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 Oindrila Mukherjee

2. Outline • Why Soft Timers? • What are Soft Timers? • Conventional Timers and Associated Problems • Soft Timer Mechanism • Problems with Soft Timers • Soft Timer Operations • Soft Timer Applications • Rate-Based Clocking • Network Polling • Experimental Results • Conclusion

3. Why Soft Timers ? • I/O Management can be done by: • Interrupts • + Low latency • - High context switching overhead • Polling • + Avoids Interrupts • - Possibility of high latency • Neither overheads acceptable for high performance time-sensitive applications. • Solution : Use Soft Timers

4. What are Soft Timers ? • New OS facility that allows efficient scheduling of software events. • Avoids interrupts and reduces context switches. • Enables efficient performance of rate-based clocking. • Can be used to perform network polling.

5. Conventional Timers • Invokes designated event handlers periodically in the context of a hardware interrupt. • Causes high CPU overhead due to: • Saving state and restoring state. • Cache/TLB pollution.

6. Soft Timer Mechanism • Before returning to user mode the kernel checks for and executes pending events. • Uses concept of trigger states. • System calls. • Exception handling (eg. TLB or page fault). • Interrupt handler associated with a device interrupt. • The CPU going idle. • No hardware interrupt.

7. Soft Timer Mechanism (contd.) • Invoking event handler equivalent to function call. • Soft timer facility checks for pending soft timer events . • Associated handler invoked without cost of hardware interrupt. • Soft Timer checks have no significant impact on system performance.

8. Problems with Soft Timers • Invoking of event handler may be delayed past its scheduled time. • Trade-off between accuracy and overhead. • Solution: Maximal delay experienced by a soft timer is bounded - low frequency hardware timer schedules overdue events. • Events scheduled at a much finer granularity than periodic hardware interrupts Soft timer polling latency Trigger states Event Scheduled

9. Soft Timer Operations • Measure_resolution() – returns the 64-bit resolution (in Hz) of the clock . • Measure_time() – returns the 64-bit current real time . • Schedule_soft_event (T, h) – schedules handler h to be called at least T ticks in future. • Interrupt_clock_resolution() – gives the expected minimal resolution (in Hz). Equal to frequency of “back up” timer interrupt. ( Interval > T + 1 ) => event fired X = measure_time() Event Scheduled

10. Soft Timer Applications - Rate-Based Clocking • Packet Transmission at a scheduled rate. • Current TCP implementations are self- clocking. • Essential for TCP implementations: • Allows sender to skip slow-start phase. • Overcomes effects of ACK compression and big ACKs. • Increases network utilization and performance on high speed WANs. • Conventional approach – schedule periodic hardware timer events – unacceptable overheads.

11. Rate Based Clocking using Soft Timers • Soft timers allow clocked transmission of packets with low overhead. • Algorithm used: • Protocol maintains running average of actual transmission rate. • Events scheduled to achieve target transmission rate. • If actual rate < target rate, then next transmission interval = maximum allowable rate.

12. Soft Timer Applications – Network Polling • System periodically reads network interfaces’ status registers. • Avoids interrupts. • May increase latency. • Use hybrid approach – interrupts under normal network load and polling under overload.

13. Soft Timer Based Network Polling • Algorithm used: • Soft timer event polls network interface. • Avoids network interrupts. • Improves memory access locality. • Communication latency similar to interrupt-driven network processing. • Aggregation quota dynamically selected.

14. Experimental Results off-chip timer on-chip timer • Hardware timer overhead does not scale with CPU speed. • Relative cost increases as CPU gets faster. • Base overhead imposed by soft timer is negligible.

15. Experimental Results (contd.)

16. Conclusion • Soft timers allow the efficient scheduling of events at granularity below that of conventional timers. • Useful range of soft timer event granularity appears to widen as CPUs get faster. • They take advantage of trigger states and invokes handlers at low cost. • Soft timers can be integrated with an existing conventional interval timer facility.