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

Limin Wang
2006-05-24T14:01:46Z

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@mail.ustc.edu.cn> [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@mail.ustc.edu.cn>
> ---
>
> 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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