From: Joel VanderWerf <vjoel@...>
Date: 2006-05-23T03:20:30+09:00
Subject: Re: sysread changes behavior in the presence of threads?

Francis Cianfrocca wrote:
> We're threadjacking, so I'll keep it short. The point of taking a very

I like this thread, but I'll nudge it in the ruby direction...

> restrictive view of synchronization is to prevent incorrect concurrency.
> Your example depends on the implementation of aMap and of calculateValue
> not
> to do evil things. (One of the evil things they can do is simply to run for
> several milliseconds, or make a blocking I/O call. This can give you a bad
> case of mutex contention, which is exceptionally costly in many modern
> implementations.) This means that the program may change behavior with
> respect to concurrency across platforms, hardware, and also across time (as
> the code inside those called functions changes). Ruby adds the further
> dimension that the code you call under lock may have been metaprogrammed on
> the fly.

If you're talking about ruby threads, sync mechanisms are expensive with
or without contention, since they are built on top of Thread.critical.

require 'thread'
require 'benchmark'

N = 1_000_000

Benchmark.bmbm(12) do |bm|

  bm.report("no Thread.critical") do
    x = 0
    N.times do
      x += 1
    end
  end

  bm.report("Thread.critical") do
    x = 0
    N.times do
      Thread.critical = true
      x += 1
      Thread.critical = false
    end
  end

  bm.report("Thread.exclusive") do
    x = 0
    N.times do
      Thread.exclusive do
        x += 1
      end
    end
  end

end

__END__

Rehearsal ------------------------------------------------------
no Thread.critical   1.040000   0.010000   1.050000 (  1.063126)
Thread.critical      1.130000   0.000000   1.130000 (  1.153935)
Thread.exclusive     2.660000   0.000000   2.660000 (  2.704054)
--------------------------------------------- total: 4.840000sec

                         user     system      total        real
no Thread.critical   0.360000   0.000000   0.360000 (  0.366529)
Thread.critical      0.910000   0.010000   0.920000 (  0.922671)
Thread.exclusive     2.670000   0.000000   2.670000 (  2.692268)


-- 
      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407