Memory and Interrupts Memory Management Exception/Interrupt Handling ���������������
Outline 2 ■ Real�Time & Embedded Memory Management □ Retro: Classical Approaches □ Retro: Classical Approaches □ Challenges & Requirements □ Dynamic Allocation Algorithms ■ Exceptions and Interrupts □ Programmable Interrupt Controller □ Interrupt Processing □ Interrupt Processing □ Interrupt Nesting Memory and Interrupts | Matthias Richly | 19. Mai 2010
Classical Approaches 3 Paging & Swapping ■ break virtual memory in eqally sized pages ■ break virtual memory in eqally sized pages ■ break physical memory in frames of the same size ■ map logical adresses to physical by mapping pages to frames ■ mapping done by HW (MMU) ■ pages may be swapped to disk; brought back into main memory, when needed Memory and Interrupts | Matthias Richly | 19. Mai 2010
MM�Challenges in Embedded & RT Systems 4 ■ missing HW support (MMU, TLB) ■ small memories ■ small memories ■ missing secondary storage ■ swapping pages from disc into memory in case of a page fault takes much longer than usual memory access □ unpredictable ■ garbage collection? ■ garbage collection? □ RT Java … Memory and Interrupts | Matthias Richly | 19. Mai 2010
Virtual Memory Locking 5 ■ swapping destroys hard real�time behavior ■ should be turned off for real�time applications ■ should be turned off for real�time applications ■ POSIX: □ mlock(), mlockall() ■ Windows: □ VirtualLock() □ VirtualLock() Memory and Interrupts | Matthias Richly | 19. Mai 2010
Real�Time Memory Management 6 ■ requirements: □ minimal overhead □ minimal overhead □ minimal fragmentation □ predictable behavior ◊ no swapping ◊ deterministic (timely bounded) allocation / deallocation of memory Memory and Interrupts | Matthias Richly | 19. Mai 2010
Fragmentation 7 ■ internal Fragmentation □ application gets more memory than requested because of □ application gets more memory than requested because of fixed block sizes ■ external Fragmentation □ non�contiguous free space: free blocks split across memory □ none may be able to satisfy memory request internal external Memory and Interrupts | Matthias Richly | 19. Mai 2010
Static vs. Dynamic Memory Allocation 8 static ■ segmentation ■ segmentation ■ fixed memory region for each task ■ predictable, but inflexible dynamic ■ memory allocation at runtime from global heap ■ allocator keeps track of used and free memory ■ allocator keeps track of used and free memory □ linked lists, bitmaps, static allocation array, segregated lists, buddy systems ■ difficult to design predictable ■ creates fragmentation � compaction / recombination of free blocks required Memory and Interrupts | Matthias Richly | 19. Mai 2010
Linked List / Bitmap 9 Linked List memory list blocks head ■ “first�fit”, “best�fit”, “worst�fit” Bitmap Bitmap ■ both ���� ■ recombination? Memory and Interrupts | Matthias Richly | 19. Mai 2010
Static Allocation Array 10 ■ integer array with special �������� : find neighboring blocks easier □ +x placed in first and last entry � x ���� blocks □ +x placed in first and last entry � x ���� blocks □ �x in first and 0 in last entry � x ��������� blocks ■ no. of array entries = no. of fixed size memory blocks ■ speed up search for free blocks by using a heap structure Memory and Interrupts | Matthias Richly | 19. Mai 2010
Static Allocation Array – Merging Free Blocks 11 ■ blocks in front can be merged, if positive number in array at index�of�current�block � 1 index�of�current�block � 1 ■ blocks behind can be merged, if positive number in array at index� of�current�block + number�of�blocks Memory and Interrupts | Matthias Richly | 19. Mai 2010
Fixed�size MM with Segregated Free Lists 12 ■ memory pools with fixed size blocks ■ block sizes and number of blocks per size statically assigned ■ block sizes and number of blocks per size statically assigned ■ no external fragmentation, internal fragmentation depends ■ ���� 32 list memory 56 heads blocks 128 ■ only useful for applications with well�known memory needs □ networking code, protocol stacks Memory and Interrupts | Matthias Richly | 19. Mai 2010
Buddy systems 13 ■ block sizes are powers of 2 (binary buddy) ■ block sizes 2 min � 2 max ■ block sizes 2 � 2 ■ (max�min) lists for free blocks ■ at beginning one block of size 2 max ■ on memory request for size s, s ≤ 2 max 1. calculate smallest k > min, such that 2 k ≥ s, i.e. k = log 2 s 2. look for smallest free j�block, j ≥ k 3. while j > k, split j�block � two blocks of size j=(j�1) arise, so called ������� called ������� 4. now j = k � return block ■ on deallocation, join buddies to get larger free blocks ■ ��������� ■ but high degree of fragmentation (internal & external) Memory and Interrupts | Matthias Richly | 19. Mai 2010
Buddy systems � Example 14 Memory and Interrupts | Matthias Richly | 19. Mai 2010
Questions so far? 15 MM end. Memory and Interrupts | Matthias Richly | 19. Mai 2010
Exceptions and Interrupts 16 ��������� ■ event, which disrupts the normal execution of the processor ■ event, which disrupts the normal execution of the processor ■ forces execution of special instructions in a privileged state ■ synchronous vs. asynchronous ��������� ■ asynchronous exception triggered by an external HW device ■ � a means to communicate between HW and application ■ � a means to communicate between HW and application ■ maskable Memory and Interrupts | Matthias Richly | 19. Mai 2010
Programmable Interrupt Controller (PIC) 17 ■ typically multiple sources of interrupts ■ PIC is a HW device ■ PIC is a HW device □ prioritizes the sources □ presents highest priority interrupt to core CPU HIGH MED MED Interrupt Interrupt Interrupt ISR Vector LOW Interrupt request line Memory and Interrupts | Matthias Richly | 19. Mai 2010
Interrupt Processing 18 ■ CPU checks for pending interrupts at the beginning of every instruction instruction ■ if interrupt was raised: □ save current processor state (PC, …) □ set PC to ISR address (calculated from interrupt vector) □ execute ISR ◊ mask other interrupts ◊ perform required operations on the device ◊ execute interrupt return instruction (iret) □ restore state and resume previous execution Memory and Interrupts | Matthias Richly | 19. Mai 2010
Nested Interrupts 19 ■ allow preemption of ISRs by higher�priority interrupts ■ ISRs need to preemptable ■ ISRs need to preemptable ■ reentrant ISRs ■ challenge: prevent stack overflows □ increase the tasks’ stack space □ better: use own stack for ISRs Memory and Interrupts | Matthias Richly | 19. Mai 2010
Interrupt Latency 20 ■ delay between interrupt raising and beginning of the ISRs execution execution ■ must be bounded in RT systems response time ISR time interrupt processing time latency request ■ sources: □ HW latency □ completion of running instruction □ handling of higher priority interrupts □ context saving Memory and Interrupts | Matthias Richly | 19. Mai 2010
Hints 21 ■ interrupt processing affects RT behavior ■ ISRs run at higher priority than all OS tasks ■ ISRs run at higher priority than all OS tasks ■ ISRs should be short □ re�enable interrupts as fast as possible □ consider deferring parts of the processing to some OS task(s) ■ don’t use blocking calls in ISRs Memory and Interrupts | Matthias Richly | 19. Mai 2010
References 22 � Real�Time Concepts for Embedded Systems. Qing Li, C. Yao. CMP Books Books � Memory Management for Embedded Systems. A. Polze, A. Rasche, B. Rabe. Betriebssysteme für Embedded Computing WT 07/08 � Interrupts and Exceptions. A. Polze, A. Rasche, B. Rabe. Betriebssysteme für Embedded Computing WT 07/08 � The Art of Computer Programming. Volume 1: Fundamental Algorithms, Second Edition. Knuth, D.E. Addison�Wesley, pp. 442� 445. 445. Memory and Interrupts | Matthias Richly | 19. Mai 2010
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