Introduction to Systems Research at SFU Dr. Alexandra Fedorova August 2007
Introduction • Systems: software systems, hardware systems, the interaction between them • New research area at SFU, before December 2006 there were no faculty members at SFU doing systems research (not counting networking) • Research opportunities at undergraduate and graduate level: • Undergraduate honours thesis • CMPT 415 • Paid research assistanships • Master’s and Ph.D.
What is Systems Research? • System – a collection of software and hardware components that accomplish a certain goal • Usually this does not include applications, but includes system software: • The operating system • System libraries • Systems research concerns with building these components and structuring their interaction
Systems Research at SFU System software design for chip multithreading processors Computer Architecture Distributed Systems
System Software Design for Chip Multithreading Processors • What is chip multithreading? • Why is this research relevant? • What research problems are we addressing?
Level-1cache Chip Multithreading (CMT) • CMT processors: multiple threads runs on the same chip simultaneously: • Conventional processor: one software thread runs on a chip at a given instant: A CHIP Level-2 cache
CMT: The Dominant Architecture • Most new processors are CMT: • Intel: 100% of new server processors and 90% of high-performance desktop processors are CMT by the end of 2007 • All major hardware vendors are in the CMT business: • Sun Microsystems Niagara (32 threads on the chip) • IBM Power4, Power5, Power6 • Intel Hyper-threaded Xeon (servers, desktops) • Intel Core Duo (desktops and laptops) • Dell Quad core systems (2x Intel Dual-core processors) • AMD Quad core (coming up in Fall 2007)
Why CMT? • Running one thread per chip is inefficient • Due to nature of modern applications, computational hardware is underutilized • Modern applications spend 50-60% of their CPU time accessing memory • While memory is accessed CPU pipeline is stalled – it is idle, not doing anything useful • But while it is stalled, CPU is still consuming power • So there’s power waste with no benefit • Idea behind CMT: while one thread stalls the pipeline, let another thread use it • Sort of like overlapping I/O and computation but at the micro level
1:load datafrom memory 2:add 3:subtract 4:add thread 1 time CMT: More Efficient CPU Utilization Stall the pipeline Pipeline is busy stall the pipeline 1:add 2:subtract 3:load data from cache 4:load data from memory thread 0
How to Enable CMT? • How to enable running multiple threads on the same chip? • Hardware multithreading • Multicore processing • Combination of the two
Level-1cache Hardware Multithreading • Run at least two threads on the same processing core • Some hardware is duplicated, some is shared • Shared hardware: • Pipeline: i.e., functional units, register files, queues • Caches: Level-1 (L1) instruction and data caches, Level-2 (L2) unified cache • Interconnects • Multithreaded processors: • Intel Hyper-threaded Xeon • IBM Power5, Power6, Cell • Sun Microsystems Niagara A CHIP Level-2 cache
L1cache L1cache Multicore Processing • Multiple processing cores on the same chip • Threads share the L2 cache (and other lower-level caches), and interconnects • Multicore processors: • Intel Core Duo • AMD Quad Core • IBM Power4, 5, 6 • Sun Microsystems Niagara A CHIP L2 cache
L1cache L1cache Multicore + Multithreading • A multicore processor • Each core is multithreaded • Multicore and multithreaded processors: • Sun Microsystems Niagara • IBM Power5, Power6 A CHIP L2 cache
Research on CMT Processors • Computer architecture research: • How to design a CMT processor to achieve a good combination of: CPU utilization, application performance, power efficiency • System software research: • How to design system software, i.e., the operating system, that enables applications to perform well on these processors?
OS Design for CMT Processors • Operating systems are traditionally responsible for the allocation of hardware resources • On CMT processors, on-chip resources are shared among threads that run simultaneously • How you allocate those resources among threads determines the performance that those threads will achieve • Let’s look at a few examples…
L1cache L1cache Constructing Optimal Co-schedules • Blue suffers when it does not have enough L1 cache, • Red uses lots of L1 cache • Green does not use much L1 cache • Yellow does not suffer when it does not have much L1 cache A CHIP L2 cache
Constructing Optimal Co-schedules (cont.) • How do we find out applications’ cache behaviour? • Turns out you need to consider memory access patterns - this is not trivial to measure • How do you model interactions among applications? • How do you know if one application’s cache usage patterns are incompatible with another’s? • These patterns/relationships cannot be measured directly • Can they be modeled? • Simple models are inaccurate • Complex models are too inefficient to use inside an operating system scheduler • Approach of my group: use learning methods, feedback-directed scheduling
L1cache L1cache Heterogeneous Multicore Systems • One size does not fit all • Application class A runs best on core with feature set X • Application class B runs best on core with feature set Y • Rather than designing a homogeneous multicore system that attempts to satisfy everyone but satisfies no one, design a heterogeneous multicore system (HMC) A CHIP L2 cache
Core 1 Core 2 L1cache L1cache Scheduling On HMC Systems Set A: Want to run on Core 1 A CHIP Set B: Want to run on Core 2 L2 cache
Scheduling On HMC Systems • If you schedule all threads in Set A on their preferred core, those threads will suffer from: • Low amount of CPU time • High response time • Because there is high demand for that core, and they’d have to share it with others • So you might want to schedule threads on their non-preferred core once in a while • How do you balance between performance, fair CPU allocation and good response time?
Summary • CMT systems are new and cool, yet prevalent enough for people to care about them • Companies are desperate to hire students with experience on CMT systems • If you are thinking about academic career: new and hot research area • Many problems • Many opportunities to publish • Talk to me if you are interested in research opportunities • Tell your friends who might be interested