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Re: Guile's I/O procedures should *not* do thread synchronization
From: |
Andy Wingo |
Subject: |
Re: Guile's I/O procedures should *not* do thread synchronization |
Date: |
Wed, 26 Mar 2014 20:45:09 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) |
Hi,
On Wed 26 Mar 2014 16:32, Mark H Weaver <address@hidden> writes:
> Andy Wingo <address@hidden> writes:
>
>> It seems to work for glibc streams. Why do you think that thread
>> synchronization is inappropriate for Guile if it works for glibc?
>
> In the Scheme world, things are very different. The Scheme standards
> provide only one set of I/O primitives, and do not mandate that they do
> thread synchronization.
Do you think they should crash when used from two threads at once? I
don't think that this acceptable for *any* Guile data structure.
>>> However, if we promise to do thread synchronization, we will condemn
>>> Guile to forever having dog slow 'read-char', 'peek-char', 'write-char',
>>> 'get-u8', 'peek-u8', and 'put-u8' operations.
>>
>> I think you are wrong about "dog slow". Uncontended mutexes are fast,
>
> I did some benchmarks of 'putchar' vs 'putchar_unlocked' in C, without
> contention. I think it's fair to assume that the GCC and GLIBC folks
> did a reasonably good job of making both of these as fast as they could.
>
> With gcc -O2, I tested two variants of this program: one with 'putchar'
> and one with 'putchar_unlocked'. On my YeeLoong (mips64el w/ N32 ABI),
> the 'putchar_unlocked' version is faster by a factor of 26.3.
On my i7-2620M, the difference is only a factor of 3.0.
Now I think I understand your perspective; 26x is terrible. But surely
this is an architecture problem, and not a Guile problem? The world
will only get more multithreaded, and ignoring that does no one any
service.
> Finally, robust programs will have to do their own explicit
> synchronization anyway. Multiple threads writing to the same port
> without explicit synchronization would lead to garbled output that is
> interleaved at unspecified points. The situation is even worse on the
> read side.
>
> In order to do proper I/O on the same port from multiple threads, the
> locking _must_ be done within code that understands the meaning of the
> data being read or written, because only such code can know where the
> data can be interleaved without producing garbage.
This is a good point but not germane to the crashing issue.
Andy
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