Hi! On 11:15 Sat 10 Sep , Zheng Da wrote: > On Sat, Sep 10, 2011 at 3:35 AM, Michael Blizek > <michi1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > >> I need to allocate fairly a large piece of memory, about 128KB or > >> 256KB. I tried to use kmalloc, and it sometimes fails, but vmalloc in > >> this case usually still work. > >> But I'm not sure how costly vmalloc is. As far as I know, vmalloc > >> needs to change the page table, and thus might need to invalidate TLB. > >> It seems quite expensive. If it is, maybe I can allocate memory with > >> kmalloc first, and then try to use vmalloc if kmalloc fails. Any > >> suggestions? > > > > If it is at all possible, I would suggest avoiding to allocate more than > > PAGE_SIZE (usually 4KB) in one continuous chunk. You can use e.g. > > scatterlists (which are basically lists of pointers) if you need a bigger > > buffer. > Actually, I have seen some problems when allocating large pieces of > memory constantly. Occasionally, I see alloc_pages and kmalloc fails > to allocate the memory I want. This does not really suprise me. Once the memory fragments, allocating large continuous memory regions gets harder. > The whole point of allocating a large chunk of memory is to avoid > extra memory copy because I need to run decompression algorithms on > it. In this case scatterlists solves 2 problems at once. First, you will not need to allocate large continuous memory regions. Second, you avoid wasting memory. > Or memory copy is cheaper? This is hard to say. But by passing the scatterlist you should be able to avoid the copying. > I'm also thinking of using slab allocator. The size of memory I need > to allocate can change from 20KB to 128KB. Then I need quite a few > slab allocators. The slab allocator is good if you have a large number of allocations which are the same size. To me it does not really sound like a good idea to use the slab allocator in your case. -Michi -- programing a layer 3+4 network protocol for mesh networks see http://michaelblizek.twilightparadox.com _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
