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Eliminating Receive Livelock in an Interrupt-driven Kernel

Eliminating Receive Livelock in an Interrupt-driven Kernel. Jeffrey C. Mogul mogul@wrl.dec.com K. K. Ramakrishnan AT&T Bell Laboratories kkrama@research.att.com 1996 USENIX. Problem: receive livelock the system spends all its time processing interrupts, to the exclusion of other necessary tasks

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Eliminating Receive Livelock in an Interrupt-driven Kernel

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  1. Eliminating Receive Livelockin an Interrupt-driven Kernel Jeffrey C. Mogulmogul@wrl.dec.comK. K. RamakrishnanAT&T Bell Laboratorieskkrama@research.att.com1996 USENIX

  2. Problem: receive livelock • the system spends all its time processing interrupts, to the exclusion of othernecessary tasks • Solution: load dependant scheduling • The OS lowers the priority of network interrupt handling as load increases

  3. OS Scheduling Techniques • Interrupts • When a task requires service, it generates an interrupt. The interrupt handler provides some service immediately. • Polling • When a task requires service, it turns on a flag. The OS polls the flags periodically, and provides some service to the tasks with flags turned on.

  4. Benefits of Scheduling Techniques • Interrupts • Much lower overhead • Rapid response • Polling • Perform better under heavy load

  5. How interrupt-driven scheduling causes excess latency under overload • Timeline of Ethernet packet processing • link-level processing at device IPL, which includes copying the packet into kernel buffers • further processing following a software interrupt, which includes locating the appropriate user process, and and queueing the packet for delivery to this process • finally, awakening the user process, which then (in kernel mode) copies the received packet into its own buffer.

  6. Under Load, Avoid Livelock • Use interrupts only to initiate polling. • Use round-robin polling to fairly allocate resources among event sources. • Temporarily disable input when feedback from a full queue, or a limit on CPU usage, indicates that other important tasks are pending. • Drop packets early, rather than late, to avoid wasted work. Try to process packets that are received to completion. • Unloaded, Maintain High Performance • Re-enable interrupts when no work is pending, to avoid polling overhead and to keep latency low. • Let the receiving interface buffer bursts, to avoid dropping packets. • Eliminate the IP input queue, and associated overhead.

  7. Livelock in a Unix router • filled circles: kernel-based forwarding performance • open squares: the screend program, a firewall application

  8. Why livelock occurs in the 4.2BSD Router

  9. Fixing the livelock problem • Do as much work as possible in a kernel thread • Eliminate the IP input queue and its associated queue manipulations and software interrupt

  10. Guaranteeing progress for user-level processes

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