THREADS WINDOWS XP SCHEDULING

Transcription

1 Threads Windows, ManyCores, and Multiprocessors THREADS WINDOWS XP SCHEDULING (c) Peter Sturm, University of Trier, D Trier, Germany 1

2 Classification Two level scheduling 1 st level uses priorities 2 nd level is round robin with varying time quantum Preemptive scheduling in all cases Some feedback mechanisms included Boosting of time quantum Boosting priorities Processor affinity Threads can be tight to certain CPUs only 1st Level Scheduling 31 priorities (0 through 31) Highest Realtime levels ( ) Medium Variable levels (1.. 15) Lowest System level (0) Realtime Level Variable Level 1 System Level 0 (c) Peter Sturm, University of Trier, D Trier, Germany 2

3 Priority Level Assignment 6 priority classes are defined by OS for the process and mapped to priority levels Class Realtime (base priority 24) Class High (base priority 13) Class Above Normal (base priority 10) Class Normal (base priority 8) Class Below Normal (base priority 6) Class Idle (base priority 4) Each thread inside process may have relative priority level Time-Critical (31 or 15) Highest (base priority +2) Above Normal (base priority +1) Normal (base priority of process) Below Normal (base priority -1) Lowest (base priority -2) Idle (16 or 1) 31 Realtime Time Critical In Detail Highest Above Normal Normal Below Normal Lowest Idle High Time Critical/Highest Above Normal Normal (13) Below Normal Lowest Idle Above Normal Time Critical Highest Above Normal Normal (10) Below Normal Lowest Idle Normal Time Critical Highest Above Normal Normal (8) Below Normal Lowest Idle Below Normal Time Critical Highest Above Normal Normal (6) Below Normal Lowest Idle Idle Time Critical Highest Above Normal Normal (4) Below Normal Lowest Idle (c) Peter Sturm, University of Trier, D Trier, Germany 3

4 View Priority Information Many tools can be used to view (and change) priority information for processes Some allow you to access thread-specific priority information Try the following tools Windows Task Manager Process Explorer from Observe your system as a whole Check individual programs Be careful when going to realtime priority class since this might interfere with sensible system processes and may lead to an instable system and may for you to reboot! Windows Task Manager Set and get base priority for process Get: Processes tab, Menu View, Select Columns Set: Right-Click on process, Set Priority (c) Peter Sturm, University of Trier, D Trier, Germany 4

5 Process Explorer 2 nd Level Scheduling Round Robin Time quantum is based on clock intervals of timer Clock interval on most x86 monoprocessors is ~10ms on most x86 multiprocessors is ~15ms You may try clockres from to determine the length of your machines clock interval Two different time quantums possible 2 clock intervals (useful for desktop systems) 12 clock intervals (default for server systems) Decrease the number of context switches Improve system performance (c) Peter Sturm, University of Trier, D Trier, Germany 5

6 Boosting Time Quantum Favor interactive applications Process with input focus receives a time quantum boost Time quantum will be tripled Doesn t happen with background processes Why? Default on server systems Boosting Priority In certain situations, the system can increase the priorities of some threads dynamically Value will be decreased with each time quantum until the original priorities is reached again Priority boosting happens, when A I/O operation is completed Some higher priority thread waits for something occupied by a lower priority thread Foreground threads are ready again GUI threads have to react on windowing activities Ready thread hasn t been running for some time (c) Peter Sturm, University of Trier, D Trier, Germany 6

7 References M.E. Russinovich, D.A. Solomon Microsoft Windows Internals 4 th edition, MS Press, pp. 325ff Thanks to Mark for clarifying some issues concerning thread priorities THREADS MANYCORES (c) Peter Sturm, University of Trier, D Trier, Germany 7

8 Gordon Moore Gene Amdahl AMDAHL UND MOORE Moores Gesetz Annahmen: Verdopplung Kernzahl alle 18 Monate 4-Kern-Prozessor in 2009 (c) Peter Sturm, University of Trier, D Trier, Germany 8

9 Moores Gesetz Verdopplung der Core-Zahl alle 24 Monate? The Fifth Paradigm Nach Kurzweil (1999) und Moravec (1998) 2010? (c) Peter Sturm, University of Trier, D Trier, Germany 9

10 Intel TeraScale CPU Quelle: techresearch.intel.com Weiter? Cores Cores 8192 Cores Many Cores Enough Cores J (c) Peter Sturm, University of Trier, D Trier, Germany 10

11 Umbruchphase Nicht-parallele Software wird nie mehr schneller laufen als heute! Amdahls Gesetz Annahmen: Zu 95% parallelisierbare Software Gleichbleibende Taktfrequenz Speedup gegenüber SingleCore (c) Peter Sturm, University of Trier, D Trier, Germany 11

12 Amdahl: SingleCore s = 0.2 T MultiCore ( s, n) = T SingleCore ( s) + T SingleCore 1 s n (1-s) = 0.8 t Exe = 1 s Amdahl: DualCore s = 0.2 (1-s) = 0.8 t Exe = 0.6 s (c) Peter Sturm, University of Trier, D Trier, Germany 12

13 Amdahl: OctCore s = 0.2 (1-s) = 0.8 t Exe = 0.3 s Amdahl: 32-Core s = 0.2 (1-s) = 0.8 t Exe = s (c) Peter Sturm, University of Trier, D Trier, Germany 13

14 THREADS MULTIPROCESSOR SCHEDULING Aspects Variations on the thread control lists Local lists for each processor Complicated switching of threads, no load sharing Global lists for all processors Contra: concurrency control, e.g. by locking Pro: easy to share load Lifetime of thread/processor assignment (affinity) Static: Thread to CPU assignment remains constant Dynamic: Select best CPU during each assignment step Shift in scheduling goals Maximum utilization is not of utmost importance if there is a vast number of CPUs J How to support cooperating threads running in parallel? (c) Peter Sturm, University of Trier, D Trier, Germany 14

15 Multiple Threads / Multiple CPUs Competing threads Cooperating threads Within address space Via shared memory Between address spaces Message exchange Goal Exploit inherent parallelism Simultaneous thread execution is of utmost importance Avoid superfluous context switches Identify groups of cooperating threads Thread A" Thread A" CPU 1" CPU 1" Possible! Context! Switches! CPU 2" Thread B" CPU 2" Thread B" Concurrency Control Global list, Master/Slave Propagate state changes to master Simple extension to OS for monoprocessors Failure of master CPU fatal Master will be performance bottleneck Global list, CPUs are peers Concurreny control required Varying granularity possible Local list for each CPU No solution at all Running Ready Blocked Running Ready Blocked (c) Peter Sturm, University of Trier, D Trier, Germany 15

16 Influence of Scheduling Strategy Additional CPUs available Resource CPU not scarce anymore Some negative effects such as convois disappear Simple strategies sufficient Example C. Sauer, K. Chandy (1981), Computer Systems Performance Modelling, Prentice Hall worse RR Throughput FCFS Throughput 1.15" Mono Processor" 1.10" 1.05" Dual Processor" 1.00" 1" 2" 3" 4" 5" Load Sharing Global ready list Idle CPUs select next thread to run Advantages Even distribution of load possible No centralized scheduler All monoprocessor strategies can be employed FCFS is sufficient in many cases Disadvantages Scalability: All CPUs have to access ready list Frequent change of CPU may lead to cold caches and TLBs Simultaneous execution of cooperating threads not guaranteed (c) Peter Sturm, University of Trier, D Trier, Germany 16

17 Gang Scheduling A group of threads will be scheduled as a single entity on a set of CPUs Also known as group scheduling, coscheduling Advantages Performance improvement (parallel programs) Less context switches Reduced scheduling cost Single scheduling decision valid for multiple processors Disadvantages Application must identify thread group Trade-off application performance vs. system utilization Fixed CPU Affinity CPUs are assigned to application for a fixed period of time Advantages: No superfluous context switches No cold caches and TLBs Easy to implement Disadvantages: Waste of CPU resources? On systems with many CPUs acceptable Sensible thread to CPU assignment hard to find In restricted situations possibly done by compiler Not all applications can be modeled this way (c) Peter Sturm, University of Trier, D Trier, Germany 17