| … | |
… | |
| 40 | points in your program, so locking and parallel access are rarely an |
40 | points in your program, so locking and parallel access are rarely an |
| 41 | issue, making thread programming much safer and easier than using other |
41 | issue, making thread programming much safer and easier than using other |
| 42 | thread models. |
42 | thread models. |
| 43 | |
43 | |
| 44 | Unlike the so-called "Perl threads" (which are not actually real threads |
44 | Unlike the so-called "Perl threads" (which are not actually real threads |
| 45 | but only the windows process emulation ported to unix, and as such act |
45 | but only the windows process emulation (see section of same name for more |
| 46 | as processes), Coro provides a full shared address space, which makes |
46 | details) ported to unix, and as such act as processes), Coro provides |
| 47 | communication between threads very easy. And Coro's threads are fast, |
47 | a full shared address space, which makes communication between threads |
| 48 | too: disabling the Windows process emulation code in your perl and using |
48 | very easy. And Coro's threads are fast, too: disabling the Windows |
| 49 | Coro can easily result in a two to four times speed increase for your |
49 | process emulation code in your perl and using Coro can easily result in |
| 50 | programs. A parallel matrix multiplication benchmark runs over 300 times |
50 | a two to four times speed increase for your programs. A parallel matrix |
| 51 | faster on a single core than perl's pseudo-threads on a quad core using |
51 | multiplication benchmark runs over 300 times faster on a single core than |
| 52 | all four cores. |
52 | perl's pseudo-threads on a quad core using all four cores. |
| 53 | |
53 | |
| 54 | Coro achieves that by supporting multiple running interpreters that share |
54 | Coro achieves that by supporting multiple running interpreters that share |
| 55 | data, which is especially useful to code pseudo-parallel processes and |
55 | data, which is especially useful to code pseudo-parallel processes and |
| 56 | for event-based programming, such as multiple HTTP-GET requests running |
56 | for event-based programming, such as multiple HTTP-GET requests running |
| 57 | concurrently. See L<Coro::AnyEvent> to learn more on how to integrate Coro |
57 | concurrently. See L<Coro::AnyEvent> to learn more on how to integrate Coro |
| … | |
… | |
| 854 | works. |
854 | works. |
| 855 | |
855 | |
| 856 | =back |
856 | =back |
| 857 | |
857 | |
| 858 | |
858 | |
|
|
859 | =head1 WINDOWS PROCESS EMULATION |
|
|
860 | |
|
|
861 | A great many people seem to be confused about ithreads (for example, Chip |
|
|
862 | Salzenberg called me unintelligent, incapable, stupid and ingullible, |
|
|
863 | while in the same mail making rather confused statements about perl |
|
|
864 | ithreads (for example, that memory or files would be shared), showing his |
|
|
865 | lack of understanding of this area - if it is hard to understand for Chip, |
|
|
866 | it is probably not obvious to everybody). |
|
|
867 | |
|
|
868 | What follows is an ultra-condensed version of my talk about threads in |
|
|
869 | scripting languages given onthe perl workshop 2009: |
|
|
870 | |
|
|
871 | The so-called "ithreads" were originally implemented for two reasons: |
|
|
872 | first, to (badly) emulate unix processes on native win32 perls, and |
|
|
873 | secondly, to replace the older, real thread model ("5.005-threads"). |
|
|
874 | |
|
|
875 | It does that by using threads instead of OS processes. The difference |
|
|
876 | between processes and threads is that threads share memory (and other |
|
|
877 | state, such as files) between threads within a single process, while |
|
|
878 | processes do not share anything (at least not semantically). That |
|
|
879 | means that modifications done by one thread are seen by others, while |
|
|
880 | modifications by one process are not seen by other processes. |
|
|
881 | |
|
|
882 | The "ithreads" work exactly like that: when creating a new ithreads |
|
|
883 | process, all state is copied (memory is copied physically, files and code |
|
|
884 | is copied logically). Afterwards, it isolates all modifications. On UNIX, |
|
|
885 | the same behaviour can be achieved by using operating system processes, |
|
|
886 | except that UNIX typically uses hardware built into the system to do this |
|
|
887 | efficiently, while the windows process emulation emulates this hardware in |
|
|
888 | software (rather efficiently, but of course it is still much slower than |
|
|
889 | dedicated hardware). |
|
|
890 | |
|
|
891 | As mentioned before, loading code, modifying code, modifying data |
|
|
892 | structures and so on is only visible in the ithreads process doing the |
|
|
893 | modification, not in other ithread processes within the same OS process. |
|
|
894 | |
|
|
895 | This is why "ithreads" do not implement threads for perl at all, only |
|
|
896 | processes. What makes it so bad is that on non-windows platforms, you can |
|
|
897 | actually take advantage of custom hardware for this purpose (as evidenced |
|
|
898 | by the forks module, which gives you the (i-) threads API, just much |
|
|
899 | faster). |
|
|
900 | |
|
|
901 | Sharing data is in the i-threads model is done by transfering data |
|
|
902 | structures between threads using copying semantics, which is very slow - |
|
|
903 | shared data simply does not exist. Benchmarks using i-threads which are |
|
|
904 | communication-intensive show extremely bad behaviour with i-threads (in |
|
|
905 | fact, so bad that Coro, which cannot take direct advantage of multiple |
|
|
906 | CPUs, is often orders of magnitude faster because it shares data using |
|
|
907 | real threads, refer to my talk for details). |
|
|
908 | |
|
|
909 | As summary, i-threads *use* threads to implement processes, while |
|
|
910 | the compatible forks module *uses* processes to emulate, uhm, |
|
|
911 | processes. I-threads slow down every perl program when enabled, and |
|
|
912 | outside of windows, serve no (or little) practical purpose, but |
|
|
913 | disadvantages every single-threaded Perl program. |
|
|
914 | |
|
|
915 | This is the reason that I try to avoid the name "ithreads", as it is |
|
|
916 | misleading as it implies that it implements some kind of thread model for |
|
|
917 | perl, and prefer the name "windows process emulation", which describes the |
|
|
918 | actual use and behaviour of it much better. |
|
|
919 | |
| 859 | =head1 SEE ALSO |
920 | =head1 SEE ALSO |
| 860 | |
921 | |
| 861 | Event-Loop integration: L<Coro::AnyEvent>, L<Coro::EV>, L<Coro::Event>. |
922 | Event-Loop integration: L<Coro::AnyEvent>, L<Coro::EV>, L<Coro::Event>. |
| 862 | |
923 | |
| 863 | Debugging: L<Coro::Debug>. |
924 | Debugging: L<Coro::Debug>. |
