linux-kernel - Re: [PATCH] mm: slowly shrink slabs with a relatively small number of objects

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Message-ID: <3b05579f964cca1d44551913f1a9ee79d96f198e.camel@surriel.com>
Date:   Fri, 31 Aug 2018 17:15:39 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Roman Gushchin <guro@...com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        Josef Bacik <jbacik@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: slowly shrink slabs with a relatively small number
 of objects

On Fri, 2018-08-31 at 13:34 -0700, Roman Gushchin wrote:

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index fa2c150ab7b9..c910cf6bf606 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -476,6 +476,10 @@ static unsigned long do_shrink_slab(struct
> shrink_control *shrinkctl,
>  	delta = freeable >> priority;
>  	delta *= 4;
>  	do_div(delta, shrinker->seeks);
> +
> +	if (delta == 0 && freeable > 0)
> +		delta = min(freeable, batch_size);
> +
>  	total_scan += delta;
>  	if (total_scan < 0) {
>  		pr_err("shrink_slab: %pF negative objects to delete
> nr=%ld\n",

I agree that we need to shrink slabs with fewer than
4096 objects, but do we want to put more pressure on
a slab the moment it drops below 4096 than we applied
when it had just over 4096 objects on it?

With this patch, a slab with 5000 objects on it will
get 1 item scanned, while a slab with 4000 objects on
it will see shrinker->batch or SHRINK_BATCH objects
scanned every time.

I don't know if this would cause any issues, just
something to ponder.

If nobody things this is a problem, you can give the
patch my:

Acked-by: Rik van Riel <riel@...riel.com>

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