Alex Harden aharden

Latency numbers every programmer should know

L1 cache reference ......................... 0.5 ns
Branch mispredict ............................ 5 ns
L2 cache reference ........................... 7 ns
Mutex lock/unlock ........................... 25 ns
Main memory reference ...................... 100 ns             
Compress 1K bytes with Zippy ............. 3,000 ns  =   3 µs
Send 2K bytes over 1 Gbps network ....... 20,000 ns  =  20 µs
SSD random read ........................ 150,000 ns  = 150 µs

Read 1 MB sequentially from memory ..... 250,000 ns = 250 µs

How to configure multiple deploy keys for different private github repositories on the same computer without using ssh-agent

Let's say alice is a github.com user, with 2 or more private repositories repoN. For this example we'll work with just two repositories named repo1 and repo2

https://github.com/alice/repo1

https://github.com/alice/repo2

You need to be to pull from these repositories without entering a passwords probably on a server, or on multiple servers.

Ansible invocation with assumed IAM role

How it works

boto3 initializes a session using the specified profile, for which it assumes a role as configured in your ~/.aws/config
Python script with above session initialization prints out shell-compatible environment variables of the temporary credentials
Wrapper script sets these a la eval
By the time Ansible runs, the AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY and AWS_SECURITY_TOKEN (for boto2) / AWS_SESSION_TOKEN (for boto3) are all set, and are consumed by boto2 in the inventory script and other boto2-based modules

Notes

I wasn't able to git fetch from Azure DevOps repos after upgrading openssh on my computer.

ssh -V
  OpenSSH_8.8p1, OpenSSL 1.1.1l  24 Aug 2021

Turns out that OpenSSL has now turned off ssh-rsa by default:

This release disables RSA signatures using the SHA-1 hash algorithm by default.

But Azure DevOps doesn't support anything else... Add the following to your config file to re-enable ssh-rsa for Azure DevOps:

	wget http://stedolan.github.io/jq/download/linux64/jq
	aws ec2 describe-instances --filters "Name=tag:Name,Values=$NAME" \
	"Name=instance-state-name,Values=running" \
	\| jq -r \
	".Reservations[] \| .Instances[] \| .InstanceId" \

	aws ec2 describe-volumes --filters \
	"Name=status,Values=available" \
	\| jq -r ".Volumes[] \| .VolumeId" \

	- action: ec2_facts

	- apt: pkg=lvm2 state=present
	- apt: pkg=mdadm state=present

	- pip: name=boto state=latest

	- ec2_vol: instance="{{ hostvars[inventory_hostname]['ansible_ec2_instance-id'] }}"
	volume_size=20
	device_name="{{ item }}"

	New-Item -Path "HKLM:Software\Policies\Microsoft\Windows\WindowsUpdate"
	New-Item -Path "HKLM:Software\Policies\Microsoft\Windows\WindowsUpdate\AU"
	Set-ItemProperty -Path "HKLM:\software\policies\Microsoft\Windows\WindowsUpdate" -Name WUServer -Value "http://wsus.domain.com:8530" -Type String -force
	Set-ItemProperty -Path "HKLM:\software\policies\Microsoft\Windows\WindowsUpdate" -Name WUStatusServer -Value "http://wsus.domain.co:8530" -Type String -force
	Set-ItemProperty -Path "HKLM:\software\policies\Microsoft\Windows\WindowsUpdate\AU" -Name UseWUServer -Value "1" -Type DWORD -force

	#! /bin/bash
	### BEGIN INIT INFO
	# Provides: dns-sync
	# Required-Start:
	# Required-Stop:
	# Default-Start: S
	# Default-Stop:
	# Short-Description: Synchronizes /etc/resolv.conf in WLS with Windows DNS - Matthias Brooks
	### END INIT INFO