Re: MPS: out-of-memory

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Eli Zaretskii <eliz@gnu.org> writes:

> Btw, the current GC in Emacs has a feature for when Emacs runs out of
> memory: it sets some small amount of memory aside, and uses it when
> "memory-full" condition happens, because some memory is needed to show
> the memory-full error message, and then shut down in an orderly
> fashion:
>
>   void
>   memory_full (size_t nbytes)
>   {
...
>
> See also refill_memory_reserve.
>
> Do we perhaps need something like that with MPS?

Yes, I think that's important, and also handling the case of getting
errors codes from MPS in general.

Here is what I've done so far. The function below is the central
allocation function. Everything being allocated comes from here.

  static mps_addr_t
  alloc_impl (size_t size, enum igc_obj_type type, mps_ap_t ap)
  {
    mps_addr_t p UNINIT;
    size = alloc_size (size);
    switch (igc_state)
      {
      case IGC_STATE_USABLE_PARKED:
      case IGC_STATE_USABLE:
        do
          {
            mps_res_t res = mps_reserve (&p, ap, size);
            if (res != MPS_RES_OK)
              memory_full (0);
            /* Object _must_ have valid contents before commit. */
            memclear (p, size);
            set_header (p, type, size, alloc_hash ());
          }
        while (!mps_commit (ap, p, size));
        break;

      case IGC_STATE_DEAD:
        p = xzalloc (size);
        set_header (p, type, size, alloc_hash ());
        break;

      case IGC_STATE_INITIAL:
        emacs_abort ();
      }
    return base_to_client (p);
  }

As one can see, igc as a whole is in a certain state, igc_state. The
idea is that the state changes to IGC_STATE_DEAD when something fatal
happens. In that state, malloc is used is used for allocating Lisp
objects instead of MPS. That lets Emacs shut down gracefully without
entering MPS recursively as it was before I added the state.

Such state changes are currently done when checking the return code of
an MPS API function and when assertions fail.

Seems to work as expected, as I could see in a couple of backtraces from
Ihor I believe showing the set_state (IGC_STATE_DEAD), and from my own
experience. But maybe someone has an idea how to improve it.