linux-kernel - Re: [RFC 0/3] Add madvise(..., MADV

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Message-ID: <CALCETrVZBpZDQ7-QjDYYQyGGtCEAk-ydb0DUUA9gcFtj4JYv6w@mail.gmail.com>
Date:	Thu, 8 Aug 2013 17:11:30 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Jan Kara <jack@...e.cz>
Cc:	Dave Hansen <dave.hansen@...el.com>, linux-mm@...ck.org,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/3] Add madvise(..., MADV_WILLWRITE)

On Thu, Aug 8, 2013 at 12:25 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> Whoops -- I read your email too quickly.  I haven't tried
> MADV_WILLNEED, but I think I tried reading each page to fault them in.
>  Is there any reason to expect MADV_WILLNEED to do any better?  I'll
> try to do some new tests to see how well this all works.
>
> (I imagine that freshly fallocated files are somehow different when
> read, since there aren't zeros on the disk backing them until they get
> written.)

Well, this will teach me to write code based on an old benchmark from
memory.  It seems that prefaulting for read is okay on Linux 3.9 --
latencytop isn't do_wp_page or ext4* at all, at least not for the last
couple minutes on my test box.

I wonder if ext4 changed its handling of fallocated extents somewhere
between 3.5 and 3.9.  In any case, please consider these patches
withdrawn for the time being.

* With file_update_time stubbed out.  I need to dust off my old
patches to fix that part.
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