Scheduling

Required reading: Eliminating receive livelock

Case study: UNIX in 1990

• What did UNIX scheduling look like at time of Livelock paper?
  • Uni-processor
  • No spin-locks in kernel
• Different contexts:
  • Process, user half
  • Process, kernel half
  • Soft interrupts
  • Device ("hard") interrupts
  • (Diagram.)
• User half, kernel half, and device interrupts are analogous to xv6. Soft interrupts delay the work from device interrupts until later, and are pre-emptable, unlike device interrupts themselves. Typically soft intr runs when last hard intr finishes.
• Rules for context transitions:
  • Hardware can force hard interrupt any most times except in a hard intr (same as xv6)
  • Soft interrupt can be invoked any time except during a soft or hard intr
  • Interrupt must complete before interrupted code can continue, since interrupts share stack (same as xv6)
  • Timer interrupt does *not* switch kernel halves, only voluntary switch via sleep()
  • Never runs a user half if any kernel half wants to run
  • User halfs pre-emptable via timer interrupt (same as xv6)
• Thus, priority structure: hard intr > soft intr > kernel half > user half
• Why no pre-emptable kernel halves?
  • To avoid most locks (remember -- uniprocessor)
  • Only have to worry about concurrency from interrupts
  • Kernel halves disable interrupts (up to certain priority level) in critical sections
• Why soft interrupts?
  • For time-consuming processing in response to hard interrupts, which should run relatively soon, and might not be associated with any specific kernel half (e.g. TCP ACKs, ARP responses, etc.)
  • Hard interrupt does time-critical stuff, schedules soft interrupt, more hard interrupts can occur during soft int.
  • Short-running hard interrupts avoid input overruns.
• Paper's application is packet forwarding -- let's look at how that works. (Diagram.)
• Receive interrupt (hard intr)
  • Copy incoming packet to memory
  • Append packet pointer to IP input queue (ipintrq)
  • Schedule IP soft interrupt
• IP soft interrupt
  • For each packet in IP input queue,
  • Decide what to do with it
  • Forward out a different network interface?
    • Append packet pointer to output interface's if_snd queue
    • Feed packet to output interface hardware, if it was idle
  • Deliver to a user program?
    • Append packet pointer to socket Q
    • Wake up process
• Transmit interrupt (hard intr)
  • Free last packet buffer
  • Copy packet from if_snd queue to h/w
  • Tell h/w to transmit
• Network interface hardware (e.g. ethernet)
  • Executes concurrent to the main CPU
  • Has small receive buffer on interface h/w (a few packets)
  • Raises receive interrupt for each received packet
  • Has small output buffer (a few packets)
  • Raises transmit-complete interrupts when buffers open up
• Why the input queue (ipintrq)?
• Why the output queue (if_snd)?
• Is this a good arrangement?

Paper discussion

• Explain Figure 6-1. Why does it go up? What determines how high the peak is? Why does it go down? What determines how fast it goes does? (The peak at 5000 means total processing time is 200 us. The x-intercept at 13000 means input processing alone takes 77 us. So each additional four inputs/second eliminates one output/second.)
• Would livelock show up for disk interrupts?
• Why not tell the O/S scheduler to give interrupts lower priority? Non-preemptible. Could you fix this by making interrupts faster? (No)
• Why not completely process each packet in the interrupt handler? (I.e. forward it?) Still need an output queue. Recv intr routine might never return, thus no tx complete interrupts, thus no transmissions. Also maybe interface h/w overruns during short input bursts. Wouldn't be fair if multiple input interfaces.
• Big picture: we've surrendered all control over scheduling various actions to the CPU's interrupt mechanism. And it doesn't give us the control we need to enforce our desired policy: output has precedence over input.
• What about using polling instead of interrupts? Solves overload problem, but under low low either wastes CPU or has high latency.

• What's the paper's solution?
  • No IP input queue.
  • Device input polling and IP input processing in kernel thread.
  • Device receive interrupt just wakes up thread. And leaves interrupts *disabled* for that device.
  • Thread does all input and output processing, then re-enables interrupts.
  • [show pseudo-code; where can we make scheduling decisions?]
• Why does this work? What happens when packets arrive too fast? What happens when packets arrive slowly?

• Explain Figure 6-3.
  • Why does "Polling (no quota)" work badly? (Input still starves xmit complete processing.)
  • Why does it quickly fall to zero, rather than gradually decreasing? Previously transmit had priority over IP, but not anymore. Estimate: x-intercept at 6500 means 150 usec for RX+IP. Made problem worse by doing more work before discarding packet.
• Explain Figure 6-4.
  • Why does "Polling, no feedback" behave badly? There's a queue in front of screend. We can still give 100% to input thread, 0% to screend.
  • Why does "Polling w/ feedback" behave well? Input thread yields when queue to screend fills.
  • What if screend hangs, what about other consumers of packets? (e.g., can you ssh to machine to fix screend?) Fortunately screend typically is only application. Also, re-enable input after timeout.
• Big picture: polling loop gives us more control -- but only knows about device and IP packet processing, not about other activities on host.

• Why are the two solutions different?
  1. Polling thread with quotas.
  2. Feedback from full queue.
• In theory, two solutions appears to be mostly interchangeable. Can use polling thread, with callback to schedule screend to run on some quota of packets, or use feedback for ipintrq/if_snd.
• Feedback more general, no magic numbers.
• In practice, passing feedback from IF_ENQUEUE(if_snd) all the way back to receive processing might require a lot of changes.

• If we apply the proposed fixes, does the phenomemon totally go away? (e.g. for web server, waits for disk, &c.)
  • Can the net device throw away packets without slowing down host?
  • Problem: We want to drop packets only for applications with big queues. But requires work to determine which application a packet belongs to. Perhaps interface hardware should look at packets and decide...
  • Solution worked well in paper for linear flow, clear feedback path.
• What if processing has more complex structure?
  • Chain of processing stages with queues? Does feedback work? What happens when a late stage is slow?
  • Split at some point, multiple parallel paths? No so great; one slow path blocks all paths.
• Can we formulate any general principles from paper?
  • Don't spend time on new work before completing existing work.
  • Or give new work lower priority than partially-completed work.
  • Corollary: If you might discard, do it as early as possible.