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Date: Mon, 4 Dec 2017 17:55:23 +0300
From: Timofey Titovets <nefelim4ag@...il.com>
To: linux-mm@...ck.org
Cc: Linux Kernel <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
Timofey Titovets <nefelim4ag@...il.com>
Subject: Re: [RFC v2 PATCH] ksm: add offset arg to memcmp_pages() to speedup comparing
Gentle ping.
P.S.
If someone can tests that on "big" machine, that will be cool (i just
have not one).
I think may be, that need more tuning, as example set max_offset_error
to 128, instead of 8, to trade off precision with performance.
I'm also wander that possible of more profit on machines with "big pages (64k)".
Thanks!
2017-10-02 15:58 GMT+03:00 Timofey Titovets <nefelim4ag@...il.com>:
> Currently while search/inserting in RB tree,
> memcmp used for comparing out of tree pages with in tree pages.
>
> But on each compare step memcmp for pages start at
> zero offset, i.e. that just ignore forward progress.
>
> That make some overhead for search in deep RB tree and/or with
> bit pages (4KiB+), so store last start offset where no diff in page content.
>
> Added: memcmpe()
> iter 1024 - that a some type of magic value
> max_offset_error - 8 - acceptable error level for offset.
>
> With that patch i get ~ same performance in bad case (where offset useless)
> on tiny tree and default 4KiB pages.
>
> So that just RFC, i.e. does that type of optimization make a sense?
>
> Thanks.
>
> Changes:
> v1 -> v2:
> Add: configurable max_offset_error
> Move logic to memcmpe()
>
> Signed-off-by: Timofey Titovets <nefelim4ag@...il.com>
> ---
> mm/ksm.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++++++------
> 1 file changed, 55 insertions(+), 6 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 15dd7415f7b3..780630498de8 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -991,14 +991,58 @@ static u32 calc_checksum(struct page *page)
> return checksum;
> }
>
> -static int memcmp_pages(struct page *page1, struct page *page2)
> +
> +/*
> + * memcmp used to compare pages in RB-tree
> + * but on every step down the tree forward progress
> + * just has been ignored, that make performance pitfall
> + * on deep tree and/or big pages (ex. 4KiB+)
> + *
> + * Fix that by add memcmp wrapper that will try to guess
> + * where difference happens, to only scan from that offset against
> + * next pages
> + */
> +
> +static int memcmpe(const void *p, const void *q, const u32 len,
> + u32 *offset)
> +{
> + const u32 max_offset_error = 8;
> + u32 iter = 1024, i = 0;
> + int ret;
> +
> + if (offset == NULL)
> + return memcmp(p, q, len);
> +
> + if (*offset < len)
> + i = *offset;
> +
> + while (i < len) {
> + iter = min_t(u32, iter, len - i);
> + ret = memcmp(p, q, iter);
> +
> + if (ret) {
> + iter = iter >> 1;
> + if (iter < max_offset_error)
> + break;
> + continue;
> + }
> +
> + i += iter;
> + }
> +
> + *offset = i;
> +
> + return ret;
> +}
> +
> +static int memcmp_pages(struct page *page1, struct page *page2, u32 *offset)
> {
> char *addr1, *addr2;
> int ret;
>
> addr1 = kmap_atomic(page1);
> addr2 = kmap_atomic(page2);
> - ret = memcmp(addr1, addr2, PAGE_SIZE);
> + ret = memcmpe(addr1, addr2, PAGE_SIZE, offset);
> kunmap_atomic(addr2);
> kunmap_atomic(addr1);
> return ret;
> @@ -1006,7 +1050,7 @@ static int memcmp_pages(struct page *page1, struct page *page2)
>
> static inline int pages_identical(struct page *page1, struct page *page2)
> {
> - return !memcmp_pages(page1, page2);
> + return !memcmp_pages(page1, page2, NULL);
> }
>
> static int write_protect_page(struct vm_area_struct *vma, struct page *page,
> @@ -1514,6 +1558,7 @@ static __always_inline struct page *chain(struct stable_node **s_n_d,
> static struct page *stable_tree_search(struct page *page)
> {
> int nid;
> + u32 diff_offset;
> struct rb_root *root;
> struct rb_node **new;
> struct rb_node *parent;
> @@ -1532,6 +1577,7 @@ static struct page *stable_tree_search(struct page *page)
> again:
> new = &root->rb_node;
> parent = NULL;
> + diff_offset = 0;
>
> while (*new) {
> struct page *tree_page;
> @@ -1590,7 +1636,7 @@ static struct page *stable_tree_search(struct page *page)
> goto again;
> }
>
> - ret = memcmp_pages(page, tree_page);
> + ret = memcmp_pages(page, tree_page, &diff_offset);
> put_page(tree_page);
>
> parent = *new;
> @@ -1760,6 +1806,7 @@ static struct page *stable_tree_search(struct page *page)
> static struct stable_node *stable_tree_insert(struct page *kpage)
> {
> int nid;
> + u32 diff_offset;
> unsigned long kpfn;
> struct rb_root *root;
> struct rb_node **new;
> @@ -1773,6 +1820,7 @@ static struct stable_node *stable_tree_insert(struct page *kpage)
> again:
> parent = NULL;
> new = &root->rb_node;
> + diff_offset = 0;
>
> while (*new) {
> struct page *tree_page;
> @@ -1819,7 +1867,7 @@ static struct stable_node *stable_tree_insert(struct page *kpage)
> goto again;
> }
>
> - ret = memcmp_pages(kpage, tree_page);
> + ret = memcmp_pages(kpage, tree_page, &diff_offset);
> put_page(tree_page);
>
> parent = *new;
> @@ -1884,6 +1932,7 @@ struct rmap_item *unstable_tree_search_insert(struct rmap_item *rmap_item,
> struct rb_root *root;
> struct rb_node *parent = NULL;
> int nid;
> + u32 diff_offset = 0;
>
> nid = get_kpfn_nid(page_to_pfn(page));
> root = root_unstable_tree + nid;
> @@ -1908,7 +1957,7 @@ struct rmap_item *unstable_tree_search_insert(struct rmap_item *rmap_item,
> return NULL;
> }
>
> - ret = memcmp_pages(page, tree_page);
> + ret = memcmp_pages(page, tree_page, &diff_offset);
>
> parent = *new;
> if (ret < 0) {
> --
> 2.14.2
>
--
Have a nice day,
Timofey.
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