Parallel
Run any code in parallel Processes(> use all CPUs) or Threads(> speedup blocking operations).
Best suited for map-reduce or e.g. parallel downloads/uploads.
Install
gem install parallel
Usage
# 2 CPUs -> work in 2 processes (a,b + c)
results = Parallel.map(['a','b','c']) do, one_letter, expensive_calculation(one_letter)
end
# 3 Processes -> finished after 1 run
results = Parallel.map(['a','b','c'], in_processes: 3) {, one_letter, ... }
# 3 Threads -> finished after 1 run
results = Parallel.map(['a','b','c'], in_threads: 3) {, one_letter, ... }
Same can be done with each
Parallel.each(['a','b','c']) {, one_letter, ... }
or each_with_index, map_with_index, flat_map
Produce one item at a time with lambda (anything that responds to .call) or Queue.
items = [1,2,3]
Parallel.each( -> { items.pop, Parallel::Stop }) {, number, ... }
You can also call any? or all?, which work the same way as Array#any? and Array#all?.
Parallel.any?([1,2,3,4,5,6,7]) {, number, number == 4 }
# => true
Parallel.all?([1,2,nil,4,5]) {, number, number != nil }
# => false
Processes/Threads are workers, they grab the next piece of work when they finish.
Processes
- Speedup through multiple CPUs
- Speedup for blocking operations
- Variables are protected from change
- Extra memory used
- Child processes are killed when your main process is killed through Ctrl+c or kill -2
Threads
- Speedup for blocking operations
- Variables can be shared/modified
- No extra memory used
ActiveRecord
Connection Lost
- Multithreading needs connection pooling, forks need reconnects
- Adjust connection pool size in
config/database.ymlwhen multithreading
# reproducibly fixes things (spec/cases/map_with_ar.rb)
Parallel.each(User.all, in_processes: 8) do, user, user.update_attribute(:some_attribute, some_value)
end
User.connection.reconnect!
# maybe helps: explicitly use connection pool
Parallel.each(User.all, in_threads: 8) do, user, ActiveRecord::Base.connection_pool.with_connection do
user.update_attribute(:some_attribute, some_value)
end
end
# maybe helps: reconnect once inside every fork
Parallel.each(User.all, in_processes: 8) do, user, @reconnected, = User.connection.reconnect!, true
user.update_attribute(:some_attribute, some_value)
end
NameError: uninitialized constant
A race happens when ActiveRecord models are autoloaded inside parallel threads
in environments that lazy-load, like development, test, or migrations.
To fix, autoloaded classes before the parallel block with either require '<modelname>' or ModelName.class.
Break
Parallel.map(User.all) do, user, raise Parallel::Break # -> stops after all current items are finished
end
Kill
Only use if whatever is executing in the sub-command is safe to kill at any point
Parallel.map([1,2,3]) do, x, raise Parallel::Kill if x == 1# -> stop all sub-processes, killing them instantly
sleep 100 # Do stuff
end
Progress / ETA
# gem install ruby-progressbar
Parallel.map(1..50, progress: "Doing stuff") { sleep 1 }
# Doing stuff, ETA: 00:00:02, ====================, Time: 00:00:10
Use :finish or :start hook to get progress information.
:starthas item and index:finishhas item, index, result
They are called on the main process and protected with a mutex.
Parallel.map(1..100, finish: -> (item, i, result) { ... do something ... }) { sleep 1 }
NOTE: If all you are trying to do is get the index, it is much more performant to use each_with_index instead.
Worker number
Use Parallel.worker_number to determine the worker slot in which your
task is running.
Parallel.each(1..5, :in_processes => 2) {, i, puts "Item: #{i}, Worker: #{Parallel.worker_number}" }
Item: 1, Worker: 1
Item: 2, Worker: 0
Item: 3, Worker: 1
Item: 4, Worker: 0
Item: 5, Worker: 1
Tips
Here are a few notable options.
- [Benchmark/Test] Disable threading/forking with
in_threads: 0orin_processes: 0, great to test performance or to debug parallel issues - [Isolation] Do not reuse previous worker processes:
isolation: true - [Stop all processses with an alternate interrupt signal]
'INT'(fromctrl+c) is caught by default. Catch'TERM'(fromkill) withinterrupt_signal: 'TERM' - [Process count via ENV]
PARALLEL_PROCESSOR_COUNT=16will use16instead of the number of processors detected. This is used to reconfigure a tool usingparallelwithout inserting custom logic.
TODO
- Replace Signal trapping with simple
rescue Interrupthandler
Authors
Contributors
- Przemyslaw Wroblewski
- TJ Holowaychuk
- Masatomo Nakano
- Fred Wu
- mikezter
- Jeremy Durham
- Nick Gauthier
- Andrew Bowerman
- Byron Bowerman
- Mikko Kokkonen
- brian p o'rourke
- [Norio Sato]
- Neal Stewart
- Jurriaan Pruis
- Rob Worley
- Tasveer Singh
- Joachim
- yaoguai
- Bartosz Dziewoński
- yaoguai
- Guillaume Hain
- Adam Wróbel
- Matthew Brennan
- Brendan Dougherty
- Daniel Finnie
- Philip M. White
- Arlan Jaska
- Sean Walbran
- Nathan Broadbent
- Yuki Inoue
- Takumasa Ochi
Michael Grosser
michael@grosser.it
License: MIT