xfgavin

[Unit]
Description=Keeps a tunnel to 'remote.com' open
After=network.target network-online.target sshd.service
#####################
#Reference: https://transang.me/create-a-remote-ssh-background-service-with-autossh-and-systemctl/

[Service]
User=YOUR_USER_NAME
Environment="AUTOSSH_GATETIME=30"
Environment="AUTOSSH_POLL=30"

xfgavin

If you run VMware Workstation 11 or above you may encounter high CPU usage from process khugepaged on Ubuntu 15.04+

The fix is to disable transparent hugepages. It seems Ubuntu has it enabled by default.

You can check the current status on your system by running:

cat /sys/kernel/mm/transparent_hugepage/enabled

cat /sys/kernel/mm/transparent_hugepage/defrag

xfgavin

System Design Cheatsheet

Picking the right architecture = Picking the right battles + Managing trade-offs

Basic Steps

1. Clarify and agree on the scope of the system

• User cases (description of sequences of events that, taken together, lead to a system doing something useful)
  • Who is going to use it?
  • How are they going to use it?

xfgavin

Latency Comparison Numbers (~2012)
----------------------------------
L1 cache reference                           0.5 ns
Branch mispredict                            5   ns
L2 cache reference                           7   ns                      14x L1 cache
Mutex lock/unlock                           25   ns
Main memory reference                      100   ns                      20x L2 cache, 200x L1 cache
Compress 1K bytes with Zippy             3,000   ns        3 us
Send 1K bytes over 1 Gbps network       10,000   ns       10 us
Read 4K randomly from SSD*             150,000   ns      150 us          ~1GB/sec SSD