On Thu, Nov 11, 2010 at 05:47:46PM -0800, Ben Goldberg wrote: > I would like to know, is perl6 going to have something like select > (with arguments created by fileno/vec), or something like IO::Select > (with which the user doesn't need to know about the implementation, > which happens to be done with fileno/vec/select), or only an event > loop.
I will be unhappy if Perl 6 doesn't provide all three. However, the spec in question doesn't really exist. The "IO" synopsis is garbage and should be deleted and rewritten from scratch by someone who knows what they're talking about. > I would recommend that there NOT be any sort of fileno exposed to the > user, unless he goes out of the way to get it -- any function (in > particular, posix functions) should simply take a perl filehandle, and > that function in turn would pull out the fileno (or fail > appropriately, if the filehandle doesn't have a fileno). If users > want to know if filehandles correspond to the same underlying file, > then there could be a method -- perhaps $fh.uses_same_desciptor($fh2), > or somesuch. This goes without saying. One caveat - it should be possible to pass integer file descriptors. > If there's a select() builtin (and I'd much rather that there not be > -- it should be hidden away in a class, like perl5's IO::Select), I'd > very much hope that it would take and return Sets of filehandles, not > vec packed strings. I'd prefer there not be one[**] Absolutely not. TIMTOWDI. There will be a select() function (in the POSIX module, not the default namespace!), and it will take parameters as close as possible to POSIX.1's definition. Perl without system calls is not Perl. I'm totally fine with discouraging casual use, though, which is why it shouldn't be in the default namespace. > If there's something like perl5's IO::Select, it should be able to > "just work" regardless of whether the perl filehandles are sockets, > regular files, or user-created pure-perl filehandles (which might > never block, or which might use one or more normal filehandles > internally, which in turn might potentially block). This is what I'd > prefer. That is a good doctoral thesis topic. > Lastly, if perl6 has an efficient enough built-in event loop, and > sufficiently lightweight coroutines (or maybe I should say fibers?), > then we might not need to have any kind of explicit multiplexing. TIMTOWDI. Perl without system calls is not Perl. > For example, any time user code does a read operation on a handle that > isn't (from the user code's point of view) in nonblocking mode, the > filehandle implementation would tell the the event loop to yield to it > when the handle becomes readable, then it would yield to the event > loop, then (once it gets back control) read from the handle.[*] This is why S16 is junk - too much blue-sky thinking, not enough pragmatism and practical experience. > This provides lots of convenience, but it would resemble Java IO > before the NIO -- except with one fiber per handle instead of one > thread per handle. Coroutines/green threads/fibers are much lighter > weight than "real" threads, but often aren't as fast as a well-written > select() loop specially written for the user's task. > > Thus, I'd hope for perl6 to have an IO::Select, and automatically- > yielding [*] blocking IO, and not have a select() builtin. [**] TIMTOWDI. > [*] This is a simplification: > A) If a user explicitly marks a filehandle as not yielding to other > coroutines, it would do a blocking read (or whatever) instead of going > through the event loop rigmarole. > B) If perl6 was compiled with an asynchronous IO library (or is on > windows and is not using stdio and has (Read|Write)FileEX support), > then it might start the Async IO operation, tell the event loop to > wake it when the operation completes, then yield to the event loop. > C) Depending on circumstances, it *may* be more efficient to have the > event loop itself do the reading or other IO itself, and schedule the > fibers for which the IO was done, than to have the fibers do the IO. > TMTOWTDI. This would be especially important if perl is compiled with > async IO -- the event loop might first wait for the fds to be readable/ > etc, *then* start the async IO for those fds, then schedule the fibers > for which the performed IO has completed, thus minimizing the number > of outstanding async io operations. > > [**] The main reason I'd prefer that perl6 not have a select() builtin > is that every time it's called perl would need to convert user-level > Sets of filehandles into the underlying implementations' versions of > them (fd_sets on unixy, fd_sets and/or an event queue handle on > windows), and then back to perl Set objects, and free up the > implementation version of the filehandle set... this is inefficient. > > A well written IO::Select-like object could create (potentially empty) > versions of the OS's set of filehandles when it's created, add to that > set as needed, and NOT destroy that implementation-specific set until > the IO::Select object itself is destroyed. Perl5's IO::Select does > this with the packed bitsets that it creates to pas to select. It > could do improve it's efficiency by using fd_sets instead of > bitstrings, and not use the perl select(), but the C select(2) > instead. > > Better still would be epoll. In this case, avoiding repeated setup > makes an object multiplexer model enormously more efficient than > something like select(). > > Similarly, on windows, if we WSAEventSelect or WSAAsyncSelect to > create readability/ writability/ etc events for IO operations we want > to wait on, and [WSA]WaitForMultipleEvents as the blocking operation, > then having an object multiplexer (which keeps events between one call > to the next) is far better than a simple subroutine (which needs to > cancel those events after it blocks and before it returns). Forbidding things out of idealistic concerns like API purity is not the Perl way. -sorear
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