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    SubjectRe: [RFC] genalloc != generic DEVICE memory allocator
    On Thursday 22 December 2005 20:18, Andrey Volkov wrote:
    > Hi Jes,
    > Jes Sorensen wrote:
    > >>>>>>"Andrey" == Andrey Volkov <> 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?

    Yes. rheap seems to cover your cases...

    > --
    > Regards
    > Andrey Volkov
    > _______________________________________________
    > Linuxppc-embedded mailing list


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