Technology and Software, Tips

How to ssh into Samsung’s Linux on Dex

Problem: you have a new Samsung tablet or phone with Linux on Dex and you want to ssh into it from your computer. Why? To work with a better keyboard, to use `rsync`, `scp`, `sftp`, move files over sftp from Ubuntu’s Nautilus, etc.


Step 1) Edit /etc/ssh/sshd_config in Linux on Dex and change:

Port 22 to something above 1024. Example: Port 9022
Uncomment ListenAddress and make it listen to the IP address of the tablet (use ifconfig to get it). Example: ListenAddress
At the very end of the file: UsePAM no

If you connect to another network you’ll have to change the address for ListenAddress

I suggest these other configurations:

PermitRootLogin No
PubkeyAuthentication yes
PermitEmptyPassword no
PasswordAuthentication no

Step 2) Every time you start the Linux on Dex container, run the command
sudo mkdir /var/run/sshd

Unfortunately this directory doesn’t survive the termination of the container.
It must be owned by root.

Step 3) Change the ownership of the site certificates
sudo chown dextop /etc/ssh/ssh_host_*

Step 4) Start the ssh daemon in a terminal with /usr/sbin/sshd -D

Unfortunately starting it with systemd won’t work (this is a LXD container, not a VM) and starting it with root fails too (same problem)

You can create a bash script for the steps from 2 to 4 and run it

Step 5) Create the directory ~dextop/.ssh in Linux of Dex
mkdir ~dextop/.ssh
chmod 700 ~dextop/.ssh

Step 6) Create a public/private key pair on your computer. I recommend
ssh-keygen -t ed25519 -f ~/.ssh/linux_on_dex

Step 7) Copy your computer’s ~/.ssh/ into the ~/.ssh/authorized_keys in Linux of Dex

Step 8) I recommend to create a ~/.ssh/config file on your computer with this content:

Host *
Protocol 2
ServerAliveInterval 60
Host linux_on_dex
HostName # the address in your network
User dextop
Port 9022 # the one you choose
PubkeyAuthentication yes
IdentitiesOnly yes
IdentityFile ~/.ssh/linux_on_dex

Step 9) Connect from your computer with ssh linux_on_dex


I noticed that ssh freezes when the tablet is locked so the screen must be on and unlocked. This is far from ideal. If you know a workaround please add a comment to this post.

Technology and Software, Tips

Named pipes, ports and Erlang code in an Elixir project

I needed to read from a named pipe in an Elixir program, I made a mistake and I learned a few thing. Follow me.

First create the named pipe:

$ mkfifo "pipe"

In Erlang you read from it like this

$ erl
1> Fifo = open_port("pipe", [eof]).
2> receive
2>   {Fifo, {data, Data}} ->
2>     io:format("Got some data: ~p~n", [Data])
2>   end.

You can check that it works by executing echo hi > pipe from another shell.

So I expected to able to write this in Elixir

$ iex
 iex(1)> fifo ="pipe", [:eof])

However this is what I get

** (ArgumentError) argument error
:erlang.open_port({"pipe"}, [:eof])

If you can see my noob mistake in the call to don’t tell anybody yet, read on.

After trying many permutations of the arguments and read twice both and I gave up. I resolved to write it as an Erlang module and call it from my Elixir project. It was pretty easy:

1) Create an erlang directory in the main directory of the project. The name is not magical, you can name it as you wish.

2) Add this line into the project options of mix.exs

erlc_paths: ["erlang"],

That’s the directory you created.

3) Create an erlang/namedpipe.erl file with this code


read(Pipe) ->
  Fifo = open_port(Pipe, [eof]),
    {Fifo, {data, Data}} ->

See how it can almost map 1 to 1 to Elixir. Variables are capitalized, symbols are lowercased and there are statement terminators (comma and full stop). All functions are private to the module except the explicitly exported ones.

4) Run mix and see that it compiles the Erlang file. Great!

But now

$ iex
** (ArgumentError) argument error
:erlang.open_port("pipe", [:eof])

Oh oh, what’s going on? I finally realized that it’s because of the wrong quote character! Single quotes are needed when passing strings to Erlang. Single quotes in Elixir are character lists, which is what Erlang needs. Double quotes are UTF-8 encoded binary data, which Erlang doesn’t understand.

So this works:

$ iex

Now go to another shell and run echo hi > pipe and confirm that it works for you too.

But wait, I did use double quotes in my first failed Elixir attempt. So did I do all of this for nothing? Embarrassingly, yes. This works:

$ iex
iex(1)> fifo ='pipe', [:eof])
iex(2)> receive do
...(2)>   {fifo, {:data, data}} ->
...(2)>     IO.puts("Got some data: #{data}")
...(2)> end

At least I learned how to embed Erlang code in an Elixir project and to care about single and double quotes.

Finally, you can embed that code in an Elixir module and call it both with a single quoted char list or a double quoted binary. Write this into lib/namedpipe.ex

defmodule NamedPipe do
  def read(pipe) when is_binary(pipe) do

  def read(pipe) do
    fifo =, [:eof])
      receive do
        {fifo, {:data, data}} ->

It uses guards to decide which version of the read function to call. Now

$ iex -S mix


Technology and Software

What’s the [5.0] in Rails 5’s ActiveRecord::Migration

The migration examples in the Rails Edge Guide are all like this

class MyMigration < ActiveRecord::Migration[5.0]

What’s that [5.0]? It can’t possibly be Ruby. Or is it?

With a little experimenting in rails c it’s easy to understand that [ is a class method of ActiveRecord::Migration. It’s defined in activerecord-5.0.0.beta1/lib/active_record/migration.rb and it accepts the 4.2 and 5.0 arguments. It allows us to select which version of the migrations we want to use. Production ready versions of ActiveRecord don’t have that method so it should go away as soon as Rails 5 goes out of beta.


Technology and Software

Configuring nginx for letsencrypt

Letsencrypt comes with a plugin for Apache. The one for nginx is still experimental. The manual configuration is pretty easy. On the server to protect with SSL:

git clone
cd letsencrypt
letsencrypt-auto certonly -a manual --rsa-key-size 4096 \
--email -d -d

This creates a directory /etc/letsencrypt with your account data and your certificates in the live/ subdirectory.

Edit you nginx configuration file and add

ssl_certificate /etc/letsencrypt/live/;
ssl_certificate_key /etc/letsencrypt/live/;

to the SSL configuration of your server. It’s important that you use fullchain.pem for the certificate, and not the cert.pem in the letsencrypt directory. Programs like curl and wget won’t work if you use cert.pem. The reason is explained in the first answer to an issue I wrongly opened to letsencrypt. A more detailed explanation is here.

Restart nginx to test your new certificate.

Remember to setup a cron job to renew the certificate before it expires in 90 days. You should also check Mozilla’s SSL Configuration Generator to improve the security of your https server.

Technology and Software

Ruby 2.3.0 InstructionSequence

Ruby 2.3 has been released on Christmas day as every Ruby version. It comes with a nice present: the RubyVM::InstructionSequence class with methods to compile scripts, save them and load them later. A quick example:

rvm install ruby-2.3.0
rvm use ruby-2.3.0
cat > test.rb
class Christmas

cat > compile.rb
instruction_sequence = 
  RubyVM::InstructionSequence.compile_file("test.rb")"test.iseq", "wb") do |file|

cat > instruction_sequence = nil"test.iseq", "rb") do |file|
  instruction_sequence =

ruby compile.rb
compile.rb  execute.rb  test.iseq  test.rb
ruby execute.rb

It works!

InstructionSequence comes with a caveat:

The goal of this project is to provide “machine dependent” binary file to achieve:

  • fast bootstrap time for big applications
  • reduce memory consumption with several techniques

“Machine dependent” means you can’t migrate compiled binaries to other machines.

Does it means that the compiled code won’t work on another machine? I generated the .iseq file on a Ubuntu 12.04 machine and uploaded it to a Ubuntu 14.04 one, both 64 bit. It keeps working and the directory structure of the two machines can be different, despite the presence in the compiled code of metadata about the source file.

I invite the readers to check the other methods of the class. They let allow for the compilation of strings of text and procs, setting compilation options, disassembling iseq code plus several instance methods that operate on an instruction sequence.

Technology and Software

Ruby performances with PostgreSQL and MySQL

(original post in the Italian Ruby Forum)

I had to convert a database seeding script from PostgreSQL 9.4 to MariaDB 10 (customer’s choice and with little enthusiasm I had to comply). This lead to a number of interesting discoveries about the pg and mysql2 Ruby drivers. Apart a few minor issues [1] [2] [3] [4] I immediately noticed that the script with MariaDB run 20 times (twenty) more slowly than the  PostgreSQL one: 21 minutes vs 1 minute and 3 seconds. Unusable and inexplicable.

Such a big difference can not be due to the database, so I started to investigate the configuration. Even the MySQL coming with Ubuntu 12.04 was too slow and I can expect that the distributors set it up reasonably well. At this point the suspect becomes the driver. I opened this issue and they gave me two valuable tips: use a profiler ( ) and the gem-import activerecord ( ). I knew both of them but sometimes you have to be reminded about tools you don’t use often. Ops.

The profiler show that the driver uses pg prepared statements that give obvious benefits with the number of records created by my script (a little over 32,000). The version of mysql2 I had to use (0.3.x) does not have prepared statement (but the newer version does) and that seems to make the difference. I rewrote the script to use activerecord-import, which  inserts a whole array of objects at once. The script looks a bit unnatural, because I repeatedly needed the ids of the record I created to pass them along the associations, but the execution times for mysql2 dropped from 21 minutes to 1 minute and 33″. It was worth it. There are only 1,045 calls to the db and yet is always slower than 32k calls made by the original script with pg. The script with pg and activerecord-import dropped to 47 seconds.

Despite all the enhancements introduced in the import-activerecord calls my script’s calls to PostgreSQL add up to 9.4 seconds. The calls to MariaDB are  49.8 seconds. Ruby accounts for 40 seconds, regardless of the database used.


1) Work on PostgreSQL has performance advantages with Ruby due to drivers.

2) mysql2 0.4.0+ has prepared statements but if you’re working with Rails you must be careful. There are issues [A] [B] and it seems you need Rails 4.2.5+ to use it. I didn’t test the combination yet.

3) For details of my profiling research (tables, times, calls) read 


Finally, the issues I run into:

[1] https: // …

[2] For MariaDB install the gem mysql2 with

bundle config \
build.mysql2 --with-mysql-config=/path/to/mariadb/bin/mysql_config

Careful: this is globals so use –with-mysql-config=/usr/bin/mysql_config when you need to connect to MySQL.

[3] My script would clear the db before seeding using TRUNCATE CASCADE, but MySQL and MariaDB don’t have it. This  is the workaround

 connection = ActiveRecord :: Base.connection
 Connection.Execute ("SET foreign_key_checks = 0;")
 [all models] .each do | model |
   Connection.Execute ("TRUNCATE model.table_name # {}")
 Connection.Execute ("SET foreign_key_checks = 1;")

[4] But neither ActiveRecord has TRUNCATE, so either you use some gems that add it to AR or even for PostgreSQL you need a loop like that, but you don’t need the SET foreign_ley_checks statements.

Technology and Software

LibreOffice, Python3 and AttributeError: ‘NoneType’ object has no attribute ‘supportsService’

I had to run a modified version of the famous script on Ubuntu 14.04. It run on 12.04 well but Ubuntu 14.04 comes with Python3 and the Uno library to interface Python with LibreOffice or OpenOffice doesn’t work.

Solution: convert the script to Python3 (exceptions, print, has_key have been changed) then install these libraries:

sudo apt-get install libreoffice-dev libreoffice-script-provider-python python3-uno

and the program will work. If you fail to install them you’ll get the AttributeError: ‘NoneType’ object has no attribute ‘supportsService’ error because loadComponentFromURL won’t be able to read the input file.

Technology and Software

Compile your own Ruby and use it with RVM

Prompted by the news about how gcc 4.9 makes Ruby 2.1 faster I decided to compile my own Ruby 2.2.0 and pit it against the one coming with rvm. I also want to keep switching between Rubies using RVM. I had to google a little to learn how to do it so I want to share.

rvm install 2.2.0
rvm use ruby-2.2.0
# find out the compilation options
ruby -r rbconfig -e 'puts RbConfig::CONFIG["configure_args"]'
 'optflags=-O2' '--enable-load-relative' '--sysconfdir=/etc'
 '--disable-install-doc' '--enable-shared'
# important, always compare to the hash advertised at
md5sum ruby-2.2.0.tar.gz
tar xzf ruby-2.2.0.tar.gz
cd ruby-2.2.0
mkdir -p /home/me/compiled-rubies/2.2.0p0
# configure with the same compilation options
# of the standard binary
CFLAGS=-O2 ./configure --enable-load-relative \
  --sysconfdir=/etc \
  --disable-install-doc --enable-shared \
make test
make install
# make it available to rvm as ext-ruby-2.2.0-gcc4.9_O2
rvm mount /home/me/compiled-rubies/2.2.0p0 \
  -n ruby-2.2.0-gcc4.9_O2
rvm list
    ext-ruby-2.2.0-gcc4.9_O2 [ x86_64 ]
 => ruby-2.2.0 [ x86_64 ]
rvm use ext-ruby-2.2.0-gcc4.9_O2

The files in ~/.rvm/rubies/ext-ruby-2.2.0-gcc4.9_O2 will be symlinks to the ones in compiled-rubies/2.2.0p0 so don’t remove that directory.

The point of this post is already made but as a bonus here are the benchmarks of the two Rubies using Antonio Cangiano’s tests.

git clone git://
cd ruby-benchmark-suite
rvm use ruby-2.2.0 # for the standard one
rvm use ext-ruby-2.2.0-gcc4.9_O2 # for the compiled one
rake # This might fail, see the note at the end

Here are the results: ruby-2.2.0 and ruby-2.2.0-gcc4.9_O2 (YAML), summary (CSV). TL;DR: the compiled Ruby is a little bit faster overall. It’s much faster in a few tests, a bit slower in some others. It’s a difficult choice and it probably depends on what you do.  Please notice all those tests that ended with errors (look at the YAML files). They could make a difference for the overall assessment of which version is faster but I didn’t dig into that issue yet.

In case of failure

Rake could end with a weird syntax error for the compiled Ruby. There are two possible fixes. One is to replace `which rake` with the version from the 2.2.0 binary distribution. The other is to really understand what’s going on. The key is: that rake is a bash script which execs a Ruby interpreter on itself using ruby’s -x switch which strips away the bash script at the beginning. But Ruby doesn’t seem to honour that. No time to investigate any further now…

Technology and Software

Running Ruby on Rails tests with a ramdisk backed PostgreSQL

I’m not a fan of mocking objects in my Ruby On Rails tests so my tests always hit the database, which is PostgreSQL anytime I can make the choice and a customer doesn’t dictate MySQL.

Hitting the DB means that the test suite eventually slows down as tests pile up as the application grows.
It’s been a long time since I wanted to check what happens if I run my tests on a database backed on RAM instead of by a spinning disk. Are they going to be much faster?

Tl;dr No, they run at the same speed.

Creating the DB on the ramdisk

The setup is based on information provided at and

The test system is my laptop, an Ubuntu 12.04, i7-4700MQ CPU @ 2.40GHz, 16 GB RAM, SDD for the OS, HDD for my home and databases. The DB is PostgreSQL 9.3.

Linux has 16 ramdisks already created as /dev/ram* at boot time. Let’s take one and mount it.

mkdir ~/tmp/ram
sudo mkfs.ext4 -m 0 /dev/ram0
sudo mount /dev/ram0 ~/tmp/ram/
df -h ~/tmp/ram/
Filesystem Size Used Avail Use% Mounted on
/dev/ram0 58M 1.3M 56M 3% /home/me/tmp/ram

It’s a tiny disk and it turned out to be barely enough to accommodate my tests but it’s OK for experimenting.
You can make it larger if you need to. explains how.

We create a DB there now.

cd ~/tmp/ram
sudo bash
mkdir postgresql
chown postgres.postgres postgresql/
su - postgres
/usr/lib/postgresql/9.3/bin/initdb --locale=en_US.UTF-8 -D ~/tmp/ram/postgresql/
mkdir postgresql/log
chown postgres.postgres postgresql/log/

We make it use a different port from the one used by the default PostgreSQL DB on the laptop.

vi postgresql/postgresql.conf

port = 5433

We start the DB and connect to it

sudo -u postgres /usr/lib/postgresql/9.3/bin/pg_ctl \
  -D ~/tmp/ram/postgresql/
  -l ~/tmp/ram/postgresql/log/postgresql-9.3-main.log start
psql -p 5433 -U postgres


Running tests

We edit config/database.yml to use the ramdisk DB

port: 5433

We create the test user and the test db

psql -p 5433 -U postgres
create role testuser login password 'password';
alter user testuser with createdb;
create database myapp_test owner testuser encoding='UTF8' lc_collate='en_US.UTF-8' lc_ctype='en_US.UTF-8';

We create the DB schema

cd the/rails/directory
RAILS_ENV=test rake db:migrate

And finally we benchmark the tests over the two databases.

rake spec:controllers


Finished in 1 minute 5.34 seconds (files took 1.76 seconds to load)
Finished in 1 minute 4.26 seconds (files took 1.75 seconds to load)
Finished in 1 minute 2.07 seconds (files took 1.75 seconds to load)


Finished in 1 minute 7.09 seconds (files took 1.76 seconds to load)
Finished in 1 minute 6.01 seconds (files took 1.72 seconds to load)
Finished in 1 minute 4.68 seconds (files took 1.74 seconds to load)

2 seconds are not worth the trouble. Let’s benchmark the models.

rake spec:models


Finished in 1 minute 36.8 seconds (files took 1.69 seconds to load)
Finished in 1 minute 38.08 seconds (files took 1.72 seconds to load)
Finished in 1 minute 37.9 seconds (files took 1.73 seconds to load)


Finished in 1 minute 38.64 seconds (files took 1.79 seconds to load)
Finished in 1 minute 32.73 seconds (files took 1.69 seconds to load)
Finished in 1 minute 41.89 seconds (files took 1.71 seconds to load)

No difference at all, only a bit more variance in the durations of the HDD tests.

This is my conjecture. The data go first into the OS file buffer, then are synced to the disks. Syncing to ramdisk is faster but if there is enough RAM data is staying in RAM anyway and it doesn’t matter if we’re using a ramdisk or a HDD. Remember: this is a test DB with a handful of data, not a large production DB with high I/O loads.

Let’s stop the DB and change the configuration to do without syncing. If there is no speedup my conjecture should be confirmed.

sudo -u postgres /usr/lib/postgresql/9.3/bin/pg_ctl \
  -D ~/tmp/ram/postgresql/ \
  -l ~/tmp/ram/postgresql/log/postgresql-9.3-main.log stop
sudo vi ~/tmp/ram/postgresql/postgresql.conf
sudo -u postgres /usr/lib/postgresql/9.3/bin/pg_ctl \
  -D ~/tmp/ram/postgresql/ \
  -l ~/tmp/ram/postgresql/log/postgresql-9.3-main.log start

Run the test on the ramdisk again.

rake spec:models


Finished in 1 minute 36.52 seconds (files took 1.71 seconds to load)
Finished in 1 minute 35.45 seconds (files took 1.7 seconds to load)
Finished in 1 minute 36.59 seconds (files took 1.68 seconds to load)

No difference, so my tests didn’t move enough data to make the syncing operations relevant.


You can keep your test DB on a spinning disk and the OS buffering will make it fast.
If you want quick tests you probably have to mock everything and do without the DB.

Alternative: run tests in parallel with the parallel_tests gem.


Could I have done something better to make the ramdisk based DB run faster?

Technology and Software

Ruby’s Influence over the Elixir Language

This pictures shows exactly what you’d expect a Ruby conference to be. Don’t you? Wait, it’s not what it looks. I can explain.

Ruby Day 2014 / Lunch Break

Ruby Day 2014 / Lunch Break

That was the lunch break and we had a wonderful sun and a wonderful lawn :-)

That was in Roncade, Italy at the premises of H-FARM.

I was there for the usual stuff: learn new things, meet people I knew and know new people. Plus, my first time at a Ruby conference, to give a talk. A talk about Ruby. No, a talk about a language designed to look like Ruby regardless of the huge differences beneath. This language is Elixir and this is my presentation on Slideshare (sorry for the fonts, some of them didn’t survive the conversion after the upload).

2014-10-23 – Update: we’ve got the video!

The original presentation files (odt, ppt, pdf) with the speaker notes and some tutorials are at

Visit also my GitHub repository for a demo Phoenix application (a RoR-like web framework for Elixir) at