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-[[!meta copyright="Copyright © 2010, 2013 Free Software Foundation, Inc."]]
-
-[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
-id="license" text="Permission is granted to copy, distribute and/or modify this
-document under the terms of the GNU Free Documentation License, Version 1.2 or
-any later version published by the Free Software Foundation; with no Invariant
-Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license
-is included in the section entitled [[GNU Free Documentation
-License|/fdl]]."]]"""]]
-
-[[!tag open_issue_hurd]]
-
-IRC, #hurd, 2010-10-05
-
-    <sdschulze> antrik: Erlang-style parallelism might actually be interesting
-      for Hurd translators.
-    <sdschulze> There are certain similarities between Erlang's message boxes
-      and Mach ports.
-    <sdschulze> The problem is that all languages that implement the Erlang
-      actor model are VM-based.
-    <antrik> sdschulze: I guess that's because most systems don't offer this
-      kind of message passing functionality out of the box... perhaps on Hurd
-      it would be possible to implement an Erlang-like language natively?
-    <sdschulze> That would be quite attractive -- having the same API for
-      in-process parallelism and IPC.
-    <sdschulze> But I don't see why Erlang needs a VM...  It could also be
-      implemented in a library.
-    [...]
-    <sdschulze> BTW, Scala doesn't require a VM by design.  Its Erlang
-      implementation is a binary-compatible abstraction to Java.
-    [...]
-    <sdschulze> My point was that Erlang employs some ideas that might be
-      usable in the Hurd libraries.
-    <sdschulze> concerning multithreading stuff
-    <sdschulze> Unfortunately, it will not contribute to readability if done in
-      C.
-    <antrik> perhaps it's worth a look :-)
-    <sdschulze> Actually, a Mach port is pretty close to an Erlang actor.
-    <sdschulze> Currently, your I/O callbacks have to block when they're
-      waiting for something.
-    <sdschulze> What they should do is save the Mach port and respond as soon
-      as they can.
-    <sdschulze> So there should be a return status for "call me later, when I
-      tell you to" in the callbacks.
-    <sdschulze> Then the translator associates the Mach port with the summary
-      of the request in some data structure.
-    <sdschulze> As soon as the data is there, it tells the callback function to
-      appear again and fulfills the request.
-    <sdschulze> That's -- very roughly -- my idea.
-    <sdschulze> Actually, this eliminates the need for multithreading
-      completely.
-    <antrik> sdschulze: not sure whether you are talking about RPC level or
-      libc level here...
-    <sdschulze> It should be transparent to libc.
-    <sdschulze> If the client does a read() that cannot be answered immediatly,
-      it blocks.
-    <sdschulze> The difference is that there is no corresponding blocking
-      thread in the translator.
-    <antrik> ah, so you are talking about the server side only
-    <sdschulze> yes
-    <antrik> you mean the callback functions provided by the translator
-      implementation should return ASAP, and then the dispatcher would call
-      them again somehow
-    <sdschulze> allowing the server to be single-threaded, if desired
-    <sdschulze> exactly
-    <sdschulze> like: call_again (mach_port);
-    <antrik> but if the functions give up control, how does the dispatcher know
-      when they are ready to be activated again? or does it just poll?
-    <sdschulze> The translator knows this.
-    <sdschulze> hm...
-    <antrik> well, we are talking about the internal design of the translator,
-      right?
-    <antrik> I'm not saying it's impossible... but it's a bit tricky
-    <antrik> essentially, the callbacks would have to tell the dispatcher,
-      "call me again when there is an incoming message on this port"
-    <sdschulze> Say we have a filesystem translator.
-    <antrik> (or rather, it probably should actually call a *different*
-      callback when this happens)
-    <sdschulze> The client does a "read(...)".
-    <sdschulze> => A callback is called in the translator.
-    <antrik> let's call it disfs_S_io_read() ;-)
-    <antrik> err... diskfs
-    <sdschulze> The callback returns: SPECIAL_CALL_ME_LATER.
-    <sdschulze> yes, exactly that :)
-    <sdschulze> But before, it saves the position to be read in its internal
-      data structure.
-    <sdschulze> (a sorted tree, whatever)
-    <sdschulze> The main loop steps through the data structure, doing a read()
-      on the underlying translator (might be the disk partition).
-    <sdschulze> "Ah, gotcha, this is what the client with Mach port number 1234
-      wanted!  Call his callback again!"
-    <sdschulze> Then we're back in diskfs_S_io_read() and supply the data.
-    <antrik> so you want to move part of the handling into the main loop? while
-      I'm not fundamentally opposed to that, I'm not sure whether the
-      dispatcher/callback approach used by MIG makes much sense at all in this
-      case...
-    <antrik> my point is that this probably can be generalised. blocking
-      operations (I/O or other) usually wait for a reply message on a port --
-      in this case the port for the underlying store
-    <antrik> so the main loop would just need to wait for a reply message on
-      the port, without really knowing what it means
-    <sdschulze> on what port?
-    <antrik> so disfs_S_io_read() would send a request message to the store;
-      then it would return to the dispatcher, informing it to call
-      diskfs_S_io_read_finish() or something like that when there is a message
-      on the reply port
-    <antrik> main loop would add the reply port to the listening port bucket
-    <antrik> and as soon as the store provides the reply message, the
-      dispatcher would then call diskfs_S_io_read_finish() with the reply
-      message
-    <sdschulze> yes
-    <antrik> this might actually be doable without changes to MIG, and with
-      fairly small changes to libports... though libdiskfs etc. would probably
-      need major rewrites
-    <sdschulze> What made me think about it is that Mach port communication
-      doesn't block per se.
-    <antrik> all this is however ignoring the problem I mentioned yesterdays:
-      we need to handle page faults as well...
-    <sdschulze> It's MIG and POSIX that block.
-    <sdschulze> What about page faults?
-    <antrik> when the translator has some data mapped, instead of doing
-      explicit I/O, blocking can occur on normal memory access
-    <sdschulze> antrik: Well, I've only been talking about the server side so
-      far.
-    <antrik> sdschulze: this *is* the server side
-    <antrik> sdschulze: a filesystem translator can map the underlying store
-      for example
-    <antrik> (in fact that's what the ext2 translator does... which is why we
-      had this 2G partition limit)
-    <sdschulze> antrik: Ah, OK, so in other words, there are requests that it
-      can answer immediatly and others that it can't?
-    <antrik> that's not the issue. the issue is the the ext2 translator doesn't
-      issue explicit blocking io_read() operations on the underlying
-      store. instead, it just copies some of it's own address space from or to
-      the client; and if the page is not in physical memory, blocking occurs
-      during the copy
-    <antrik> so essentially we would need a way to return control to the
-      dispatcher when a page fault occurs
-    <sdschulze> antrik: Ah, so MIG will find the translator unresponsive?  (and
-      then do what?)
-    <antrik> sdschulze: again, this is not really a MIG thing. the main loop is
-      *not* in MIG -- it's provided by the tranlator, usually through libports
-    <sdschulze> OK, but as Mach IPC is asynchronous, a temporarily unresponsive
-      translator won't cause any severe harm?
-    <sdschulze> antrik: "Easy" solution: use a defined number of worker
-      threads.
-    <antrik> sdschulze: well, for most translators it doesn't do any harm if
-      they block. but if we want to accept that, there is no point in doing
-      this continuation stuff at all -- we could just use a single-threaded
-      implementation :-)
-
-[[continuation]].
-
-    <sdschulze> Hard solution: do use explicit I/O and invent a
-      read_no_pagefault() call.
-    <antrik> not sure what you mean exactly. what I would consider is something
-      like an exception handler around the copy code
-    <antrik> so if an exception occurs during the copy, control is returned to
-      the dispatcher; and once the pager informs us that the memory is
-      available, the copy is restarted. but this is not exacly simple...
-    <sdschulze> antrik: Ah, right.  If the read() blocks, you haven't gained
-      anything over blocking callbacks.
-    * sdschulze adopted an ML coding style for his C coding...
-    <sdschulze> antrik: Regarding it on the Mach level, all you want to do is
-      some communication on some ports.
-    <sdschulze> antrik: Only Unix's blocking I/O makes you want to use threads.
-    <sdschulze> Unless you have a multicore CPU, there's no good reason why you
-      would *ever* want multithreading.
-    <sdschulze> (except poor software design)
-    <sdschulze> antrik: Is there a reason why not to use io_read?
-    <antrik> sdschulze: I totally agree about multithreading...
-    <antrik> as for not using io_read(): some things are easier and/or more
-      efficient with mapping
-    <antrik> the Mach VM is really the most central part of Mach, and it's
-      greatest innovation...
-    <sdschulze> antrik: If you used explicit I/O, it would at least shift the
-      problem somewhere else...
-    <antrik> sure... but that's a workaround, not a solution
-    <sdschulze> I'm not sure how to deal with page faults then -- I know too
-      little about the Hurd's internal design.
-    <sdschulze> Non-blocking io_read only works if we address the client side,
-      too, BTW.
-    <sdschulze> which would be quite ugly in C IMHO
-    <sdschulze> announce_read (what, to, read, when_ready_callback);
-    <antrik> sdschulze: POSIX knows non-blocking I/O
-    <antrik> never checked how it works though
-    <sdschulze> Yes, but I doubt it does what we want.
-    <antrik> anyways, it's not too hard to do non-blocking io_read(). the
-      problem is that then you have to use MIG stubs directly, not the libc
-      function
-    <sdschulze> And you somehow need to get the answer.
-    <sdschulze> resp. get to know when it's ready
-    <antrik> the Hurd actually comes with a io_request.defs and io_reply.defs
-      by default. you just need to use them.
-    <sdschulze> oh, ok
-    <antrik> (instead of the usual io.defs, which does a blocking send/receive
-      in one step)
-    <sdschulze> I'd be interested how this works in Linux...
-    <antrik> what exactly?
-    <sdschulze> simultaneous requests on one FS
-    <antrik> ah, you mean the internal threading model of Linux? no idea
-    <sdschulze> if it uses threading at all
-    <antrik> youpi probably knows... and some others might as well
-    <sdschulze> Callbacks are still ugly...