linux-kernel - Re: [PATCH 8/9] readahead: basic support for backwards prefetching

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Date:	Tue, 29 Nov 2011 16:35:52 +0100
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Li Shaohua <shaohua.li@...el.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 8/9] readahead: basic support for backwards prefetching

On Tue 29-11-11 21:09:08, Wu Fengguang wrote:
> Add the backwards prefetching feature. It's pretty simple if we don't
> support async prefetching and interleaved reads.
> 
> Here is the behavior with an 8-page read sequence from 10000 down to 0.
> (The readahead size is a bit large since it's an NFS mount.)
> 
> readahead-random(dev=0:16, ino=3948605, req=10000+8, ra=10000+8-0, async=0) = 8
> readahead-backwards(dev=0:16, ino=3948605, req=9992+8, ra=9968+32-0, async=0) = 32
> readahead-backwards(dev=0:16, ino=3948605, req=9960+8, ra=9840+128-0, async=0) = 128
> readahead-backwards(dev=0:16, ino=3948605, req=9832+8, ra=9584+256-0, async=0) = 256
> readahead-backwards(dev=0:16, ino=3948605, req=9576+8, ra=9072+512-0, async=0) = 512
> readahead-backwards(dev=0:16, ino=3948605, req=9064+8, ra=8048+1024-0, async=0) = 1024
> readahead-backwards(dev=0:16, ino=3948605, req=8040+8, ra=6128+1920-0, async=0) = 1920
> readahead-backwards(dev=0:16, ino=3948605, req=6120+8, ra=4208+1920-0, async=0) = 1920
> readahead-backwards(dev=0:16, ino=3948605, req=4200+8, ra=2288+1920-0, async=0) = 1920
> readahead-backwards(dev=0:16, ino=3948605, req=2280+8, ra=368+1920-0, async=0) = 1920
> readahead-backwards(dev=0:16, ino=3948605, req=360+8, ra=0+368-0, async=0) = 368
> 
> And a simple 1-page read sequence from 10000 down to 0.
> 
> readahead-random(dev=0:16, ino=3948605, req=10000+1, ra=10000+1-0, async=0) = 1
> readahead-backwards(dev=0:16, ino=3948605, req=9999+1, ra=9996+4-0, async=0) = 4
> readahead-backwards(dev=0:16, ino=3948605, req=9995+1, ra=9980+16-0, async=0) = 16
> readahead-backwards(dev=0:16, ino=3948605, req=9979+1, ra=9916+64-0, async=0) = 64
> readahead-backwards(dev=0:16, ino=3948605, req=9915+1, ra=9660+256-0, async=0) = 256
> readahead-backwards(dev=0:16, ino=3948605, req=9659+1, ra=9148+512-0, async=0) = 512
> readahead-backwards(dev=0:16, ino=3948605, req=9147+1, ra=8124+1024-0, async=0) = 1024
> readahead-backwards(dev=0:16, ino=3948605, req=8123+1, ra=6204+1920-0, async=0) = 1920
> readahead-backwards(dev=0:16, ino=3948605, req=6203+1, ra=4284+1920-0, async=0) = 1920
> readahead-backwards(dev=0:16, ino=3948605, req=4283+1, ra=2364+1920-0, async=0) = 1920
> readahead-backwards(dev=0:16, ino=3948605, req=2363+1, ra=444+1920-0, async=0) = 1920
> readahead-backwards(dev=0:16, ino=3948605, req=443+1, ra=0+444-0, async=0) = 444
> 
> CC: Andi Kleen <andi@...stfloor.org>
> CC: Li Shaohua <shaohua.li@...el.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
  Someone already mentioned this earlier and I don't think I've seen a
response: Do you have a realistic usecase for this? I don't think I've ever
seen an application reading file backwards...

> --- linux-next.orig/include/linux/fs.h	2011-11-29 20:55:27.000000000 +0800
> +++ linux-next/include/linux/fs.h	2011-11-29 20:57:07.000000000 +0800
...
> @@ -676,6 +677,20 @@ ondemand_readahead(struct address_space 
>  	}
>  
>  	/*
> +	 * backwards reading
> +	 */
> +	if (offset < ra->start && offset + req_size >= ra->start) {
> +		ra->pattern = RA_PATTERN_BACKWARDS;
> +		ra->size = get_next_ra_size(ra, max);
> +		max = ra->start;
> +		if (ra->size > max)
> +			ra->size = max;
> +		ra->async_size = 0;
> +		ra->start -= ra->size;
  IMHO much more obvious way to write this is:
ra->size = get_next_ra_size(ra, max);
if (ra->size > ra->start) {
  ra->size = ra->start;
  ra->start = 0;
} else
  ra->start -= ra->size;


								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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