This wiki article contains incomplete and informal notes about the MPS, the precursor to more formal documentation. Not confidential. Readership: MPS users and developers.

How to choose the right MPS pool class

We would like a 'map' from a client's requirements to the recommended pool class:

Do you need garbage collection?
Do you have objects with no references in them?
Do you need to pass buffers to system calls?
etc etc
You need mps_class_lo.

For now, the closest we have to this is a survey from 1998 that still contains confidential material: guide.mps.pool-classes by RIT ∅.

Some notes about MPS pool classes

mps_class_amc

General-purpose automatic (collecting) pool class. This is the most 'advanced' pool class in the MPS, intended for most client objects.

AMC is "Automatic, Mostly Copying": it uses copying collection except when prevented by ambiguous references. It is generational.

Chain: specify capacity and mortality of generations 0..N-1. Survivors from N-1 get promoted into an arena-wide topGen (often anachronistically called the "dynamic" generation).

mps_class_amcz

General-purpose leaf-only pool class (for objects that contain no references). A variant of AMC (the Z means "Zero rank", ie. containing no references).

mps_class_ams

Non-moving automatic (collecting) pool class.

AMS is "Automatic Mark & Sweep". Not generational.

Chain: specify capacity and mortality of 'generation' 0. [Why? Perhaps to trigger nursery-like collections? New 2001-03-02. RHSK 2006-11-27]

mps_class_lo

Non-moving, non-protecting, automatic (collecting), Leaf Only pool class (for objects that contain no references). Unit test: locv.c. RefMan: -undocumented-.

This pool class should really be called "flat data that needs to be accessed by foreign code" -- it's ideal for allocating a buffer to pass to an operating system I/O call. It is not very fast. [Conversation with DRJ, 2006-07].

(For a general-purpose leaf-only pool see AMCZ).

[Internal note: has a dummy chain: 1MiB 20%]

mps_class_awl

For weak references, linked to normal references. See further AWL notes below.

[Internal note: has a dummy chain: 2^32KiB 50%]

AWL: FindDependentMethod and format scan method

You need to pass a FindDependentMethod to mps_pool_create. Here are some notes:

...
The "dependent object" is an object which
the scan method is permitted to update.  The awlScanObject function
Exposes that object before the scan and Covers it afterwards, to allow
the update to take place.  It also discards the MPS "summary" for that
object's segment (the "summary" of a segment is a conservative
approximation of the objects pointed to by objects in that segment;
since the scan method is now allowed to update the segment to point to
anything else, the summary has to be discarded).

If the keys and values of hash tables are stored in separate linear
tables (a key table and a value table), then for a weak-key table the
scan method for the key table may need to update the value table (to
null out the corresponding value if a key goes away on fix).  So the
dependent object for the key table is the value table.  For weak-value
tables, the relationship is the other way around.

The dylan format works like that, and here's the relevant code from my
auto-header variant of that.  First dylan_weak_dependent, which is the
dependent-object method.  Then dylan_scan_contig_weak, which is the
innermost part of the scan method for those weak tables.  The comment
"delete corresponding entry" is where the scan method updates the
value table.


/* dylan_weak_dependent -- returns the linked object, if any.
 */

extern mps_addr_t dylan_weak_dependent(mps_addr_t parent)
{
  mps_word_t *object;
  mps_word_t *wrapper;
  mps_word_t fword;
  mps_word_t fl;
  mps_word_t ff;

  assert(parent != NULL);
  object = (mps_word_t *)parent;
  wrapper = (mps_word_t *)object[0];
  assert(dylan_wrapper_check(wrapper));
  fword = wrapper[3];
  ff = fword & 3;
  /* traceable fixed part */
  assert(ff == 1);
  fl = fword & ~3uL;
  /* at least one fixed field */
  assert(fl >= 1);
  return (mps_addr_t) object[1];
}

/* Scan weakly a contiguous array of references in [base, limit). */
/* Only required to scan vectors for Dylan Weak Tables. */
/* Depends on the vector length field being scannable (ie a tagged */
/* integer). */
/* When a reference that has been fixed to NULL is detected the */
/* corresponding reference in the associated table (pointed to be the */
/* assoc variable) will be deleted. */

static mps_res_t
dylan_scan_contig_weak(mps_ss_t mps_ss,
                       mps_addr_t *base, mps_addr_t *limit,
                       mps_addr_t *objectBase, mps_addr_t *assoc)
{
  mps_addr_t *p;
  mps_res_t res;
  mps_addr_t r;

  MPS_SCAN_BEGIN(mps_ss) {
    p = base;
    goto skip_inc;
  loop:
    ++p;
  skip_inc:
    if(p >= limit)
      goto out;
    r = *p;
    if(((mps_word_t)r & 3) != 0) /* non-pointer */
      goto loop;
    if(!MPS_FIX1(mps_ss, r))
      goto loop;
    res = MPS_FIX2(mps_ss, p);
    if(res == MPS_RES_OK) {
      if(*p == 0 && r != 0) {
        if(assoc != NULL) {
          assoc[p-objectBase] = 0;      /* delete corresponding entry */
        }
      }
      goto loop;
    }
    return res;
  out:
    assert(p == limit);
  } MPS_SCAN_END(mps_ss);

  return MPS_RES_OK;
}

If your scan method won't need to update any other object while
scanning a weak object, the dependent-object method should return
NULL.

Source: //info.ravenbrook.com/mail/2002/04/12/15-52-29/0.txt ∅ //info.ravenbrook.com/mail/2002/04/12/15-56-15/0.txt ∅ .

B. Document History

  2006-06-02  RHSK  Created.
  2006-06-23  RHSK  Link to RIT's 1998 guide.mps.pool-classes.
  2006-06-28  RHSK  Notes on AWL dependent object and scan method.
  2006-08-02  RHSK  LO is for foreign code; for leaf-only use AMCZ.
                    How to choose the right pool class.
  2006-08-07  RHSK  AMC.
  2006-11-27  RHSK  Chains.

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