RFS（Receive Flow Steering）扩展了 RPS 的性能以增加 CPU 缓存命中率，以此减少网络延迟。
RFS中的有个配置参数：/proc/sys/net/core/rps_sock_flow_entries
设置此文件至同时活跃连接数的最大预期值。对于中等服务器负载，推荐值为 32768 。所有输入的值四舍五入至最接近的2的幂。
当echo一个值设置到rps_sock_flow_entries时，会被设置为离这个值最近的2的次幂。由于一些需要，我需要用Shell实现这个类似的功能：给定一个正整数，计算出离它最近的2的次幂整数是多少。
写了个shell脚本：https://github.com/smilejay/shell/blob/master/sh2017/round_to_2_power_num.sh
#!/bin/bash
num=$1
for (( i=1; i<=64; i++ ))
do
low=$(( 2 ** (($i-1)) ))
high=$(( 2 ** $i ))
avg=$(( (( $low + $high )) / 2 ))
if [ $num -le 0 ]; then
echo "error! we only support positive integer."
exit 1
fi
if [ $num -eq $low -o $num -eq $high ]; then
echo "num:$num 2-power-num:$num"
break
fi
if [ $num -gt $low -a $num -lt $high ]; then
if [ $num -lt $avg ]; then
echo "num:$num 2-power-num:$low"
break
else
echo "num:$num 2-power-num:$high"
break
fi
fi
done
运行效果：
Jay-Ali:~ jay$ ./round_to_2_power_num.sh 33 num:33 2-power-num:32 Jay-Ali:~ jay$ ./round_to_2_power_num.sh 60 num:60 2-power-num:64 Jay-Ali:~ jay$ ./round_to_2_power_num.sh 64 num:64 2-power-num:64 Jay-Ali:~ jay$ ./round_to_2_power_num.sh 3 num:3 2-power-num:4 Jay-Ali:~ jay$ ./round_to_2_power_num.sh 0 error! we only support positive integer.
RFS说明资料：
https://access.redhat.com/documentation/zh-cn/red_hat_enterprise_linux/7/html/performance_tuning_guide/sect-red_hat_enterprise_linux-performance_tuning_guide-networking-configuration_tools#sect-Red_Hat_Enterprise_Linux-Performance_Tuning_Guide-Configuration_tools-Configuring_Receive_Flow_Steering_RFS