Pantelis Antoniou wrote: > On Thursday 22 December 2005 20:18, Andrey Volkov wrote: > >>Hi Jes, >> >>Jes Sorensen wrote: >> >>>>>>>>"Andrey" == Andrey Volkov <avolkov at 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? >> > > > Yes. rheap seems to cover your cases... > Agree, I couldn't see nothing better for a basement of generic dev. alloc.
So, it will be much better if it will be moved to lib/. Anyone have some more comments about subj. ? -- Regards Andrey Volkov
