Soft Timers: Efficient Microsecond Software Timer Support For Network Processing

1. Soft Timers: Efficient Microsecond Software Timer Support For Network Processing Mohit Aron and Peter Druschel Rice University Presented by Reinette Grobler

2. Managing I/O and other events • Asynchronous via Interrupts • Pro: Low latency - occurs immediately after event • Con: High overhead due to context switching • Synchronous via Polling • Pro: Avoids interrupts – no context switch • Con: Potential high latency - average latency is (polling interval)/2 • Neither overhead of interrupts nor latency of polling is acceptable for high performance time-sensitive applications • For example, Gigabit Ethernet network capable of accepting or delivering a full size (1500 byte) packet every 12sec

3. Hardware Interrupts • Control transfer expensive • Interrupt handler usually higher priority than running process • Save and restore CPU state of interrupted process • Cache and TLB misses experienced upon entry and exit from interrupt handler • Causes cache and TLB pollution for interrupted process • Expensive even on modern CPUs • Longer pipelines to improve execution rates • Possibly multiple levels of caches • Costs usually acceptable if context switch intervals in millisecond range – not microsecond intervals as in high-performance networks

4. Hardware Interrupt Overhead off-chip timer on-chip timer • Null-event handler without memory references – no cache or TLB pollution • Relative cost increases as CPUs get faster • Overhead found to be independent of the event frequency

5. Efficient Event Scheduling • Invoke event handler with minimal overhead: avoid interrupts • When kernel is already running (for another reason) • Before returning to user mode the kernel checks for and executes pending events • No hardware interrupt: invoking event handler  function call • No need to save or restore CPU state • Memory access locality cost already incurred • Termed: trigger states • How often ? • system calls, TLB misses, page faults, I/O interrupts, CPU idle • unpredictable as it depends on workload

6. Soft Timers • Soft timer facility checks for pending soft timer events in trigger states • Invokes associated handler when appropriate • Can schedule events at much finer granularity than would be feasible using periodic hardware interrupt – as long as system reaches trigger states with sufficient frequency • Pending events executed without cost of hardware timer interrupt • Overdue events scheduled with hardware interrupt

7. Hardware Interrupt Cost with Memory References Soft timers: lower cache pollution and lower overhead than hardware timers

8. Frequency of trigger states • When soft timer event is due the associated handler is executed at the earliest time when the system reaches a trigger state • Granularity and precision of soft timer facility depends on frequency of trigger states

9. Optimizations in Network Subsystem • Rate-based clocking • Goal: high utilization of bandwidth (depends on measurement or estimation of available bandwidth) • Transmit packets at given rate: potentially one packet every 12sec in Gigabit Ethernet • Packet transmission scheduled at very fine granularity using soft timers • Soft-timer-based network polling • Soft-timer events used to trigger reading of network interfaces’ status registers • Scheduled at sec granularity • Improved memory access locality • May delay packet processing (configurable)

10. Rate-based clocking

11. Network polling • Soft timers initiate polling • Polling interval adaptively set aggregation quotas from 1 to 15

12. Tighter delay bounds • Soft timer events experience probabilistic delays • On-chip timer can be integrated with soft timers – tighter worst-case delays for soft timer events at low cost • Soft timer event is scheduled with a scheduled time and a deadline • Schedule hardware timer event at earliest deadline of all scheduled soft timer events • Hardware timer event handler invokes all expired soft timer events

13. Conclusion • Soft timers allow the efficient scheduling of events at granularity below conventional timers • Useful range of soft timer event granularity appears to widen as CPUs get faster • Solution for events that require fine granularity and can tolerate probabilistic delays