GHC status report
Dominick Samperi
djsamperi at gmail.com
Fri May 2 00:09:43 UTC 2014
It turns out that the 7.8 update fixed problems that were NOT
fixed by using -fno-ghci-sandbox with earlier versions of ghci,
like 7.6.3, so this flag by itself did not address the issue. I
assumed (perhaps incorrectly) that a full explanation must
include the new dynamic linking support.
With 7.6.3 the problem arises when using FFI and embeded R, which
involves linking to its shared library libR.so. Linking seems to be
successful, but the program crashes when run, with a message
about C stack overflow and/or a segfault. (Note that this does not
happen when the program is compiled using ghc...only ghci has
the problem.)
On the stack space question, embedded R tries to detect when
C stack space is exhausted by inspecting the address of variables
allocated in a function call, using code something like
this (in R src/main/errors.c):
void (R_CheckStack)(void)
{
int dummy;
intptr_t usage = R_CStackDir * (R_CStackStart - (uintptr_t)&dummy;
...
}
The seg fault happens with ghci 7.6.3 with or without the flag
-fno-ghci-sandbox.
With 7.8.2 the seg fault happens if the flag -fno-ghci-sandbox is
omitted; with the flag everything works fine.
If I understand your last comment correctly linking to libR should
continue to work, even if you revert to static linking of Haskell compiled
code via RTS linker. Since you say this is the way things have always
been, perhaps the real explanation for why 7.8 fixed the issue is that
some bug was fixed along the way...
Note that R is a C library, so the C++ issues that Carter mentions are
not a factor here.
Thanks,
Dominick
On Thu, May 1, 2014 at 5:27 PM, Simon Marlow <marlowsd at gmail.com> wrote:
> On 01/05/14 14:48, Dominick Samperi wrote:
>>
>> The problem with some graphics libraries used via FFI (and other libraries
>> that are not thread-safe), if I understand the situation correctly, is
>> that ghci
>> forks a thread when it shouldn't, causing some programs to miscalculate
>> the available stack space (because they think there is only one thread).
>
>>
>>
>> The new dynamic linking support and the flag -fno-ghci-sandbox fixes
>> this problem, and I would not vote for the removal of these features.
>
>
> So I understand how -fno-ghci-sandbox avoids problems with GUI libraries
> that use thread-local state. But how is dynamic linking involved here?
> What improved in GHC 7.8 relative to 7.6 for you? And could you clarify
> "miscalculate the available stack space"? What needs to calculate stack
> space? Why? C stack space?
>
> We can certainly make a smoother experience around -fno-ghci-sandbox for
> using GUI libraries.
>
>
>> It is not clear to me from Simon's original post how linking to all of
>> those C++ libs can continue to work if dynamic linking is removed
>> in 7.10? Perhaps I misunderstand what you mean by "revert to
>> static linking"?
>
>
> When I say "revert to static linking" I mean make GHCi static linked, and
> have it load Haskell code compiled for static linking using the RTS linker
> (like it did in 7.6). Foreign libraries would still be loaded using the
> system linker, as they always have been.
>
> To be clear, I'm not officially proposing that we drop dynamic linking, but
> I think it's worthwhile exploring the design space again, given that we know
> dynamic linking has been tougher than we expected.
>
> Cheers,
> Simon
>
>
>
>> Thanks,
>> Dominick
>>
>>
>> On Wed, Apr 30, 2014 at 9:26 PM, George Colpitts
>> <george.colpitts at gmail.com> wrote:
>>>
>>> To elaborate, in the past, I had a lot of problems using libraries from
>>> the
>>> ghci prompt on the Mac but I haven't tried recently.
>>>
>>> As an example, on the web page for the book the Haskell School of
>>> Expression
>>> it says:
>>>
>>> Note for OS X users: running graphics applications from GHCi is no longer
>>> supported. Instead, one has to compile a graphics program using GHC in
>>> order
>>> to run it (see example/GMIExamples.lhs for an example).
>>>
>>> I had similar problems using the Yale Euterpea music program from ghci.
>>> When
>>> I inquired I was referred to https://ghc.haskell.org/trac/ghc/ticket/4244
>>> and https://ghc.haskell.org/trac/ghc/ticket/781. I see that the latter is
>>> now scheduled for 7.10.1
>>>
>>>
>>> On Wed, Apr 30, 2014 at 1:45 PM, Simon Marlow <marlowsd at gmail.com> wrote:
>>>>
>>>>
>>>> On 30/04/2014 01:35, George Colpitts wrote:
>>>>>
>>>>>
>>>>> It doesn't have anything about the dynamic linking changes made for
>>>>> 7.8.
>>>>> I think it's worth mentioning the improvements we expect to get from
>>>>> that. The highlights of the release notes do mention it, so maybe that
>>>>> suffices.
>>>>>
>>>>> In particular, I'm hoping that it is going to fix a lot of problems
>>>>> with
>>>>> using foreign libraries such as OpenGL from ghci. I could be wrong
>>>>> about
>>>>> that though.
>>>>
>>>>
>>>>
>>>> I'd like to understand more about what those problems are. As a data
>>>> point, at Facebook we're using static linking (I compiled GHC with
>>>> DYNAMIC_GHC_PROGRAMS=NO), we're loading upwards of 50 3rd-party C++
>>>> libraries and one gigantic shared library consisting of a ton of
>>>> in-house
>>>> C++ code, together with all our Haskell code into GHCi, and it works
>>>> perfectly. The key to using the static linker is to not use it for C++
>>>> code
>>>> - you want all your external C++ code in shared libraries and load those
>>>> using the system linker.
>>>>
>>>> Dynamic linking has been a huge headache in GHC, and it's not clear that
>>>> it's an overall improvement compared with the static linker. Now that
>>>> 7.8
>>>> is out of the way, it's time to have a conversation about whether we
>>>> want to
>>>> do dynamic linking again for 7.10, or revert to static linking. I think
>>>> Austin is going to update
>>>> https://ghc.haskell.org/trac/ghc/wiki/DynamicGhcPrograms, and then we'll
>>>> see
>>>> where we stand.
>>>>
>>>> Cheers,
>>>> Simon
>>>>
>>>>
>>>>
>>>>>
>>>>> On Tue, Apr 29, 2014 at 6:13 PM, Simon Peyton Jones
>>>>> <simonpj at microsoft.com <mailto:simonpj at microsoft.com>> wrote:
>>>>>
>>>>> As Austin has told us, there's a draft of the *GHC Status Report
>>>>> for
>>>>> the HCAR*, here:____
>>>>>
>>>>> https://ghc.haskell.org/trac/ghc/wiki/Status/May14____
>>>>>
>>>>>
>>>>> Have we missed out something you have been working hard on? Do
>>>>> take a moment to add a bullet in an appropriate place (it's a
>>>>> wiki). I'd like to be sure that we are giving credit to all the
>>>>> appropriate people, so please help us fix that too. GHC is a team
>>>>> effort.____
>>>>>
>>>>> Deadline is 1 May I think.____
>>>>>
>>>>> Thanks____
>>>>>
>>>>> Simon____
>>>>>
>>>>> __ __
>>>>>
>>>>>
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>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
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