Real Time Operating Systems Lecture 25 1 Memory Manager http users ece utexas edu valvano arm Heap 4F120 zip Definitions Internal fragmentation convenient of the operating system contains no information wasted in order to improve speed or provide for a simpler implementation wasted storage is inside the allocated region External fragmentation largest block that can be allocated is less than the total amount of free space occurs because memory is allocated in contiguous blocks occurs over time as free storage becomes divided into many small pieces Worse when application allocates deallocates blocks of storage of varying sizes unusable storage is outside the allocated regions Explain what happens if char v1 short v2 char v3 long v4 Think of the four storage categories give examples of each Private scope temporary allocation Private scope permanent allocation Public scope temporary allocation Public scope permanent allocation heap large piece of memory managed by the operating system used for temporary allocation initialization Heap Init called by OS during the initialization process allocation Heap Malloc called by user or OS deallocation Heap Free called by user or OS statically allocated storage assigned by the compiler 2000 byte heap static long Heap 500 Real Time Operating Systems Lecture 25 2 Initial heap Heap 498 498 Figure 3 7 An initial heap of 2000 bytes is one block of 498 words each box is 32 bits The user or OS itself calls Heap Malloc when it needs a contiguous blocks of memory If the memory is needed for a long time a pointer to it should be stored in permanent memory For example if a 20 byte buffer is needed we could call char Pt void UserStart void called at the beginning Pt Heap Malloc 20 When the program is finished with the block it is released by calling Heap Free void UserFinish void called at the end Heap Free Pt Checkpoint 3 7 What happens if a function allocates a block stores a pointer to the block in a local variable and then returns from the function without deallocating the block Saving the pointer to an allocated block in a local variable does not make sense If the memory is needed for the duration of one function call the block should be allocated on the stack For example if a 20 byte buffer is needed we could call void User void char buffer 20 use 20 byte buffer Free block Used block Header 100 Header 5 Trailer 5 Trailer 32 bits Figure 3 8 Each block has a header and a trailer 100 Real Time Operating Systems Lecture 25 3 When allocating blocks we can use a number of algorithms to choose which block to allocate Let n be the number of bytes requested by Heap Malloc First fit uses the first free block with a size greater than or equal to n Best fit uses the smallest free block with a size greater than or equal to n Worst fit uses the largest free block with a size greater than or equal to n Depending on the allocation pattern of the user program these three allocation methods will have differing levels of external fragmentation The implementation on the book web site as Heap xxx zip uses first fit Checkpoint 3 8 How would you change the way free blocks are organized to implement best fit block is allocated with Heap Malloc find a free block free block is divided to two parts new free block is smaller a pointer to the allocated block is returned block may not be large enough to split allocate the big block internal frag Before Header 100 After Pt 20 Pt Heap Malloc 40 20 78 Trailer 100 78 Figure 3 9 Example the user calls Pt Heap Malloc 80 Checkpoint 3 9 In Figure 3 9 why does the sum of the parts not equal the whole In particular 20 78 does not equal 100 When deallocating a block with Heap Free there are four cases no merge merge above merge below and merge both above and below Real Time Operating Systems Lecture 25 4 Before 10 Pt 10 20 After 10 Heap Free Pt 10 20 Used Free 20 10 20 10 10 10 Figure 3 10 Example the user calls Heap Free Pt no merge Before 10 Pt 10 20 After 32 Heap Free Pt Free Used 20 10 32 10 10 10 Figure 3 11 Two blocks are merged during a call to Heap Free Checkpoint 3 10 What happens if you continue to access a memory block after the block is deallocated The Knuth buddy allocation maintains the heap as a collection of blocks each with a size of 2m When the user requests a block of size n it will find the smallest block with 2m greater than or equal to n For example if the smallest block is size 1024 and the user requests a block of 100 bytes the 1024 byte block will be divided into two 128 byte blocks one 256 byte block and one 512 byte blocks The user will be given the 128 byte block The 28 extra bytes allocated to the user is internal fragmentation Real Time Operating Systems Lecture 25 5 Final exam 2010 20 Question 8 Implement a memory manager for fixed sized blocks Let the block size be define SIZE 100 size in bytes Let the number of blocks be define NUM 10 number of blocks For these two definitions 1000 bytes of memory will be managed Create three functions initialization allocate and deallocate