Fork actually copy the memory from one process to another

That depends on the operating system. I don't know of any Unix-like OSes worth using that don't have copy-on-write for forking.

With threads the memory *is* shared

That depends on the threading model. With a shared-everything model, threads can safely share most of everything (except for parts of the execution context). Of course, then you have the potential for deadlocks and weird execution order if you want atomic operations where it matters. With a shared-almost-nothing model, you won't even have the potential memory savings of copy-on-write unless the implementors did a lot of work.

if it did fork, these procesess that eat memory would run in a forked procecss and the memory would be given back to the Operating System.

The OS should reclaim the copied memory pages, yes, but not the shared memory pages.

In reply to Re^4: Threading vs perl by chromatic
in thread Threading vs perl by Eyck

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


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.