From: Andrey Ryabinin <aryabinin@virtuozzo.com>
To: josef@toxicpanda.com, minchan@kernel.org, linux-mm@kvack.org,
hannes@cmpxchg.org, riel@redhat.com, akpm@linux-foundation.org,
david@fromorbit.com, kernel-team@fb.com
Cc: Josef Bacik <jbacik@fb.com>
Subject: Re: [PATCH][v2] mm: use sc->priority for slab shrink targets
Date: Fri, 25 Aug 2017 23:54:56 +0300 [thread overview]
Message-ID: <d2f483b6-670a-54ef-e997-96777163241f@virtuozzo.com> (raw)
In-Reply-To: <1503589176-1823-1-git-send-email-jbacik@fb.com>
On 08/24/2017 06:39 PM, josef@toxicpanda.com wrote:
> From: Josef Bacik <jbacik@fb.com>
>
> Previously we were using the ratio of the number of lru pages scanned to
> the number of eligible lru pages to determine the number of slab objects
> to scan. The problem with this is that these two things have nothing to
> do with each other, so in slab heavy work loads where there is little to
> no page cache we can end up with the pages scanned being a very low
> number. This means that we reclaim next to no slab pages and waste a
> lot of time reclaiming small amounts of space.
>
> Consider the following scenario, where we have the following values and
> the rest of the memory usage is in slab
>
> Active: 58840 kB
> Inactive: 46860 kB
>
> Every time we do a get_scan_count() we do this
>
> scan = size >> sc->priority
>
> where sc->priority starts at DEF_PRIORITY, which is 12. The first loop
> through reclaim would result in a scan target of 2 pages to 11715 total
> inactive pages, and 3 pages to 14710 total active pages. This is a
> really really small target for a system that is entirely slab pages.
> And this is super optimistic, this assumes we even get to scan these
> pages. We don't increment sc->nr_scanned unless we 1) isolate the page,
> which assumes it's not in use, and 2) can lock the page. Under
> pressure these numbers could probably go down, I'm sure there's some
> random pages from daemons that aren't actually in use, so the targets
> get even smaller.
>
> Instead use sc->priority in the same way we use it to determine scan
> amounts for the lru's. This generally equates to pages. Consider the
> following
>
> slab_pages = (nr_objects * object_size) / PAGE_SIZE
>
> What we would like to do is
>
> scan = slab_pages >> sc->priority
>
> but we don't know the number of slab pages each shrinker controls, only
> the objects. However say that theoretically we knew how many pages a
> shrinker controlled, we'd still have to convert this to objects, which
> would look like the following
>
> scan = shrinker_pages >> sc->priority
> scan_objects = (PAGE_SIZE / object_size) * scan
>
> or written another way
>
> scan_objects = (shrinker_pages >> sc->priority) *
> (PAGE_SIZE / object_size)
>
> which can thus be written
>
> scan_objects = ((shrinker_pages * PAGE_SIZE) / object_size) >>
> sc->priority
>
> which is just
>
> scan_objects = nr_objects >> sc->priority
>
> We don't need to know exactly how many pages each shrinker represents,
> it's objects are all the information we need. Making this change allows
> us to place an appropriate amount of pressure on the shrinker pools for
> their relative size.
>
> Signed-off-by: Josef Bacik <jbacik@fb.com>
>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
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next prev parent reply other threads:[~2017-08-25 20:55 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-24 15:39 [PATCH][v2] mm: use sc->priority for slab shrink targets josef
2017-08-25 20:54 ` Andrey Ryabinin [this message]
2017-08-25 22:58 ` Dave Chinner
2017-08-29 20:40 ` Johannes Weiner
2017-08-29 20:58 ` Andrew Morton
2017-09-28 17:40 ` Josef Bacik
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