CMPT 431 Dr. Alexandra Fedorova Lecture III: OS Support

2 CMPT 431 © A. 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 We will discuss the support that the operating system needs to provide

3 CMPT 431 © A. Fedorova Direct Interaction with the OS OS Process: a DS component system calls A process directly interacts with the OS via system calls Example: a web browser, a web server

4 CMPT 431 © A. Fedorova Interaction via Middleware Layer OS Process: a DS component system calls Middleware Function calls or IPC 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

5 CMPT 431 © A. Fedorova 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 CMPT 431 © A. Fedorova Digression: Protection Implementation In the Kernel System calls are expensive Why? – Protection domains Refresh memory protection from your OS class Good thing: we get memory protection Bad thing: crossing protection domains is expensive. Why? So is this the best solution?

7 CMPT 431 © A. Fedorova Alternative: Protection Via Language Safety features are guaranteed by language/runtime Compiler checks safe memory access In addition there are manifests w.r.t. what the process will and will not do This way you get protection And no need for hardware protection domains – everything can run in a single address space Singularity: an OS from Microsoft implemented these concepts... End digression

8 CMPT 431 © A. Fedorova 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)

9 CMPT 431 © A. Fedorova The Need for Good Process/Thread Support Many distributed applications are implemented using multiple threads or processes Why?

10 CMPT 431 © A. Fedorova Motivation for Multithreaded Designs 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 block Single thread time compute Multiple threads 1 request1.6 requests

11 CMPT 431 © A. Fedorova 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?)

12 CMPT 431 © A. Fedorova 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

13 CMPT 431 © A. Fedorova Runqueues in Solaris Global kernel priority queue kpqueue User priority queues for CPU0 disp_q s User priority queues for CPU1 disp_q s …… Pri 0Pri 1Pri NPri 0Pri 1Pri N There is a user-level queue for each priority level A dispatcher runs the thread from the highest-priority non-empty queue

14 CMPT 431 © A. Fedorova 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: line 1200

15 CMPT 431 © A. Fedorova 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 setfrontdq from Solaris source code. Source: line 1381

16 CMPT 431 © A. Fedorova 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?

17 CMPT 431 © A. Fedorova 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)

18 CMPT 431 © A. Fedorova 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

19 CMPT 431 © A. Fedorova Thread Synchronization If lock is not available, threads wait Execution becomes serialized

20 CMPT 431 © A. Fedorova Next… Talk about synchronization Operating system support for efficient synchronization Transactional memory – new programming paradigm for efficient synchronization