Beefy Boxes and Bandwidth Generously Provided by pair Networks
"be consistent"
 
PerlMonks  

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??

I was recently faced with a thread that used .= on a shared variable, and I wondered if that was safe. I figured I'd write up a introductory tutorial on the answer I found. For simplicity, we'll look at ++ first.


The following code outputs 400,000:

my $count = 100_000; my $num_calls = 4; my $sum = 0; sub inc { ++$sum for 1..$count; } inc() for 1..$num_calls; print("$sum\n"); # 400000

If you ran the 4 calls to inc in parallel, would the answer still be 400,000? Not likely, if you don't change inc.

use threads; use threads::shared; my $count = 100_000; my $num_calls = 4; my $sum : shared = 0; sub inc { ++$sum for 1..$count; } $_->join for map { threads->create( \&inc ) } 1..$num_calls; print("$sum\n"); # 314813

That's because there is a race condition.

+=======================+ | CPU | +-----------+-----------+ | thread 1 | thread 2 | +===========+===========+ | ... | | T | load $sum | | i | inc | | m +-----------+-----------+ e | | ... | | | | load $sum | | | | inc | v | | save $sum | | | ... | +-----------+-----------+ | save $sum | | | ... | | +===========+===========+

The solution is to protect the critical section using a thread synchronization mechanism such as lock.

use threads; use threads::shared; my $count = 100_000; my $num_calls = 4; my $sum : shared = 0; sub inc { for (1..$count) { lock($sum); ++$sum } } $_->join for map { threads->create( \&inc ) } 1..$num_calls; print("$sum\n"); # 400000

Whenever an transformation operation (read ⇒ manipulate ⇒ write) is performed on a shared variable, locking is needed. See threads::shared for tools to do this.

The program behind the <spoiler> below outputs results similar to the following:

++s sum = 233564 (expecting 400000) s+=1 sum = 143915 (expecting 400000) c.=l length = 248149 (expecting 400000) c=c.l length = 123360 (expecting 400000)

As you can see, +=, .= and = . are also not atomic. The program can only prove that an operator isn't atomic (i.e. is interruptable). It cannot prove that an operator is atomic (i.e. is not interruptable). If you're getting the "expecting" result, try upping $count and/or $threads.

Update: Added the preface and links to Wikipedia.

Added to Tutorials by planetscape

( keep:0 edit:6 reap:0 )


In reply to Threads: why locking is required when using shared variables by ikegami

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":



  • Are you posting in the right place? Check out Where do I post X? to know for sure.
  • Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
    <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>
  • Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
  • Want more info? How to link or or How to display code and escape characters are good places to start.
Log In?
Username:
Password:

What's my password?
Create A New User
Domain Nodelet?
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others making s'mores by the fire in the courtyard of the Monastery: (4)
As of 2021-12-01 12:30 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?
    R or B?



    Results (7 votes). Check out past polls.

    Notices?