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

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Message-ID: <20121026065855.GA9179@localhost>
Date:	Fri, 26 Oct 2012 14:58:55 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Ni zhan Chen <nizhan.chen@...il.com>
Cc:	Dave Chinner <david@...morbit.com>,
	YingHang Zhu <casualfisher@...il.com>,
	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state

>  static void shrink_readahead_size_eio(struct file *filp,
>                                         struct file_ra_state *ra)
>  {
> -       ra->ra_pages /= 4;
> +       spin_lock(&filp->f_lock);
> +       filp->f_mode |= FMODE_RANDOM;
> +       spin_unlock(&filp->f_lock);
> 
> As the example in comment above this function, the read maybe still
> sequential, and it will waste IO bandwith if modify to FMODE_RANDOM
> directly.

Yes immediately disabling readahead may hurt IO performance, the
original '/ 4' may perform better when there are only 1-3 IO errors
encountered.

Thanks,
Fengguang
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