LddKernelMemory < Training2

%SECTION0{name=LddKernelMemory}% Allocating Memory

   * simplest form =kmalloc(size, flags)=
      * flags see =linux/gfp.h=
      * may sleep =GFP_KERNEL=, or not =GFP_ATOMIC=
      * fast, continuous, doesn't clear (security? -> =kzalloc=)
      * free with =kfree(ptr)=

   * more memory needed? allocate pages:
      * =get_zeroed_page(flags)=
      * =__get_free_page(flags) /* non zeroed */=
      * =__get_free_pages(flags, order)=
         * allocate 2^order pages
         * calculate oder from size: =get_order(size)=
      * freeing
         * =free_page(ptr)=
         * =free_pages(ptr, order)=
      * advantages: slightly faster and avoids fragementation

   * =vmalloc= / =vfree=
      * above functions allocate physically consecutive memory
      * =vmalloc= allocated memory is only *virtually* consecutive
      * cons:
         * slower because page tables need to be setup
         * not usable from outside of CPU (DMA)
      * pro:
         * large allocation less likely to fail due to fragementation
      * -> ok for e.g. large software only buffer 

   * Optimizations (premature opt ...)
      * Situation: driver allocates many objects of the same size over and over again
         * lookaside cache (see slab cache in !DocBook "Memory Management in Linux")
         * nice: cache usage information in /proc/slabinfo
      * Sitation: memory allocation is not allowed to fail
         * mempool can be used for reserving memory in emergencies
         * Usually bad: memory is idle. Better deal with allocation failures
5.7.4.4. Other locking techniques		1. Denx Training Topics		5.7.6. How to delay execution
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