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Date: Mon, 9 Jul 2007 15:47:25 +0300
From: "Pekka Enberg" <penberg@cs.helsinki.fi>
To: "Nick Piggin" <nickpiggin@yahoo.com.au>
Subject: Re: [patch 09/10] Remove the SLOB allocator for 2.6.23
Cc: "Andrew Morton" <akpm@linux-foundation.org>, "Ingo Molnar" <mingo@elte.hu>,
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Hi Nick,

Pekka Enberg wrote:
> > I assume with "slab external fragmentation" you mean allocating a
> > whole page for a slab when there are not enough objects to fill the
> > whole thing thus wasting memory?

On 7/9/07, Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> Yep. Without really analysing it, I guess SLOB's savings here are
> O(1) over SLUB and will relatively diminish as the machine size gets
> larger, however with the number of slabs even a small kernel creates,
> this is likely to be significant on small memory systems.

Running the included script on my little Debian on UML with 32 MB of
RAM shows anywhere from 20 KB up to 100 KB of wasted space on light
load. What's interesting is that the wasted amount seems to stabilize
around 70 KB and never goes below that.

                                               Pekka

#!/bin/bash

total_wasted=0

for i in $(find /sys/slab -type d -mindepth 1 -maxdepth 1 | sort)
do
  nr_objs=$(cat $i/objects)
  slabs=$(cat $i/slabs)
  objs_per_slab=$(cat $i/objs_per_slab)
  let "max_objs=$objs_per_slab*$slabs"

  object_size=$(cat $i/object_size)
  let "wasted=($max_objs-$nr_objs)*$object_size"

  if [ "$wasted" -ne "0" ]; then
    echo "$i: max_objs=$max_objs, nr_objs=$nr_objs, $wasted bytes wasted"
  fi
  let "total_wasted=$total_wasted+$wasted"
done

echo "Total wasted: $total_wasted"