linux-kernel - Re: [PATCH 12/15] drivers: add Contiguous Memory Allocator

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Date:	Sun, 5 Feb 2012 12:25:40 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-media@...r.kernel.org, linux-mm@...ck.org,
	linaro-mm-sig@...ts.linaro.org,
	Michal Nazarewicz <mina86@...a86.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Russell King <linux@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hillf Danton <dhillf@...il.com>
Subject: Re: [PATCH 12/15] drivers: add Contiguous Memory Allocator

On Fri, Feb 3, 2012 at 8:18 PM, Marek Szyprowski
<m.szyprowski@...sung.com> wrote:
> The Contiguous Memory Allocator is a set of helper functions for DMA
> mapping framework that improves allocations of contiguous memory chunks.
>
> CMA grabs memory on system boot, marks it with CMA_MIGRATE_TYPE and
> gives back to the system. Kernel is allowed to allocate movable pages
> within CMA's managed memory so that it can be used for example for page
> cache when DMA mapping do not use it. On dma_alloc_from_contiguous()
> request such pages are migrated out of CMA area to free required
> contiguous block and fulfill the request. This allows to allocate large
> contiguous chunks of memory at any time assuming that there is enough
> free memory available in the system.
>
> This code is heavily based on earlier works by Michal Nazarewicz.
>
> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park@...sung.com>
> CC: Michal Nazarewicz <mina86@...a86.com>
> Acked-by: Arnd Bergmann <arnd@...db.de>
> Tested-by: Rob Clark <rob.clark@...aro.org>
> Tested-by: Ohad Ben-Cohen <ohad@...ery.com>
> Tested-by: Benjamin Gaignard <benjamin.gaignard@...aro.org>
> ---

[...]

> +/*
> + * Contiguous Memory Allocator
> + *
> + *   The Contiguous Memory Allocator (CMA) makes it possible to
> + *   allocate big contiguous chunks of memory after the system has
> + *   booted.
> + *
> + * Why is it needed?
> + *
> + *   Various devices on embedded systems have no scatter-getter and/or
> + *   IO map support and require contiguous blocks of memory to
> + *   operate.  They include devices such as cameras, hardware video
> + *   coders, etc.
> + *
> + *   Such devices often require big memory buffers (a full HD frame
> + *   is, for instance, more then 2 mega pixels large, i.e. more than 6
> + *   MB of memory), which makes mechanisms such as kmalloc() or
> + *   alloc_page() ineffective.
> + *
> + *   At the same time, a solution where a big memory region is
> + *   reserved for a device is suboptimal since often more memory is
> + *   reserved then strictly required and, moreover, the memory is
> + *   inaccessible to page system even if device drivers don't use it.
> + *
> + *   CMA tries to solve this issue by operating on memory regions
> + *   where only movable pages can be allocated from.  This way, kernel
> + *   can use the memory for pagecache and when device driver requests
> + *   it, allocated pages can be migrated.
> + *

Without boot mem reservation, what is the successful rate of CMA to
serve requests of 1MiB, 2MiB, 4MiB and 8MiB chunks?
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