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Mon, 18 Sep 2023 14:49:54 -0400 (EDT) References: <20230912175228.952039-1-shr@devkernel.io> <20230912175228.952039-2-shr@devkernel.io> <20230913140750.616d3d87fe986a74d870b71f@linux-foundation.org> User-agent: mu4e 1.10.1; emacs 28.2.50 From: Stefan Roesch To: Andrew Morton Cc: kernel-team@fb.com, david@redhat.com, hannes@cmpxchg.org, riel@surriel.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v1 1/4] mm/ksm: add "smart" page scanning mode Date: Mon, 18 Sep 2023 11:47:57 -0700 In-reply-to: <20230913140750.616d3d87fe986a74d870b71f@linux-foundation.org> Message-ID: MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Queue-Id: 18A301C0009 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: gs98fbqpghjnaj8i8gw8xxpg34up43qs X-HE-Tag: 1695062995-578567 X-HE-Meta: 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 SnHIuuxF y//iGWjxTLRWZA7fKQ+YhWOh45cfoLvNcQYgi8750ALwvsIPDaMKHbVYcptowQL+uW+S7U7h+DEWhjMHYhccRmJr8bhRVhwGwnadx2e8fPeMSJgd2NF9/nG/flwrD6K82powK6N1ZbNox3XqciB54q2TZ5PZKsMTka2dzVJuUXTVVSOatQdO8BLCLkPsHsGUFLQYgqCdAurHdPAGZyG68YXWQk/fUuJ2JYzAw X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Andrew Morton writes: > On Tue, 12 Sep 2023 10:52:25 -0700 Stefan Roesch wrote: > >> This change adds a "smart" page scanning mode for KSM. So far all the >> candidate pages are continuously scanned to find candidates for >> de-duplication. There are a considerably number of pages that cannot be >> de-duplicated. This is costly in terms of CPU. By using smart scanning >> considerable CPU savings can be achieved. >> >> This change takes the history of scanning pages into account and skips >> the page scanning of certain pages for a while if de-deduplication for >> this page has not been successful in the past. >> >> To do this it introduces two new fields in the ksm_rmap_item structure: >> age and skip_age. age, is the KSM age and skip_page is the age for how > > s/skip_page/skip_age/ > Fixed in the next version. >> long page scanning of this page is skipped. The age field is incremented >> each time the page is scanned and the page cannot be de-duplicated. >> >> How often a page is skipped is dependent how often de-duplication has >> been tried so far and the number of skips is currently limited to 8. >> This value has shown to be effective with different workloads. >> >> The feature is currently disable by default and can be enabled with the >> new smart_scan knob. >> >> The feature has shown to be very effective: upt to 25% of the page scans >> can be eliminated; the pages_to_scan rate can be reduced by 40 - 50% and >> a similar de-duplication rate can be maintained. >> > > All seems nice. I'll sit out v1, see what people have to say. > > Some nits: > >> --- a/mm/ksm.c >> +++ b/mm/ksm.c >> >> ... >> >> @@ -2305,6 +2314,45 @@ static struct ksm_rmap_item *get_next_rmap_item(struct ksm_mm_slot *mm_slot, >> return rmap_item; >> } >> >> +static unsigned int inc_skip_age(rmap_age_t age) >> +{ >> + if (age <= 3) >> + return 1; >> + if (age <= 5) >> + return 2; >> + if (age <= 8) >> + return 4; >> + >> + return 8; >> +} > > "inc_skip_age" sounds like it increments something. Can we give it a > better name? > > And a nice comment explaining its role in life. > Renamed it to skip_age in the next version and added a comment. >> +static bool skip_rmap_item(struct page *page, struct ksm_rmap_item *rmap_item) >> +{ >> + rmap_age_t age; >> + >> + if (!ksm_smart_scan) >> + return false; >> + >> + if (PageKsm(page)) >> + return false; >> + >> + age = rmap_item->age++; >> + if (age < 3) >> + return false; >> + >> + if (rmap_item->skip_age == age) { >> + rmap_item->skip_age = 0; >> + return false; >> + } >> + >> + if (rmap_item->skip_age == 0) { >> + rmap_item->skip_age = age + inc_skip_age(age); >> + remove_rmap_item_from_tree(rmap_item); >> + } >> + >> + return true; >> +} > > Would a better name be should_skip_rmap_item()? > Renamed it to should_skip_rmap_item(). > But even that name implies that the function is idempotent (has no > side-effects). Again, an explanatory comment would be good. And > simple comments over each non-obvious `if' statement. > Added more comments to the function to explain the different cases. >> >> ... >>