NAME

docs/dev/infant.pod - Infant Mortality

Overview

We have a garbage collector that needs to detect all dead objects (the process is called DOD, for Dead Object Detection). Any Parrot operation that might allocate memory can potentially trigger a DOD run (if not enough memory is available, we need to free up some unused objects to give us enough room to allocate our new object. But in order to know what can be freed up, we need to know what's alive, so we do a DOD pass.)

The DOD pass begins with a "root set" of objects that are known to be in use -- the PMC and String registers, the Parrot stack, the global symbol table, etc. Each of these objects is scanned for references to other objects, recursively. All objects found during this search are regarded as live. Everything else is considered dead and so is available for reclamation.

The question of what should be in the root set is problematic. Consider the case where you've created a PMC. If you immediately stuff it into a register, then you're all set. But what if you're putting it into an aggregate? If the aggregate isn't large enough, you'll need to resize it, which might allocate memory, which might trigger a DOD run. And that DOD run may free your freshly-created PMC.

In this case, the solution is simple: resize the array, if necessary, before creating the PMC. But many situations are not so simple, and the set of operations which could conceivably require more memory is vast. This problem is referred to as the "infant mortality" problem.

Peter Gibbs, Mike Lambert, Dan Sugalski and others have considered various solutions to this problem. Most of those solutions have been implemented in some form or other. This document is an attempt to compare and contrast all such proposals.

Solution 1: Stack Walking

One possible solution is to add the C stack into the root set. This way, temporary PMCs that have not yet been anchored to the root set will be found on the stack and treated as live. Actually, the C stack is insufficient -- you must also scan through all processor registers, and for some processors there may be a separate backing store for those registers (eg the Sparc's register windows). Uninitialized data on the stack and high alignment requirements for stackframes can fool the DOD system with left over pointers from previous calls. This is especially a problem for objects which need early destruction.

+ No slowdown in the common case of no DOD
+ Convenient: the programmer does not need to code differently to
  accommodate the garbage collection system.
- Unportable. Different processors need different code for scanning
  their registers, stacks, register windows, etc. Also, on some
  architectures you must scan every possible offset into the stack to
  find all the pointers, while on others you must NOT scan every
  possible offset or you'll get a bus error due to a misaligned
  access.
- Slow DOD. A full stack walk takes quite a while, and this time
  grows with nested interpreter calls, external code's stack usage, etc.
- Complex. The stack walk is necessarily conservative, in that it
  must consider every valid pointer on the stack to potentially be a
  traceable object. But some of those pointers may be stale, in which
  case the memory they point to may have been partially reused for
  some other purpose. Everything must operate within certain
  constraints that guarantee that no invalid pointers will be
  dereferenced and trigger a segmentation fault or bus error.
- Another side effect of the conservative nature of stack walking is
  that the memory for these objects may never be returned to the
  system, because it is always possible that there will be a stale
  pointer lying around on the stack or in a register, and all such
  pointers will be dereferenced.

Solution 2: Neonate flag

There are several possible variants of the neonate flag, but the basic idea is to set a flag on newly created objects that prevents them from being collected. At some point, this flag is cleared -- either as the newborn object is anchored to the root set, or during the first DOD pass after it was anchored, or explicitly when the object is no longer needed.

+ Portable
+ Can return memory to the system when unneeded
+ Exact: the state of every object is always known precisely (no more
  "this object MIGHT still be reachable")
- The coder must remember to clear the flag before discarding
  unanchored objects
- The flag-clearing takes a small amount of time
- For some variants of this scheme, some time is consumed to clear
  the flag for the common case of rooted objects

Subspecies of neonate flags

The variants of the neonate idea all hinge on exactly how and when the flag is cleared.

Variant 1: explicit

The flag is always explicitly cleared by the coder.

+ Very simple
+ Fast DOD
- Slow for unanchored temporaries
- Slow for anchored objects
- Easy to forget to clear the flag for unanchored temporaries
- Easy to forget to clear the flag for anchored objects
- longjmp() can bypass the clearing

Variant 2: explicit for temporaries, cleared during anchoring

The flag is explicitly set for temporaries. All routines which anchor an object to the root set also clear the flag.

+ Simple
+ Fast DOD
- Slow for unanchored temporaries
- Slow for anchored objects
- Easy to forget to clear the flag for unanchored temporaries
- Forces all anchoring operations to set the flag (so this disallows
  direct assignment into a PMC register, for example)
- longjmp() can bypass the clearing

Variant 3: clear during DOD

The neonate flag is cleared during DOD when an object is encountered during the recursive root set traversal. (Leopold Toetsch's trick of setting the live_FLAG during creation is equivalent to this variation, I think.)

+ Simple
+ Fast DOD (DOD already manipulates the flags)
- If there are multiple DOD runs before the object is anchored or
  dies, it will be prematurely freed

Variant 4: generation count

This is the same as variant 3, except a "generation count" is maintained in the interpreter so that the neonate flag is only cleared during a later generation. The generation is incremented only at major control points such between opcodes, so that there is no chance of unanchored temporaries.

+ Fast DOD (DOD already manipulates the flags)
- Generation count must be maintained
- Disallows recursive opcode calls (necessary for eg implementing
  vtable methods in pasm)
- Can temporarily use more memory (dead objects accumulate during the
  current generation)

In order to allow recursive opcode calls, we could increment the generation count in more places and make sure nothing is left unanchored at those points, but that would gradually remove all advantages of this scheme and make it more difficult to call existing vtable methods (since you never know when they might start running pasm code.)

Variant 5: generation stack

Notice that when using a generational count, you really only need to test whether the current generation is _different_ from an object's creation generation (which eliminates wraparound problems, too.) So rather than testing against a single "current" generation, allow a stack of multiple "current" generations. An object encountered during DOD will have its neonate flag cleared only if it doesn't match any of the "current" generation ids. This check can be optimized using a conservative bit mask as a preliminary test.

+ Still faster DOD than stackwalking, though slower than the
  other neonate variants
- Generation count must be maintained
- Generation stack must be maintained
- Disallows longjmp()'ing out of recursive opcode calls
- Can temporarily use more memory (dead objects accumulate during all
  current generations)

Variant 6: Generation based on stack depth

Another similar idea is to use a generational system, with the "current generation" as a value on the C stack, passed as an extra argument after the interpreter. If a function creates temporary objects it calls other functions with an increased generational count. During a DOD run, any PMC with a generation less than the current generation is considered live. Any PMC with a generation greater than the current generation is considered free. This works through longjmps and recursive run_cores.

+ Simple
+ No stack-walking
+ Works through longjmps and recursive run_cores
+ No explicit setting and clearing of flags
- Needs to change to change the signature of every Parrot function
- Nested temporaries can survive if there is no DOD-run between two 
  function calls with increased generation count

Solution 3: Explicit root set augmentation

Variant 1: Temporarily anchor objects

Provide a mechanism to temporarily anchor an otherwise unanchored object to the root set. (eg, have an array of objects associated with the interpreter that are all considered to be part of the root set.) This has pretty much the same advantages and disadvantages of explicit neonate flag setting:

+ Simple
+ Fast DOD
- Slow for unanchored temporaries
- Sometimes slow for anchored objects (depending on whether they need
  to be temporarily anchored before the final anchoring)
- Easy to forget to remove temporaries from the root set
- Easy to double-anchor objects and forget to remove the temporary
  anchoring
- longjmp() can bypass the unanchoring

Many of the same or similar variations also apply: objects could be automatically removed from the temporary anchoring at generation boundaries, etc.

Variant 2: Anchor early, anchor often

First place a new PMC in the root set (e.g. a register), then initialise it. If that's too cumbersome, disable DOD; if that's suboptimal, use active anchoring to some root set linked list for temporary PMCs.

+ Simple
+ Fast DOD (No stack-walking)
- DOD might be turned off for a long time (Maybe a recursive run_core
  is called)
- Easy to forget to reenable DOD
- longjmp() can bypass reenabling of DOD (this might be hidden in the
  wrapper functions as only one value needs to be restored)

Variant 3: Use a linked list of frames

The signature of every Parrot function is extended with an extra parameter which is a parameter to a frame structure. All temporary PMCs needs to put into such a frame structure. The first parameter of this frame structure is a link to the previously used frame structure. If a function that can do a DOD run is called a pointer to the current frame is applied. The linked list of frames represents always an exact list of the active temporaries on the C-stack.

+ Fast DOD-runs (only the known PMC-pointers are walked)
+ Exact
+ works through recursive run_cores and longjmp()
- signature of every Parrot function changes
- Creation of temporaries is complicated (Need to create a frame
  first)