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

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Date:	Mon, 04 Jan 2010 11:57:16 -0700
From:	Andreas Dilger <adilger@....com>
To:	Quentin Barnes <qbarnes@...oo-inc.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"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>
Subject: Re: [RFC][PATCH v3] readahead: introduce O_RANDOM for POSIX_FADV_RANDOM

On 2010-01-04, at 09:50, Quentin Barnes wrote:
> On Sun, Jan 03, 2010 at 11:33:28PM -0800, Christoph Hellwig wrote:
>> On Mon, Jan 04, 2010 at 04:17:19PM +1100, Stephen Rothwell wrote:
>>>> @@ -80,6 +80,10 @@
>>>> #define O_NDELAY	O_NONBLOCK
>>>> #endif
>>>>
>>>> +#ifndef O_RANDOM
>>>> +#define O_RANDOM	010000000	/* random access pattern hint */
>>>> +#endif
>>>
>>> This value conflicts with O_CLOEXEC on alpha and parisc and  
>>> O_NOATIME on
>>> sparc.
>>
>> Also when I tried to use this value for O_RSYNC and tested it I could
>> not actually see it getting propagated by the open code.
>>
>> Eitherway I don't think an O_ value is a good idea for a simple  
>> access
>> pattern hint.
>
> I was surprised by Wu's O_RANDOM approach, but after thinking about
> it, I liked it.  I'm used to seeing (on non-UNIX OSes) a parameter
> as part of the open syscall that announces to the OS what the app's
> access strategy through that file descriptor will be for that file.
> An issue with the current fadvise(2) approach is for random access
> files it necessitates two syscalls (open plus fadvise) for what
> could be or should be only one syscall (open).


Given that syscall overhead is very minimal, especially since fadvise  
is only setting some in-memory state and doesn't have to flush cache  
or anything, I don't see that as a significant reason to consume an O_  
flag.  Those flags are essentially limited to 32-bit values, or 32-bit  
applications wouldn't be able to use all of the flags, and we are  
already into the mid-20's of bits.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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