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How to sleep million years

19 July 2013

Let me assure you, the "fluxcapacitor" project is very interesting. Unfortunately, I find it very difficult to describe what it does. For this project I completely fail the elevator pitch.

I won't attempt to describe it, instead I'll try present it in action.


First, you need to compile the fluxcapacitor (FC), it's supposed to be simple:

$ sudo aptitude install git gcc make
$ git clone
$ cd fluxcapacitor
$ make

If you're lucky you should have FC up and running:

$ ./fluxcapacitor --help
 fluxcapacitor [options] [ -- command [ arguments ... ] ... ]


Fluxcapacitor runs any program in an special environment which has different view on passing time than the rest of the operating system. From a point of view of a program everything is normal, but if you observe it from outside you'll see it run "faster". You can say fluxcapacitor speeds up the flow of time for the program. For example:

$ time sleep 12
real    0m12.003s

$ time ./fluxcapacitor -- sleep 12
real    0m0.057s

Both times sleep thinks 12 seconds had passed. But in the second case the observer would disagree - it only took a few milliseconds to finish. Sleep isn't able to realize that.

Notice the time command was run outside fluxcapacitor environment. If you run it inside, it will surely report 12 seconds:

$ ./fluxcapacitor -- bash -c "time sleep 12"
real    0m12.016s

Let's make the example more extreme:

$ ./fluxcapacitor -- bash -c "sleep 315360000; date"
Sat Jul 15 11:36:23 BST 2023

We briefly went to the year 2023.

How to sleep a million years

You can try sleeping for any time you wish. Unfortunately my operating system (bash?) can't express dates after the year 2550:

$ ./fluxcapacitor --idleness=0 -- bash -c \
    'for i in `seq 1 100`; do sleep 315360000; date; done'
Sat Jul 15 11:38:25 BST 2023
Tue Jul 12 11:38:25 BST 2033
Fri Jul 10 11:38:25 BST 2043
Sun Apr 28 11:38:26 BST 2543
Wed Apr 25 11:38:26 BST 2553
Tue Oct  3 12:03:52 BST 1978
Fri Sep 30 12:03:52 BST 1988

Fluxcapacitor is very powerful, let's jump to more sophisticated examples.

Speeding up sleep sort

In 2011 someone on 4chan invented the "sleep sort" algorithm:

function f() {
    sleep "$1"
    echo -n "$1 "
while [ -n "$1" ]
    f "$1" &

It sorts numbers by spawning many processes and waiting approriate number of seconds. For example:

$ time ./ 5 3 6 3 6 3 1 4 7
1 3 3 3 4 5 6 6 7
real    0m7.012s

Although this was intended as a joke, with fluxcapacitor you can run it in a fraction of a second:

$ time ./fluxcapacitor -- ./ 5 3 6 3 6 3 1 4 7
1 3 3 3 4 5 6 6 7
real    0m0.056s

For the curious: the complexity of sleep search on fluxcapacitor is O(n^2).

Advanced usage

Sleep sort forked many processes and fluxcapacitor is able to guard any number of OS processes or threads. You can spawn many commands by delimiting the command line with two dashes --:

$ time ./fluxcapacitor \
   -- bash -c "sleep 60; date" \
   -- bash -c "sleep 120; date"
Wed Jul 17 13:14:01 BST 2013
Wed Jul 17 13:15:01 BST 2013
real    0m0.179s

With many processes things get really interesting.

Memcached expiration

Let's try to use do something more complicated. Memcached, a caching daemon, can expire an item after a timeout. Let's try to test it using fluxcapacitor. We need to run two processes within FC:

/tmp/memcached-1.4.15/memcached -p 1121
# 1) set key foo with timeout of 60 seconds
echo -e "set foo 0 60 2\r\naa"|nc localhost 1121
# 2) is foo still there? (it should)
echo "get foo"|nc localhost 1121
# 3) wait for 70 seconds
sleep 70
# 4) is foo still there? (it shouldn't)
echo "get foo"|nc localhost 1121
# 5) cleanup, don't need memcached anymore
killall memcached

It would be better to write a proper script for the "client" part, but we can also run it inline as a parameter to bash -c. Here it goes:

$ time ./fluxcapacitor -- /tmp/memcached-1.4.15/memcached -p 1121 -- bash -c 'echo -e "set foo 0 60 2\r\naa"|nc localhost 1121; echo "get foo"|nc localhost 1121; sleep 70; echo "get foo"|nc localhost 1121; killall memcached'
VALUE foo 0 2

real    0m0.521s

Memcached behaves as it should. The test takes about 500ms, instead of 70 seconds it would take without the FC.

You may want to pass --idleness=5ms option to FC to make it go fast. Without this option the test takes about 4 seconds.

In fact, fluxcapacitor is intended for similar scenarios - to make network protocol tests run faster. It's especially useful when mocking a time library is not an option - just like in our memcached example.

Redis expiration

Let's repeat the same test with redis. Again, we need:

/tmp/redis-2.6.14/src/redis-server --port 7379
# 1) set key foo with expiry time of 60 seconds and check it
echo -en "SET foo aa EX 60\r\nGET foo\r\n"|nc localhost 7379
# 2) wait 70 seconds
sleep 70
# 3) is foo still there? (it shouldn't)
echo -en "GET foo\r\n"|nc localhost 7379
# 4) cleanup
killall redis-server

To run it:

./fluxcapacitor --idleness=1ms -- /tmp/redis-2.6.14/src/redis-server --port 7379 -- bash -c 'echo -en "SET foo aa EX 60\r\nGET foo\r\n"|nc localhost 7379; sleep 70; echo -en "GET foo\r\n"|nc localhost 7379; killall redis-server'

real    0m1.109s

Wrapper scripts

I wouldn't recommend stacking very long parameters to fluxcapacitor for any serious application. Instead, you should create a wrapper script that spawns all the processes you need. This is a python script to run the redis test (derived from a fluxcapacitor example):

#!/usr/bin/env python
import os, time, signal, socket

server_pid = os.fork()
if server_pid == 0:
             ['redis-server', '--port', '7379'])
    s = socket.socket()
    s.connect(('', 7379))
    s.send('SET foo aa EX 60\r\n')
    assert s.recv(1024) == '+OK\r\n'
    s.send('GET foo\r\n')
    assert s.recv(1024) == '$2\r\naa\r\n'
    s.send('GET foo\r\n')
    assert s.recv(1024) == '$-1\r\n'
    os.kill(server_pid, signal.SIGINT)

Run it as usual:

$ ./fluxcapacitor --idleness=1ms -- ./

Final note

Fluxcapacitor is really good at speeding up client/server or protocol tests.

Actually fluxcapacitor was originally created to speed up the sockjs-protocol test suite. It wasn't possible to mock up a time library - we needed to run the tests against any sockjs http server, whether it's in erlang, node.js or python. It turns out the only way to run timeout-related tests in a reasonable time is to use fluxcapacitor.

You might ask how fluxcapacitor works. In short - it uses ptrace to catch syscalls like clock_gettime or gettimeofday and overwrite the kernel response with a fake time. Additionally it short-circuits syscalls that can block for a timeout like select or poll. For technical details see the README.

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