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    SubjectRe: [RFC] genalloc != generic DEVICE memory allocator
    Hi Jes,

    Jes Sorensen wrote:
    >>>>>>"Andrey" == Andrey Volkov <avolkov@varma-el.com> writes:
    >
    >
    > Andrey> Hello Jes and all I try to use your allocator (gen_pool_xxx),
    > Andrey> idea of which is a cute nice thing. But current implementation
    > Andrey> of it is inappropriate for a _device_ (aka onchip, like
    > Andrey> framebuffer) memory allocation, by next reasons:
    >
    > Andrey,
    >
    > Keep in mind that genalloc was meant to be simple for basic memory
    > allocations. It was never meant to be an over complex super high
    > performance allocation mechanism.
    >
    > Andrey> 1) Device memory is expensive resource by access time and/or
    > Andrey> size cost. So we couldn't use (usually) this memory for the
    > Andrey> free blocks lists.
    >
    > This really is irrelevant, the space is only used within the object
    > when it's on the free list. Ie. if all memory is handed out there's
    > no space used for this purpose.

    I point out 2 reasons: ACCESS TIME was first :), let take very
    widespread case: PCI device with some onboard memory and any
    N GHz proc. - result may be terrible: each access to device mem (which
    usually uncached) will slowed down this super fast proc to 33 MHZ, i.e
    same as we made busy-wait with disabled interrupts after each read/write...

    I possible awry when use 'control structures' in 2), I've in view
    allocator's control structures (size/next etc), not device specific
    control structs.

    >
    > Andrey> 3) Obvious (IMHO) workflow of mem. allocator
    > Andrey> look like: - at startup time, driver allocate some big
    > Andrey> (almost) static mem. chunk(s) for a control/data structures.
    > Andrey> - during work of the device, driver allocate many small
    > Andrey> mem. blocks with almost identical size. such behavior lead to
    > Andrey> degeneration of buddy method and transform it to the
    > Andrey> first/best fit method (with long seek by the free node list).
    >
    > This is only really valid for network devices, and even then it's not
    > quite so. For things like uncached allocations your observation is
    > completely off.

    Could you give me some examples? Possible I overlooked something
    significant.

    >
    > For the case of more traditional devices, the control structures will
    > be allocated from one end of the block, the rest will be used for
    > packet descriptors which will be going in and out of the memory pool
    > on a regular basis.

    This was main reason why I try to modify genalloc: I needed in
    generic allocator for both short-live strictly aligned blocks and
    long-live blocks with restriction by size.

    > In most normal cases these will all be of the same
    > size and it doesn't matter where in the memory space they were
    > allocated.

    And thats also why I consider that 'buddy' is not appropriate to be
    'generic' (most cases == generic, isn't is :)?): when you're allocate
    mainly same sized blocks, 'buddy' degraded to the first-fit.

    Possible solution I see in mixed first-fit with lazy coalescent for
    short lived blocks and first-fit with immediately coalescent for
    long-lived blocks. But, again, I may overlook something significant.
    And, certainly, I could overlooked someone else allocator implementation
    in some driver.

    >
    > Andrey> 4) The simple binary buddy method is far away from perfect for
    > Andrey> a device due to a big internal fragmentation. Especially for a
    > Andrey> network/mfd devices, for which, size of allocated data very
    > Andrey> often is not a power of 2.
    >
    snip
    >
    > Andrey> I start to modify your code to satisfy above demands, but
    > Andrey> firstly I wish to know your, or somebody else, opinion.
    >
    > I honestly don't think the majority of your demands are valid.
    > genalloc was meant to be simple, not an ultra fast at any random
    > block size allocator. So far I don't see any reason for changing to
    > the allocation algorithm into anything much more complex - doesn't
    > mean there couldn't be a reason for doing so, but I don't think you
    > have described any so far.
    I disagree here, generic couldn't be very simple and slow, because in
    this case simply no one will be use it, and hence we'll get today's
    picture: reimplemented allocators in many drivers.

    >
    > You mentioned frame buffers, but what is the kernel supposed to do
    > with those allocation wise? If you have a frame buffer console, the
    > memory is allocated once and handed to the frame buffer driver.
    > Ie. you don't need a ton of on demand allocations for that and for
    > X, the memory management is handled in the X server, not by the
    > kernel.

    For video-only device this is true, but if device is a multifunctional,
    which is frequent case in embedded systems, then kernel must control of
    device memory allocation. Currently, however, even video cards for
    desktops become more and more multifunctional (VIVO/audio etc.).

    >
    > The only thing I think would make sense to implement is to allow it to
    > use indirect descriptor blocks for the memory it manages. This is not
    > because it's wrong to use the memory for the free list, as it will
    > only be used for this when the chunk is not in use, but because access
    > to certain types of memory isn't always valid through normal direct
    > access. Ie. if one used descriptor blocks residing in normal
    > GFP_KERNEL memory, it would be possible to use the allocator to manage
    > memory sitting on the other side of a PCI bus.
    I describe above, why we couldn't/wouldn't use onboard memory for
    allocator specific data.

    Pantelis, Am I answered to your question (...what are you trying to
    do...) too?

    --
    Regards
    Andrey Volkov
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