linux-ext4 - Re: [RFC 0/3] Add madvise(..., MADV

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Date:	Wed, 07 Aug 2013 10:40:52 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Jan Kara <jack@...e.cz>
CC:	Andy Lutomirski <luto@...capital.net>, linux-mm@...ck.org,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/3] Add madvise(..., MADV_WILLWRITE)

On 08/07/2013 06:40 AM, Jan Kara wrote:
>   One question before I look at the patches: Why don't you use fallocate()
> in your application? The functionality you require seems to be pretty
> similar to it - writing to an already allocated block is usually quick.

One problem I've seen is that it still costs you a fault per-page to get
the PTEs in to a state where you can write to the memory.  MADV_WILLNEED
will do readahead to get the page cache filled, but it still leaves the
pages unmapped.  Those faults get expensive when you're trying to do a
couple hundred million of them all at once.
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