为什么这些线程化的ActiveRecord查询不会同时运行？(Why won't these threaded ActiveRecord queries run concurrently?)

 我正在尝试理解生产中的问题，所以我把这段代码扔到dev中的控制器动作中进行测试： 
 
start = Time.now
num_threads = 6
results = Queue.new
saved_results = []
threads = []
connections = []
semaphore = Mutex.new

# start threads
(1..num_threads).each do |i|
  threads << Thread.new do
    #semaphore.synchronize { connections << ActiveRecord::Base.connection } # for cleanup?

    #ActiveRecord::Base.connection.execute("select sleep(1.6);")   # runs sequentially
    sleep(1.6)                                                    # runs concurrently
    result = User.find_by_id(i)
    results << [i, result]
  end
end

# end option 1 - let everyone finish
threads.each(&:join)

# end option 2 - simulate early exit condition
#while saved_results.count < 3 do saved_results << results.pop end
#threads.each(&:exit)

# cleanup/close open connections?
#connections.select(&:active?).each(&:disconnect!)

elapsed = Time.now - start
render :text => [ elapsed.to_s, saved_results.size, results.size ].join(", ")
 
 如预期的那样， sleep(1.6)在大约1.6秒内执行。 
 
 但是，ActiveRecord select sleep(1.6); 尽管mysql控制台show processlist; ，但需要6 * 1.6 = 9.6秒show processlist; 显示为每个线程打开的独立连接*。 
 
 这是怎么回事？ 为什么ActiveRecord查询不会同时运行？ 我也在生产控制台中体验过这一点。 
 
 我确实有config.threadsafe! 在config/environment.rb设置。 如果重要，我正在使用Rails 2.3。 
 
 *这些连接必须手动关闭？ 生产总是有很多无用的开放连接，导致Mysql::Error: Too many connections 。 我可能会将此问题作为另一个问题提交。 

I'm experimenting to understand a problem in production, so I've thrown this snippet inside a controller action in dev to test:
 
start = Time.now
num_threads = 6
results = Queue.new
saved_results = []
threads = []
connections = []
semaphore = Mutex.new

# start threads
(1..num_threads).each do |i|
  threads << Thread.new do
    #semaphore.synchronize { connections << ActiveRecord::Base.connection } # for cleanup?

    #ActiveRecord::Base.connection.execute("select sleep(1.6);")   # runs sequentially
    sleep(1.6)                                                    # runs concurrently
    result = User.find_by_id(i)
    results << [i, result]
  end
end

# end option 1 - let everyone finish
threads.each(&:join)

# end option 2 - simulate early exit condition
#while saved_results.count < 3 do saved_results << results.pop end
#threads.each(&:exit)

# cleanup/close open connections?
#connections.select(&:active?).each(&:disconnect!)

elapsed = Time.now - start
render :text => [ elapsed.to_s, saved_results.size, results.size ].join(", ")
 
sleep(1.6) executes in approximately 1.6 seconds, as expected.
 
However, the ActiveRecord select sleep(1.6); takes 6 * 1.6 = 9.6 seconds, despite mysql console show processlist; displaying that independent connections are opened for each thread*.
 
What's going on? Why won't the ActiveRecord queries run concurrently? I've also experienced this in production console.
 
I do have config.threadsafe! set in config/environment.rb. If it matters, I'm using Rails 2.3.
 
*These connections have to be manually closed? Production always has a lot of open connections that are doing nothing, causing Mysql::Error: Too many connections. I'll probably submit this issue as a another question.

原文：https://stackoverflow.com/questions/23689570