linux-kernel - Re: [PATCH v1 1/4] mm/ksm: add "smart" page scanning mode

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Message-ID: <qvqwa5tjb9km.fsf@devbig1114.prn1.facebook.com>
Date:   Mon, 18 Sep 2023 11:47:57 -0700
From:   Stefan Roesch <shr@...kernel.io>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     kernel-team@...com, david@...hat.com, hannes@...xchg.org,
        riel@...riel.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 1/4] mm/ksm: add "smart" page scanning mode


Andrew Morton <akpm@...ux-foundation.org> writes:

> On Tue, 12 Sep 2023 10:52:25 -0700 Stefan Roesch <shr@...kernel.io> 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.

>>
>> ...
>>