linux-kernel - Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra

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Date:	Thu, 25 Oct 2012 11:12:33 +0800
From:	YingHang Zhu <casualfisher@...il.com>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ni zhan Chen <nizhan.chen@...il.com>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state

On Thu, Oct 25, 2012 at 10:58 AM, Fengguang Wu <fengguang.wu@...el.com> wrote:
> Hi Chen,
>
>> But how can bdi related ra_pages reflect different files' readahead
>> window? Maybe these different files are sequential read, random read
>> and so on.
>
> It's simple: sequential reads will get ra_pages readahead size while
> random reads will not get readahead at all.
>
> Talking about the below chunk, it might hurt someone that explicitly
> takes advantage of the behavior, however the ra_pages*2 seems more
> like a hack than general solution to me: if the user will need
> POSIX_FADV_SEQUENTIAL to double the max readahead window size for
> improving IO performance, then why not just increase bdi->ra_pages and
> benefit all reads? One may argue that it offers some differential
> behavior to specific applications, however it may also present as a
> counter-optimization: if the root already tuned bdi->ra_pages to the
> optimal size, the doubled readahead size will only cost more memory
> and perhaps IO latency.
I agree, we should choose the reasonable solution here.

Thanks,
     Ying Zhu
>
> --- a/mm/fadvise.c
> +++ b/mm/fadvise.c
> @@ -87,7 +86,6 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice)
>                 spin_unlock(&file->f_lock);
>                 break;
>         case POSIX_FADV_SEQUENTIAL:
> -               file->f_ra.ra_pages = bdi->ra_pages * 2;
>                 spin_lock(&file->f_lock);
>                 file->f_mode &= ~FMODE_RANDOM;
>                 spin_unlock(&file->f_lock);
>
> Thanks,
> Fengguang
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