linux-kernel - Re: Old O

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Date:	Tue, 6 Jan 2015 09:53:40 +0200
From:	Leon Pollak <leon.pollak@...il.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: Old O_DIRECT story

IMHO(!):
1. It will be slower, as pre-fill takes its significant time.
2. DMA-Kernel competition makes this method unusable.

On 6 January 2015 at 04:04, Kirill A. Shutemov <kirill@...temov.name> wrote:
> 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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