run_with_n_cpus

#!/bin/bash

# This is a defunct strategy.  There's no way it will really work.

# We can try to come up with some nice value at which to run the main
# multi-threaded process, but we run into various problems.  Scheduler
# priorities are not guarantees of anything.

# When the nice is too low, the main threads sneak onto the other
# processors in spite of the dummy processes.  When the nice is too
# high, the threads fail to migrate to other processers even though
# they are available.  

# (Wait, I need to confirm that second part... I thought I'd seen it
# take only 100% in two-cpu mode, but now I'm having trouble
# reproducing.)  Ok, yes, I'm seeing it hover at 130% when in should
# use 3 cpus (at nice 19).  And it's only using 200% at 4 cpus.


# First argument is number of cpus.
N=$1
shift

TOTAL=`number_of_cpus`
DUMMIES=$((TOTAL - N))
echo "Total CPUS/Cores found: $TOTAL, disabling $DUMMIES processors"


if [ "$NOSUDO" = "" ]; 
then
  
  if [ "$SUDO" = "" ]; then SUDO=sudo; fi

  # First we make sure all these CPUs are on, in case somebody else left it in a bad state.
  for ((i=1; i < $N; i++)) do
    $SUDO sh -c "echo 1 > /sys/devices/system/cpu/cpu$i/online"
  done
  for ((i=$N; i < $TOTAL; i++)) do
    #echo Disabling $i
    $SUDO sh -c "echo 0 > /sys/devices/system/cpu/cpu$i/online"
  done

  time $* 
  for ((i=$N; i < $TOTAL; i++)) do
    #echo Enabling $i
    $SUDO sh -c "echo 1 > /sys/devices/system/cpu/cpu$i/online"
  done
  echo Reenabled extra processors.
else 
  # Tried 5 7 8 10 19
  if [ "$NICE" = "" ]; then 
    NICE=8; 
    echo Setting nice to $NICE.
  fi 

  echo SUDO unavailable so we attempt to run dummy processes to disable cpus.
  SCHEME=chez
  TMP=`mktemp`
  echo '(let loop () (loop))' > $TMP
  CHILDPIDS=""
  for ((i=1; i <= $DUMMIES; i++)) do
    $SCHEME $TMP &
    CHILDPIDS+=" $! "
  done
  echo "Running command with nice $NICE:"
  echo time nice -n $NICE $*
  time nice -n $NICE $*
  kill -9 $CHILDPIDS
  echo Killed dummy processes.
fi