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diff --git a/open_issues/multithreading/erlang-style_parallelism.mdwn b/open_issues/multithreading/erlang-style_parallelism.mdwn new file mode 100644 index 00000000..75539848 --- /dev/null +++ b/open_issues/multithreading/erlang-style_parallelism.mdwn @@ -0,0 +1,201 @@ +[[!meta copyright="Copyright © 2010 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 :-) + <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... |