Re: Re: [PATCH] ksm : use checksum and memcmp for rb_tree

[Kyeongdon Kim]

Sorry, re-send this email because of the Delivery failed message (to 
linux-kernel)

On 2017-10-30 ìí 10:22, Timofey Titovets wrote:
> 2017-10-30 15:03 GMT+03:00 Kyeongdon Kim <kyeongdon.kim@xxxxxxx>:
> > 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@xxxxxxx>
> > ---
> > 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)
 */
Âstruct stable_node {
@@ -159,6 +160,7 @@ struct stable_node {
ÂÂÂÂÂÂÂÂ */
Â#define STABLE_NODE_CHAIN -1024
ÂÂÂÂÂÂÂ int rmap_hlist_len;
+ÂÂÂÂÂÂ u32 oldchecksum;
Â#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@xxxxxxxxx>
>
> --
> 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