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Low Overhead Interrupt Handling with SMT. CS 252 Greg Gibeling Andrew Schultz. Problem. Traditional OS interrupt handling adds latency (context switch + copies) High bandwidth devices overwhelm traditional I/O architecture

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low overhead interrupt handling with smt

Low Overhead Interrupt Handling with SMT

CS 252

Greg Gibeling

Andrew Schultz

problem
Problem
  • Traditional OS interrupt handling adds latency (context switch + copies)
  • High bandwidth devices overwhelm traditional I/O architecture
    • Ethernet speed increasing faster than processor (10 Gb/s Ethernet becoming common)
    • Require interrupt coalescing, TOEs and other techniques to handle
problem cont
Problem (cont)
  • Motivation: Click
    • Click removes traditional interrupt handling code in favor of direct polling on DMA descriptors
  • Can we do better?
    • Get low latency, low overhead demanded by Click with traditional interrupt paradigm
solution
Solution
  • Re-evaluate I/O architecture
  • Allow OS more control over resources it needs for low latency
  • Use “hot” SMT thread to reduce context switch overhead
  • Zero kernel/user copy of data
slide5
How
  • Dedicate SMT thread to device handling
  • Cache pinning to give more control over what stays in cache
  • Tighter integration with hardware to support user level networking (a la U-Net)
    • Allow device handling thread to initiate DMA directly to user buffers
  • Evaluate with M5 or ML-RSIM (maybe compare Click under polling and new I/O)
cool things
Cool Things
  • Able to support high speed I/O applications like Click with commodity HW/SW (w/o rewriting int. handling)
  • Primarily aimed at I/O but has implications for improving all exception handling
  • Architectural support for application control of data unit size
what s related
What’s Related
  • Hardware
    • EMP – True zero copy on NIC
    • SMT exception handling – Done for TLB misses
    • On-chip NICs – Tighter coupling between NIC and CPU
    • TOEs and accelerators – Shift processing to special devices
  • Software (OS)
    • U-Net – ADU + fewer copies, more application control
    • Click – Change method from interrupt to polling
    • Interrupt coalescing – Add latency, reduce thrashing