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Lecture III: OS Support

Lecture III: OS Support. CMPT 401 Dr. Alexandra Fedorova. The Role of the OS. The operating system needs to provide support for implementation of distributed systems We will look at how distributed systems services interact with the operating systems

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Lecture III: OS Support

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  1. Lecture III: OS Support CMPT 401 Dr. Alexandra Fedorova

  2. The Role of the OS • The operating system needs to provide support for implementation of distributed systems • We will look at how distributed systems services interact with the operating systems • We will discuss the support that the operating system needs to provide

  3. Direct Interaction with the OS • A process directly interacts with the OS via system calls • Example: a web browser, a web server Process:a DS component system calls OS

  4. Interaction via Middleware Layer Process:a DS component • A process directly interacts with the OS via a middleware layer • A middleware layer directly interacts with the OS • Example: a peer-to-peer file system implemented over a distributed hash table Function calls or IPC Middleware system calls OS

  5. Interaction via Inclusion OS DS component • A DS component is a part of the operating system, i.e., an operating system daemon • Example: Network File System (NFS) daemon • Runs as a kernel thread, shares address space with the kernel, interacts with the rest of the OS via function calls • Why would one want to build a DS component that interacts with the OS via inclusion?

  6. Infrastructure Provided by the OS • Networking • Interface to network devices • Implementation of common protocols: TPC, UDP, IP • Processes and threads • Efficient scheduling, load balancing and thread switching • Efficient thread synchronization • Efficient inter-process communication (IPC)

  7. The Need for Good Process/Thread Support • Many distributed applications are implemented using multiple threads or processes • Why?

  8. Motivation for Multithreaded Designs Multiple threads Single thread • Servers provide access to large data sets (web servers, e-commerce servers) • Even in the presence of caching, they often need to do I/O (to access files on disk or a network FS) • I/O takes much longer than computation • Overlapping I/O with computation to improve response time • Threads make it easy to overlap I/O with computation • While one thread blocks on I/O another can perform computation time compute block 1 request 1.6 requests

  9. Process or Thread Scheduling • Will use “process” and “thread” interchangeably • A single-threaded process maps to a kernel thread • Each thread in a multithreaded process (usually) maps to a kernel thread • A scheduler decides which thread runs next on the CPU • To ensure good support for DS components, a scheduler must: • Be scalable • Balance the load well • Ensure good interactive response • Keep context switches to a minimum (why?)

  10. Case Study: Solaris™ 10 OS • Solaris is often used on server systems • Known for its good scalability, good load balancing and interactive performance • We will look at Solaris runqueues and how they are managed • A runqueue is a scheduling queue • A structure containing pointers to runnable threads – i.e., threads that are waiting for CPU

  11. Runqueues in Solaris Global kernel priority queue kpqueue User priority queues for CPU0 disp_qs User priority queues for CPU1 disp_qs Pri 0 Pri 1 Pri N Pri 0 Pri 1 Pri N … … • There is a user-level queue for each priority level • A dispatcher runs the thread from the highest-priority non-empty queue

  12. Processor Load Balancing • Load balancing ensures that the load is evenly distributed among the CPUs on a multiprocessor • This improves the overall response time • Solaris kernel ensures that queues are well balanced when it enqueues a thread into a runqueue /* * setbackdq() keeps runqs balanced such that the difference in length * between the chosen runq and the next one is no more than RUNQ_MAX_DIFF. * (…) */ A comment from Solaris source code. Source: http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/disp/disp.c, line 1200

  13. Tuning Thread Priorities For Improved Response Time • If a thread has waited too long for a processor, its priority is elevated, so no thread is starved • Threads holding critical resources are put to the front of the queue so that they release those resources as quickly as possible /* * Put the specified thread on the front of the dispatcher * queue corresponding to its current priority. * * Called with the thread in transition, onproc or stopped state * and locked (transition implies locked) and at high spl. * Returns with the thread in TS_RUN state and still locked. */ A comment on setfrontdqfrom Solaris source code. Source: http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/disp/disp.c, line 1381

  14. Ensuring Good Responsiveness in Time-Sharing Scheduler • Solaris’s time-sharing scheduler (the default scheduler) assigns priorities so as to ensure good interactive performance • Timeslice: the amount of time a thread can run on CPU before it is pre-empted • If thread T used up it’s entire timeslice on CPU: • priority(T)↓, timeslice(T)↑ • If thread T has given up CPU before using up its timeslice: • priority(T) ↑, timeslice (T) ↓ • Why is this done?

  15. Time-Sharing Scheduler: Answers • Minimizing context switch costs: • CPU-bound threads stay on CPU longer without a context switch • In compensation, they are scheduled less often, due to decreased priority • Reducing the number of context switches improves performance • Ensuring good response for interactive applications • Interactive applications usually don’t use up their entire timeslice • Example: process a network message and release the CPU before the timeslice expires • Those applications will have their priority elevated, so they will respond quickly when response is needed (e.g., the next network packet arrives)

  16. What Limits Performance of MP/MT Applications? • The cost of context switching – depends on the hardware; the OS cannot fix it alone • Save/restore the registers • Flush the CPU pipeline • If switching address spaces • May need to flush the TLB (depends on the processor) • May need to flush the cache (depends on the processor) • The cost of inter-process communication(IPC): requires context switching • The cost of inter-thread synchronization – by and large depends on the program structure; OS can fix some of it, but not all

  17. Thread Synchronization shared data If lock is not available, threads wait Execution becomes serialized

  18. Next… • Talk about synchronization • Operating system support for efficient synchronization • Transactional memory – new programming paradigm for efficient synchronization

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