linux-kernel - Re: Old O

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Message-ID: <20150106020445.GB23775@node.dhcp.inet.fi>
Date:	Tue, 6 Jan 2015 04:04:45 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Leon Pollak <leon.pollak@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: Old O_DIRECT story

On Mon, Jan 05, 2015 at 03:52:10PM +0000, One Thousand Gnomes wrote:
> > I tried in all ways to implement this with mmap(), but it does not success,
> > because I did not find a way to mmap() file as O_WRONLY. Mapping as O_RDWR
> > makes kernel to pre-fill mapped memory with partition data. So, kernel and
> > DMA actually compete on the RAM area to fill it - one with garbage, one
> > with actual data. Kernel wins.
> > 
> > So, how to implement Linus's advice?
> 
> Use O_DIRECT. There are lots of problems with the mmap() model, in
> particular with how mmu table changes scale to large numbers of CPU
> threads (ie they don't).

They do. Kinda. See split page table lock.
But, yeah, mmap() approach should not be faster anyway.

-- 
 Kirill A. Shutemov
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