <nefelim4ag@gmail.com> > > -- > Have a nice day, > Timofey. Thanks a lot :) Actually, our organization want to use this KSM feature in general, but, current logic needs too high cost. So I wish to change more light version. Please kindly give your opinion on this idea.
Thanks, Kyeongdon Kim
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