memory.text

[Marius Vollmer]

Hi,

FYI, I have just added this text as guile-core/devel/memory.text and
will go about implementing it.

- - - - - - - - - - - - - - - - - 

The following is gathered from a shortish discussion on the
guile-devel mailing list.  I plan to implement this in the next
days. -mvo

Improving memory handling in Guile
----------------------------------

I think we have a problem with the `mallocated' GC trigger.  It is not
maintained reliably and I'm afraid we need to have everybody review
their code to get it right.

I think the current interface with scm_must_malloc, scm_must_free,
scm_done_malloc, scm_done_free is too difficult to use right and too
hard to debug.

Guile itself is full of mtrigger related bugs, I'm afraid.  A typical
one is in fports.c: the buffers for a fport are allocated with
scm_must_malloc and freed with scm_must_free.  The allocation is
reported to the GC, but the freeing never is.  The result is that the
GC thinks that more and more memory is being allocated that it should
be able to free, but that never actually gets freed (although in
reality the program is very well behaved).  As a counter measure to
constant GC, the GC raises its mtrigger setting in a frenzy until it
wraps around, causing a `hallucinating GC' syndrome, effectively
stopping the program dead.

(Watch scm_mtrigger while your favorite long-running Guile program
executes, it will continuously rise.)

The problem is that scm_must_malloc registers the allocated amount
with the GC, but scm_must_free does not de-register it.  For that, one
would currently needs to use scm_done_free, or return an appropriate
number from a smob free routine.

Another problem is that scm_must_malloc is used in places where it is
probably not appropriate since the caller does not know whether that
block memory is really ending up under the control of the GC, or not.
For example scm_do_read_line in rdelim.c uses scm_must_malloc to
allocate a buffer that it returns, and scm_read_line passes this to
scm_take_string.  scm_take_string assumes that the memory has not been
under GC control previously and calls scm_done_malloc to account for
the fact that it now is.  But scm_must_malloc has _already_ increased
scm_mallocated by the proper amount.  Thus, it is now doubly
reflected.

Since the current interface is unsymmetrical (scm_must_malloc
registers, but scm_must_free doesn't de-register), I propose to change
it as follows.  Switching to this new interface will force us and
everybody else to systematically review their code.

    - the smob free routine does no longer return the number of bytes
      that have been freed.  For the transition period, free routines
      are first encourged to return 0, then required, and then their
      return type changes to void.

    - scm_must_malloc, scm_must_free are deprecated.

    - in their place, we have 

      Function: void *scm_malloc (size_t size, const char *what);

        Allocate SIZE bytes of memory.  When not enough memory is
        available, signal an error.  This function runs the GC to free
        up some memory when it deems it appropriate.

        The WHAT argument is used for statistical purposes and for
        error reporting.  It should describe the type of object that
        the memory will be used for so that users can identify just
        what strange objects are eating up their memory.

      [ Note: this function will not consider the memory block to be
        under GC control. ]

      Function: void scm_free (void *mem);

        Free the memory at MEM.  MEM must have been returned by
        scm_malloc.  MEM might be NULL in which case nothing happens.

      Function: void *scm_realloc (void *mem, size_t newsize);

        Change the size of the memory block at MEM to NEWSIZE.  A new
        pointer is returned.  When NEWSIZE is 0 this is the same as
        calling scm_free on MEM and NULL is returned.  MEM must be
        non-NULL, that is, the first allocation must be done with
        scm_malloc, to allow specifying the WHAT value.

      Function: void scm_gc_register_collectable_memory
                      (void *mem, size_t size, const char *what);

        Informs the GC that the memory at MEM of size SIZE can
        potentially be freed during a GC.  That is, MEM is part of a
        GC controlled object and when the GC happens to free that
        object, SIZE bytes will be freed along with it.  The GC will
        _not_ free the memory itself, it will just know that so-and-so
        much bytes of memory are associated with GC controlled objects
        and the memory system figures this into its decisions when to
        run a GC.

        MEM does not need to come from scm_malloc.  You can only call
        this function once for every memory block.

      Function: void scm_gc_unregister_collectable_memory 
                      (void *mem, size_t size);

        Inform the GC that the memory at MEM of size SIZE is no longer
        associated with a GC controlled object.

      Function: void *scm_gc_malloc (size_t size, const char *what);

        Like scm_malloc, but also call scm_gc_register_collectable_memory.

      Function: void scm_gc_free (void *mem, size_t size, const char *what);

        Like scm_free, but also call scm_gc_unregister_collectable_memory.

        Note that you need to explicitely pass the SIZE parameter.
        This is done since it should normally be easy to provide this
        parameter (for memory that is associated with GC controlled
        objects) and this frees us from tracking this value in the GC
        itself.  (We don't do this out of lazyness but because it will
        keep the memory management overhead very low.)

The normal thing to use is scm_gc_malloc / scm_gc_free.


Cell allocation and initialization
----------------------------------

It can happen that the GC is invoked during the code that initializes
a cell.  The half initialized cell is seen by the GC, which would
normally cause it to crash.  To prevent this, the initialization code
is careful to set the type tag of the cell last, so that the GC will
either see a completely initialized cell, or a cell with type tag
`free_cell'.  However, some slot of that `free' cell might be the only
place where a live object is referenced from (since the compiler might
reuse the stack location or register that held it before it was
stuffed into the cell).  To protect such an object, the contents of
the cell (except the first word) is marked conservatively.

What happened to me was that scm_gc_mark hit upon a completely
uninitialized cell, that was tagged a s a free cell, and still pointed
to the rest of the freelist were it came from.  (It was probably this code

     :
    SCM_NEWCELL (port);                  // port is a free cell
    SCM_DEFER_INTS;
    pt = scm_add_to_port_table (port);   // this invokes the GC
     :

that caused it.)

scm_gc_mark would conservatively mark the cdr of the free cell, which
resulted in marking the whole free list.  This caused a stack overflow
because the marking didn't happen in a tail-calling manner.  (Even if
it doesn't crash, a lot of unnecessary work is done.)

I propose to change the current practice so that no half initialized
cell is ever seen by the GC.  scm_gc_mark would abort on seeing a free
cell, and scm_mark_locations (and scm_gc_mark_cell_conservatively if
will survive) would not mark a free cell, even if the pointer points
to a valid cell.  scm_gc_sweep would continue to ignore free cells.

To ensure that initialization can not be interrupted by a GC, we
provide new functions/macros to allocate a cell that include
initialization.  For example

    SCM scm_newcell_init (scm_bits_t car, scm_bits_t cdr)
    {
      SCM z;
      if (SCM_NULLP (scm_freelist))
        z = scm_gc_for_newcell (&scm_master_freelist,
                                &scm_freelist);
      else
        {
          z = scm_freelist;
          scm_freelist = SCM_FREE_CELL_CDR (scm_freelist);
        }
      SCM_SET_CELL_WORD_1 (z, cdr);
      SCM_SET_CELL_WORD_0 (z, car);
      scm_remember_upto_here (cdr);
      return into;
    }

For performance, we might turn this into a macro, and find some
specialized ways to make the compiler remember certain values (like
some `asm' statement for GCC).  For a non-threaded Guile, and a
cooperatively threaded one, the scm_remember_upto_here call is not
even needed since we know the the function can not be
interrupted. (Signals can not interrupt the flow of control any
longer).

In the transition period, while SCM_NEWCELL is deprecated, we can make
it always initialize the first slot with scm_tc16_allocated.  Such
cells are marked conservatively by the GC.  SCM_NEWCELL can have
abysmal performance while being deprecated.

Update: SCM_NEWCELL already behaves like this.  We only need to
implement scm_newcell_init and replace uses of SCM_NEWCELL with it.