Re: [PATCH 28/33] readahead: loop case

[Limin Wang]

If the loopback files is bigger than the memory size, it may cause miss again and
may better to turn on the read ahead?


Regards,
Limin
* Wu Fengguang <wfg@xxxxxxxxxxxxxxxx> [2006-05-24 19:13:14 +0800]:

> Disable look-ahead for loop file.
> 
> Loopback files normally contain filesystems, in which case there are already
> proper look-aheads in the upper layer, more look-aheads on the loopback file
> only ruins the read-ahead hit rate.
> 
> Signed-off-by: Wu Fengguang <wfg@xxxxxxxxxxxxxxxx>
> ---
> 
> I'd like to thank Tero Grundstr?m for uncovering the loopback problem.
> 
>  drivers/block/loop.c |    6 ++++++
>  1 files changed, 6 insertions(+)
> 
> --- linux-2.6.17-rc4-mm3.orig/drivers/block/loop.c
> +++ linux-2.6.17-rc4-mm3/drivers/block/loop.c
> @@ -779,6 +779,12 @@ static int loop_set_fd(struct loop_devic
>  	mapping = file->f_mapping;
>  	inode = mapping->host;
>  
> +	/*
> +	 * The upper layer should already do proper look-ahead,
> +	 * one more look-ahead here only ruins the cache hit rate.
> +	 */
> +	file->f_ra.flags |= RA_FLAG_NO_LOOKAHEAD;
> +
>  	if (!(file->f_mode & FMODE_WRITE))
>  		lo_flags |= LO_FLAGS_READ_ONLY;
>  
> 
> --
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