linux-kernel - Re: [RFC][PATCH v3] readahead: introduce O_RANDOM for POSIX_FADV

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Date:	Tue, 5 Jan 2010 10:46:09 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Quentin Barnes <qbarnes+nfs@...oo-inc.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Steven Whitehouse <swhiteho@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jonathan Corbet <corbet@....net>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [RFC][PATCH v3] readahead: introduce O_RANDOM for 
	POSIX_FADV_RANDOM

On Mon, Jan 4, 2010 at 9:16 PM, Wu Fengguang <fengguang.wu@...el.com> wrote:
> Hi Minchan,
>
> On Mon, Jan 04, 2010 at 01:20:49PM +0800, Minchan Kim wrote:
>> > --- linux.orig/mm/readahead.c   2010-01-04 12:39:29.000000000 +0800
>> > +++ linux/mm/readahead.c        2010-01-04 12:39:30.000000000 +0800
>> > @@ -501,6 +501,12 @@ void page_cache_sync_readahead(struct ad
>> >        if (!ra->ra_pages)
>> >                return;
>> >
>> > +       /* be dumb */
>> > +       if (filp->f_flags & O_RANDOM) {
>> > +               force_page_cache_readahead(mapping, filp, offset, req_size);
>> > +               return;
>> > +       }
>> > +
>>
>> Let me have a dumb question. :)
>>
>> How about testing O_RANDOM in front of ra_pages testing?
>>
>> My intention is that although we turn off ra, it would be better to read
>> contiguous block all at once than readpage() callback doing I/O
>> one page at a time.
>>
>> Is it break some semantics or happen some problem in ondemand readahead?
>
> Yes it will have some problem with shrink_readahead_size_eio(), which
> want to disable readahead and use ->readpage() when ra_pages==0.
>
> Do you have specific use case in mind? The file systems that set
> ra_pages=0 seems to don't need readahead, too.

Never mind. It's just out of curiosity. :)

I thought although user disable readahead, we could enhance file I/O
with one readpages not multiple readpage if we know the user want to
read big contiguous blocks.

But I though it break current readahead off semantics. right?

Thanks for reply about my dumb question, Wu. :)

>
> Thanks,
> Fengguang
>



-- 
Kind regards,
Minchan Kim
--
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