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Re: [PATCH 03/10] introduce systhread layer
From: |
Ken Raeburn |
Subject: |
Re: [PATCH 03/10] introduce systhread layer |
Date: |
Mon, 13 Aug 2012 06:21:24 -0400 |
>
> On 8/9/2012 12:38 PM, Tom Tromey wrote:
>> This introduces the low-level system threading support. It also adds
>> the global lock. The low-level support is a bit over-eager, in that
>> even at the end of the present series, it will not all be used. I
>> think thiat is ok since I plan to use it all eventually.
>>
>> I've only implemented the pthreads-based version. I think it should
>> be relatively clear how to port this to other systems, though.
>>
>> I'd also like to do a "no threads" port that will turn most things
>> into no-ops, and have thread-creation fail. I was thinking perhaps
>> I'd make a future (provide 'threads) conditional on threads actually
>> working. Thoughts on this?
Unless there's a platform where the support isn't possible, I'd suggest not
doing this last bit, so that Emacs code (both Lisp and C) can assume threads
are available. Otherwise, either you can't use threads at all in the release,
which means that code won't be exercised very well, or you write code that has
to cope with both modes (which may simply mean crippling some features when
threads aren't available, and ensuring that the rest of the code still works
properly without those features), and probably only really gets tested well in
the mode where threads are available.
What benefit do you think providing this "no threads" port would have?
On Aug 9, 2012, at 21:39, Daniel Colascione wrote:
> If threads don't execute simultaneously anyway (and if I understand your
> design
> correctly, the global lock ensures they don't), then it might be worthwhile to
> also support a "green threads" implementation like GNU Pth or Windows fibers
> in
> order to avoid OS-level context switch overhead.
Where every I/O operation needs to be rewritten to call some helper function
that do the thread switching? That sounds like a fine way to make the code
really ugly. :-( I think, too, we'd probably want support libraries (X11,
image or sound processing, TLS, etc) to be able to do their thing (I/O,
parsing, encryption) while another thread runs Lisp code, and if those
libraries aren't written to use Pth or whatever, that won't happen.
I've worked on code where performance of the thread-switching support was a big
deal. I don't see Emacs falling into that category, at least not any time
soon. On the other hand, I do hope that people find more uses for thread
support than occur to me right now, so….
Ken