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

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Message-ID: <CA+55aFyqK90YJkjtHR2QGFt4Mvn=mj8a4FkB_8nbTTj3=jp3NA@mail.gmail.com>
Date:	Thu, 3 Jul 2014 11:53:57 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
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 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.

           Linus
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