linux-kernel - Re: [PATCH] mm readahead: Fix sys_readahead breakage by reverting 2MB limit (bug 79111)

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Message-ID: <53B5A769.5030108@linux.vnet.ibm.com>
Date:	Fri, 04 Jul 2014 00:26:41 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	David Cohen <david.a.cohen@...ux.intel.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Damien Ramonda <damien.ramonda@...el.com>,
	Jan Kara <jack@...e.cz>, David Rientjes <rientjes@...gle.com>,
	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm readahead: Fix sys_readahead breakage by reverting
 2MB limit (bug 79111)

On 07/04/2014 12:23 AM, Linus Torvalds wrote:
> On Thu, Jul 3, 2014 at 11:38 AM, Raghavendra K T
> <raghavendra.kt@...ux.vnet.ibm.com> wrote:
>>
>> Okay, how about something like 256MB? I would be happy to send a patch
>> for that change.
>
> I'd like to see some performance numbers. I know at least Fedora uses
> "readahead()" in the startup scripts, do we have any performance
> numbers for that?
>
> Also, I think 256MB is actually excessive. People still do have really
> slow devices out there. USB-2 is still common, and drives that read at
> 15MB/s are not unusual. Do we really want to do readahead() that can
> take tens of seconds (and *will* take tens of seconds sycnhronously,
> because the IO requests fill up).
>
> So I wouldn't go from 2 to 256. That seems like an excessive jump. I
> was more thinking in the 4-8MB range. But even then, I think we should
> always have technical reasons (ie preferably numbers) for the change,
> not just randomly change it.

Okay. I 'll take some time to do the analysis. I think we also should
keep in mind of possible remote readahead that would cause unnecessary
penalty.





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