RE: [Caml-list] Re: ocaml sefault in bytecode: unanswered questions

["ivan chollet" <ivan.chollet@free.fr>]

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Thanks for your help Sylvain, David & Michel for all these explanations.
These insightful comments have enlightened my vision of the ocaml runtime.

It all makes sense so I’ll follow your advice and stop worrying about the
GC.

Let’s come back to my problem. I played with ocaml interpreter a bit, and
there is a behavior that I cannot understand. This behavior actually creates
a fairly large range of problems all over my code base. See the following
snippet:

 

# let q = Queue.create () in

  Queue.push 0 q;

  q == q;;

- : bool = true

 

Standard behavior.

Now let see this:

 

# let q = Queue.create () in

  Queue.push 0 q;

q = q;;

 

which hangs for ever…

I would have thought physical equality implies structural equality, but it
doesn’t seem like it.

Can you please explain to me what’s wrong there?

 

Thanks again for your help!

 

 

From: David Allsopp [mailto:dra-news@metastack.com] 
Sent: dimanche 9 août 2009 10:39
To: 'ivan chollet'
Subject: RE: [Caml-list] Re: ocaml sefault in bytecode: unanswered questions

 

Chapter 18.2 of the manual is what you need – it explains the value type
used internally for the heap. In hyper-simplistic terms, when the garbage
collector runs it just assumes that anything could be a pointer and applies
a colouring to the heap to determine reachable values (the memory bit
required for this colouring is why integer values are only 31 or 63 bits in
OCaml – you get the performance of their being unboxed, but the hit of
losing a bit for the colouring). 

 

Personally, I wouldn’t spend so much time worrying about the Garbage
Collector – it just works! To answer part of your original question: using
only the standard library, the only way that you can segfault a program
(other than by abusing Marshal) is to abuse the Obj module (Obj.magic in
particular allows you to circumvent the type system).  However, the
documentation for Obj says it all – “Operations on internal representations
of values. Not for the casual user.”

 

If you’ve seen an example in the past which segfaulted the bytecode runtime,
then it was a bug in the compiler... if you can still produce a repro case
then raise a bug in mantis. However, the Garbage Collector received a lot of
attention in both 3.10 (big overhaul of lazy values) and 3.11 (change the
way the memory tables are implemented) so if it was with an older version of
OCaml that you saw it then the error may have disappeared. Similarly, the
semantics of the bytecode runtime and native runtime are supposed to be the
same – but there are a few instances where the native runtime intentionally
segfaults (for performance) where the bytecode runtime would raise an
exception (stack overflow is the principal one). 

 

Best,

 

 

D

 

 

From: ivan chollet [mailto:ivan.chollet@free.fr] 
Sent: 09 August 2009 08:59
To: 'David Allsopp'
Cc: caml-list@yquem.inria.fr; 'Edgar Friendly'
Subject: RE: [Caml-list] Re: ocaml sefault in bytecode: unanswered questions

 

Definitely.

Actually I had my real-world case in mind, so let me explain further with
the following snippet:

 

let myfun = doSomeWork (); myref := List.filter somefilterfunction !myref in

List.iter myfun !myref

 

In this case, a new linked list is created in each iteration of the
List.filter. (that is, a new list allocation)

Then, if doSomeWork () does a lot of work and lots of allocations, the GC
will be called on a regular basis while in function myfun. 

Then List.iter is tail-recursive, so it doesn’t push its successive
arguments on the stack. So the successively created myref become unreachable
while still iterating on them.

So my question is, how does the GC know whether all these myref created
throughout the iteration are collectable or not? I’m curious about how these
myref are tagged/untagged by the garbage collector. Maybe pointing me the
relevant portions of the ocamlrun source code would be nice.

 

Anyway no worries, once I get a bit more free I’ll just try to read about
this topic by myself. Also I’ll try to send you some source code for that.
All this will take me a little while, so see you next time!

 

 

 

From: David Allsopp [mailto:dra-news@metastack.com] 
Sent: samedi 8 août 2009 19:25
To: 'ivan chollet'
Cc: caml-list@yquem.inria.fr
Subject: RE: [Caml-list] Re: ocaml sefault in bytecode: unanswered questions

 

When you pass a value to a function, you create a pointer to that value in
the OCaml runtime – the GC can’t collect the old value until List.iter
completes because the value is still live (internally, it’s part of a local
root but, in practice, as List.iter is implemented in OCaml directly it’s
because an OCaml function parameter references the value). Note that in this
example:

 

let a = [1; 2; 3]

and b = [4; 5; 6]

and c = [7; 8; 9] in

let myref = ref a in

(* No allocations are done after here *)

  myref := a;

  myref := b;

  myref := c;;

 

the assignments to [myref] do not result in any memory being allocated at
all (my point is that action of assigning to a reference does not implicitly
result in an allocation).

 

 

David

 

From: caml-list-bounces@yquem.inria.fr
[mailto:caml-list-bounces@yquem.inria.fr] On Behalf Of ivan chollet
Sent: 08 August 2009 18:10
To: ivan.chollet@free.fr
Cc: caml-list@yquem.inria.fr
Subject: [Caml-list] Re: ocaml sefault in bytecode: unanswered questions

 

Yes it was a freebsd 6.4 with ocaml 3.10.2

I’ll run the program on linux later and see how it goes.

 

Thanks for your advices regarding debugging. I pretty much tried all of
these though… the thing is my error is not an ocaml error at runtime but an
error of the ocaml runtime. And to analyze a core dump of ocamlrun, I just
thought my best bet was gdb. Whatever.

 

OK I’ll try to provide you with a minimal ocaml code that produce an
ocamlrun error. Might take a little while as I’m not free.

In the meantime, I’ve got a newbie question regarding ocaml garbage
collector and the same List.iter stuff:

Say you do a “List.iter myfun !myref”, where !myref is a list (hehe…), and
where myfun is a function that does reallocations of myref (that is
affectations like myref := [some new or old objects]). The pointers myref
that are generated through this process are destroyed each time a new
reallocation of myref is done. Of course the underlying linked lists that
are not referenced anymore shouldn’t be collected by the GC before the end
of the main “List.iter”, otherwise it’s iterating through a linked list that
has been garbage collected.

My question is: does the GC know that it cannot collect the unreferenced
myref pointers before the end of the List.iter? 

Sorry, I just wanted to ask this question to rule it out.

 

Thanks again.

 

 

 

 

On 07-08-2009, ivan chollet <ivan.chollet@free.fr> wrote:

> 

> This GDB was configured as "i386-marcel-freebsd"...(no debugging symbols

> found)...

> 

> Not very informative. So here are my questions:

 

I suppose you are running freebsd ? Which version of freebsd, of ocaml ? 

 

> 

>  

> 

> -          What is the best way to produce and analyze core dumps in
ocaml?

> Should I compile in bytecode or native? Is there any special gdb "trick"

> that gives you more information? Is there any special "trick" while

> compiling the ocaml runtime to make it throw more information?

> 

 

gdb is not the perfect tool to debug ocaml program. You should give a

try to ocamldebug which is a better option for bytecode (see below for

options). Bytecode is more informative when coming to reporting

backtrace (at least with old version of ocaml). 

 

Compile every program with "-g" option (just like gcc). 

 

If you have compiled everything with "-g" option, you can also use the

environment variable OCAMLRUNPARAM="b" to get a backtrace for your

exception, at runtime.

 

> -          Then, my main question is actually: in bytecode, what can
produce

> segfaults? My ocaml code is completely standard, excepted that I use the

> Marshal module. So my question is rather: outside the Marshal module, what

> can cause segfault?

 

Some part of the bytecode are just standard C, everything can cause a

segfault just as C. These errors are not very common but it is possible

that some case are not well handled on freebsd. Most probably a porting

issue.

 

Marshal module can easily trigger a segfault when you map the loaded data

to a type which doesn't match the dumped data.

 

Example: 

List.length (Marshal.from_string (Marshal.to_string 1234 []) 0);;

 

Here the integer value is marshalled and then unmarshalled as a list ->

segfault.

 

> 

> -          Slightly unrelated question: I have been able to generate

> segfaults by running ocaml code that: 1) iterates recursively through a
list

> reference 2) changes the reference while still iterating on it. For
example,

> you just do a "List.iter myfun !myref", and within the function myfun, you

> do stuff like "myref := List.filter somefilterfunction !myref". It is not

> good to program like this, but for some reason I thought ocaml would not

> segfault on that. Is this expected behavior? If it's not, I'll be happy to

> provide some simple source code that illustrates it. (nevermind I have

> actually cleaned all my code base from these dirty uses of references)

> 

 

Could you provide a minimal example code for this error ? I don't think

this should generate a segfault.

 

 

Regards

Sylvain Le Gall

 


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