linux-kernel - Re: [00/41] Large Blocksize Support V7 (adds memmap support)

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Date:	Sat, 15 Sep 2007 22:14:44 +0200
From:	Goswin von Brederlow <brederlo@...ormatik.uni-tuebingen.de>
To:	Andrea Arcangeli <andrea@...e.de>
Cc:	Goswin von Brederlow <brederlo@...ormatik.uni-tuebingen.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joern Engel <joern@...fs.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Christoph Lameter <clameter@....com>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
	Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.com>,
	David Chinner <dgc@....com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Fengguang Wu <fengguang.wu@...il.com>,
	swin wang <wangswin@...il.com>, totty.lu@...il.com,
	hugh@...itas.com
Subject: Re: [00/41] Large Blocksize Support V7 (adds memmap support)

Andrea Arcangeli <andrea@...e.de> writes:

> On Sat, Sep 15, 2007 at 02:14:42PM +0200, Goswin von Brederlow wrote:
>> I keep coming back to the fact that movable objects should be moved
>> out of the way for unmovable ones. Anything else just allows
>
> That's incidentally exactly what the slab does, no need to reinvent
> the wheel for that, it's an old problem and there's room for
> optimization in the slab partial-reuse logic too. Just boost the order
> 0 page size and use the slab to get the 4k chunks. The sgi/defrag
> design is backwards.

How does that help? Will slabs move objects around to combine two
partially filled slabs into nearly full one? If not consider this:

- You create a slab for 4k objects based on 64k compound pages.
  (first of all that wastes you a page already for the meta infos)
- Something movable allocates a 14 4k page in there making the slab
  partially filled.
- Something unmovable alloactes a 4k page making the slab mixed and
  full.
- Repeat until out of memory.

OR

- Userspace allocates a lot of memory in those slabs.
- Userspace frees one in every 15 4k chunks.
- Userspace forks 1000 times causing an unmovable task structure to
  appear in 1000 slabs. 

MfG
        Goswin
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