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A Better Signal()POSIX Sigaction() spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth ADRIAN PERRIG & TORSTEN HOEFLER Oldskool: signal() Networks and Operating Systems (252-0062-00) Chapter 5: Memory Management void (*signal(int sig, void (*handler)(int))) (int); . Unpacking this: . A handler looks like void my_handler(int); . Signal takes two arguments… An integer (the signal type, e.g., SIGPIPE) A pointer to a handler function http://support.apple.com/kb/HT5642 (Jul. 2015) . … and returns a pointer to a handler function “Description: The iOS kernel has checks to validate that the user-mode The previous handler, pointer and length passed to the copyin and copyout functions would not result in a user-mode process being able to directly access kernel memory. The checks were not being used if the length was smaller than . “Special” handler arguments: one page. This issue was addressed through additional validation of the . SIG_IGN (ignore), SIG_DFL (default), SIG_ERR (error code) arguments to copyin and copyout.” 2 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Unix signal handlers Implications . Signal handler can be called at any time! . There is very little you can safely do in a signal handler! . Can’t safely access program global or static variables . Executes on the current user stack . Some system calls are re-entrant, and can be called . If process is in kernel, may need to retry current system call . Many C library calls cannot (including _r variants!) . Can also be set to run on a different (alternate) stack . Can sometimes execute a longjmp if you are careful . With signal, cannot safely change signal handlers… User process is in undefined state when signal delivered . What happens if another signal arrives? 3 4 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Multiple signals A better signal()POSIX sigaction() . If multiple signals of the same type are to be delivered, Unix will New action for discard all but one. signal signo #include <signal.h> . If signals of different types are to be delivered, Unix will deliver Previous action them in any order. int sigaction(int signo, is returned const struct sigaction *act, struct sigaction *oldact); . Serious concurrency problem: Signal How to make sense of this? struct sigaction { handler void (*sa_handler)(int); Signals to be blocked in this sigset_t sa_mask; handler (cf., fd_set) int sa_flags; void (*sa_sigaction)(int, siginfo_t *, void *); }; More sophisticated signal handler (depending on flags) 5 6 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Signals as upcalls Our Small Quiz . Particularly specialized (and complex) form of Upcall . True or false (raise hand) . Kernel RPC to user process . Mutual exclusion on a multicore can be achieved by disabling interrupts . Test and set can be used to achieve mutual exclusion . Other OSes use upcalls much more heavily . Test and set is more powerful than compare and swap . The CPU retries load-linked/store conditional instructions after a conflict . Including Barrelfish . The best spinning time is 2x the context switch time . “Scheduler Activations”: dispatch every process using an upcall instead of return . Priority inheritance can prevent priority inversion . The receiver never blocks in asynchronous IPC . Very important structuring concept for systems! . The sender blocks in synchronous IPC if the receiver is not ready . A pipe file descriptor can be sent to a different process . Pipes do not guarantee ordering . Named pipes in Unix behave like files . A process can catch all signals with handlers . Signals always trigger actions at the signaled process . One can implement a user-level tasking library using signals . Signals of the same type are buffered in the kernel 7 8 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Goals of Memory Management . Allocate physical memory to applications . Protect an application’s memory from others . Allow applications to share areas of memory . Data, code, etc. Create illusion of a whole address space . Virtualization of the physical address space . Create illusion of more memory than you have . Tomorrow – demand paging . Book: OSPP Chapter 8 9 http://en.wikipedia.org/wiki/Imagery 10 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth In CASP last semester we saw: What’s new this semester? . Assorted uses for virtual memory . Wider range of memory management hardware . x86 paging . Base/limit, segmentation . Page table format . Inverted page tables, etc. Translation process . How the OS uses the hardware . Translation lookaside buffers (TLBs) . Demand paging and swapping . Interaction with caches . Page replacement algorithms . Performance implications . Frame allocation policies . For application code, e.g., matrix multiply 11 12 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Terminology Memory management . Physical address: address as seen by the memory unit 1. Allocating physical addresses to applications . Virtual or Logical address: address issued by the processor 2. Managing the name translation of virtual addresses to physical . Loads addresses . Stores 3. Performing access control on memory access . Instruction fetches . Possible others (e.g., TLB fills)… . Functions 2 & 3 usually involve the hardware Memory Management Unit (MMU) 13 14 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Base and Limit Registers . A pair of base and limit registers define the logical address space 0x0000000 Operating System Simple scheme: partitioned memory 0x1000000 Process 0x5600ba0 0x5600ba0 base Process 0x39ef470 0x8ff0010 Process limit 0xB000000 0xfffffff 15 16 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Issue: address binding Dynamic relocation using a relocation register . Base address isn’t known until load time . Options: 1. Compiled code must be completely position-independent, or 2. Relocation Register maps compiled addresses dynamically to physical addresses Relocation register 14000 Logical Physical address address CPU + Memory 346 14346 MMU 17 18 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Contiguous allocation Hardware Support for Relocation and Limit Registers . Main memory usually into two partitions: . Resident OS, usually in low memory with interrupt vector . User processes in high memory . Relocation registers protect user processes from 1. each other Limit Relocation 2. changing operating-system code and data register register . Registers: . Base register contains value of smallest physical address Logical Physical . Limit register contains range of logical addresses address yes address CPU Memory each logical address must be less than the limit register < + . MMU maps logical address dynamically no trap: addressing error 19 20 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Base & Limit summary . Simple to implement (addition & compare) . Physical memory fragmentation . Only a single contiguous address range . How to share data between applications? . How to share program text (code)? . How to load code dynamically? . Total logical address space ≤ physical memory Segmentation 21 22 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Segmentation User’s View of a Program . Generalize base + limit: . Physical memory divided into segments . Logical address = (segment id, offset) stack . Segment identifier supplied by: shared library . Explicit instruction reference . Explicit processor segment register symbol . Implicit instruction or process state table heap main program logical address 23 24 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Segmentation Reality Segmentation Hardware 1 3 limit base 2 1 2 Segment 3 table CPU s d Physical Memory 0 4 yes address 4 < + 0 no logical address trap: addressing error physical memory 25 26 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Segmentation Reality Segmentation Architecture 6000 . Segment table – each entry has: 1 . base – starting physical address of segment 5000 . limit – length of the segment 3 2 4600 . Segment-table base register (STBR) limit base 1 0 300 1500 . Current segment table location in memory 1 1000 5000 3800 2 400 3400 2 . Segment-table length register (STLR) 3 3 400 4600 3400 4 1000 1800 . Current size of segment table segment 2800 table segment number s is legal if s < STLR 0 4 4 1800 0 1500 logical address physical memory 27 28 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Segmentation Summary . Fast context switch . Simply reload STBR/STLR . Fast translation . 2 loads, 2 compares . Segment table can be cached . Segments can easily be shared . Segments can appear in multiple segment tables Paging . Physical layout must still be contiguous . (External) fragmentation still a problem 29 30 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Paging Page table jargon . Solves contiguous physical memory problem . Page tables maps VPNs to PFNs . Process can always fit if there is available free memory . Page table entry = PTE . Divide physical memory into frames . VPN = Virtual Page Number . Size is power of two, e.g., 4096 bytes . Upper bits of virtual or logical address . Divide logical memory into pages of the same size . PFN = Page Frame Number . For a program of n pages in size: . Upper bits of physical or logical address . Find and allocate n frames . Same number of bits (usually). Load program . Set up page table to translate logical pages to physical frames 31 32 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Recall: P6 Page tables (32bit) x86-64 Paging Virtual address 9 9 9 9 12 20 12 VPN1 VPN2 VPN3 VPN4 VPO Logical address: VPN VPO Page Directory Page 20 12 Pointer VPN1 VPN2 PPN PPO Page Map Directory Page Table Table Table Table PDE p=1 PTE p=1 data PDBR PM4LE PDPE PDE PTE Page Page table Data page directory BR . Pages, page directories, page tables all 4kB 40 12 PPN PPO Physical address 33 34 spcl.inf.ethz.ch spcl.inf.ethz.ch @spcl_eth @spcl_eth Problem: performance Translating with the P6 TLB . Every logical memory access needs more than two physical memory accesses 1. Partition VPN into . Load page table entry → PFN CPU TLBT and TLBI. Load desired location 2. Is the PTE for VPN 20 12 virtual address .
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