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Date: Thu, 3 Jul 2014 11:53:57 -0700
Message-ID: <CA+55aFyqK90YJkjtHR2QGFt4Mvn=mj8a4FkB_8nbTTj3=jp3NA@mail.gmail.com>
Subject: Re: [PATCH] mm readahead: Fix sys_readahead breakage by reverting 2MB
 limit (bug 79111)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Fengguang Wu <fengguang.wu@intel.com>,
        David Cohen <david.a.cohen@linux.intel.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Damien Ramonda <damien.ramonda@intel.com>, Jan Kara <jack@suse.cz>,
        David Rientjes <rientjes@google.com>,
        Nishanth Aravamudan <nacc@linux.vnet.ibm.com>,
        linux-mm <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Jul 3, 2014 at 11:38 AM, Raghavendra K T
<raghavendra.kt@linux.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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