Garbage Collection and High-Level Languages Programming Languages Fall 2003

The Notion of Garbage Collection In a language like Python, we are treat high level data structures like integers. In a language like Python, we are treat high level data structures like integers. Consider a = [1,2,3,6,8,9]; … a = [6.9]; Consider a = [1,2,3,6,8,9]; … a = [6.9]; What happened to the old list? What happened to the old list? Answer from Python programmer: Huh? Answer from Python programmer: Huh?

More on the Basic Notion What do you mean Huh? What do you mean Huh? Sorry I don’t understand, if I write a = 10; … a = 23; Sorry I don’t understand, if I write a = 10; … a = 23; I don’t worry about what happens to the 10, it is just overwritten by the 23 and the 10 is gone. I don’t worry about what happens to the 10, it is just overwritten by the 23 and the 10 is gone.

More on the Basic Notion Yes, but there is a big difference between integers. Integers fit in one word, but lists require dynamic allocation and you have to make sure the dynamic memory is released and reclaimed or you have a memory leak. Yes, but there is a big difference between integers. Integers fit in one word, but lists require dynamic allocation and you have to make sure the dynamic memory is released and reclaimed or you have a memory leak. Sorry I don’t know what you are talking about, to me the two cases are identical. Sorry I don’t know what you are talking about, to me the two cases are identical.

The Point of This If we want genuine high level languages, we do indeed what to treat the two cases as the same If we want genuine high level languages, we do indeed what to treat the two cases as the same And it is not acceptable to have the programmer have to worry about low level implementation details like releasing memory. And it is not acceptable to have the programmer have to worry about low level implementation details like releasing memory.

The Solution: Garbage Collection So, at the implementation level we do indeed have to release and reclaim the memory, but it has to be done automatically (garbage collected). So, at the implementation level we do indeed have to release and reclaim the memory, but it has to be done automatically (garbage collected). Very easy to describe at the abstract level, since garbage collection is semantic free, it’s just a memory optimization. Very easy to describe at the abstract level, since garbage collection is semantic free, it’s just a memory optimization. But implementation concepts are not so clear. But implementation concepts are not so clear.

Some Approaches Try to release memory as soon as it is garbage. Try to release memory as soon as it is garbage. But then we have to be able to determine this efficiently. But then we have to be able to determine this efficiently. Clever idea, reference counts keep track of how many people are referencing a chunk of memory. Clever idea, reference counts keep track of how many people are referencing a chunk of memory. If count is zero, memory is garbage If count is zero, memory is garbage But what about cycles  But what about cycles 

Type Accurate Garbage Collection Figure out all the “roots” in the program, these are values accessible directly by the program which can reference memory blocks. Figure out all the “roots” in the program, these are values accessible directly by the program which can reference memory blocks. These blocks are not garbage, also any blocks referenced by these blocks (recursively to all levels) are not garbage These blocks are not garbage, also any blocks referenced by these blocks (recursively to all levels) are not garbage Everything else is garbage. Everything else is garbage.

Requirements for TA Garbage Collection At run time, when garbage collection occurs, we have to be able to trace all references. At run time, when garbage collection occurs, we have to be able to trace all references. This means that we need information at run-time on the exact data layouts of all data structures in use. This means that we need information at run-time on the exact data layouts of all data structures in use. Probably we have this in Python and other dynamic languages, but less likely to be true in static languages. Probably we have this in Python and other dynamic languages, but less likely to be true in static languages.

Conservative Garbage Collection Assume that if a block is not garbage then there is somewhere in memory a word containing the address of this block. Assume that if a block is not garbage then there is somewhere in memory a word containing the address of this block. Scan in use memory blocks for all addresses of memory blocks, assume all these blocks are not garbage. All other blocks are garbage Scan in use memory blocks for all addresses of memory blocks, assume all these blocks are not garbage. All other blocks are garbage But what if an integer value happens to look like the address of a block. But what if an integer value happens to look like the address of a block.

Worry Spots for Garbage Collection What if some global variables are logically dead, and will be clobbered, but have not been reassigned yet. May hold on to garbage (even with TA Gcol) What if some global variables are logically dead, and will be clobbered, but have not been reassigned yet. May hold on to garbage (even with TA Gcol) What if cycles and using reference counts What if cycles and using reference counts What if conservative gcol, and large blocks retained by accident. What if conservative gcol, and large blocks retained by accident.

Garbage Collection and Real-Time If we use stop-and-collect approach (wait till we run out of memory, then do a TC or conservative garbage collection), this may take a hard to analyze amount of time. If we use stop-and-collect approach (wait till we run out of memory, then do a TC or conservative garbage collection), this may take a hard to analyze amount of time. This hard to analyze amount of time may occur at an embarrassing moment: This hard to analyze amount of time may occur at an embarrassing moment: WARNING: MISSILE THREAD DETECTED! WARNING: MISSILE THREAD DETECTED! (system msg): please wait, garbage collection in progress, normal execution will be resumed as soon as possile (system msg): please wait, garbage collection in progress, normal execution will be resumed as soon as possile

Garbage Collection and Safety-Critical Programs How can we analyze memory use How can we analyze memory use Hard anyway in a language like Python Hard anyway in a language like Python And made harder by garbage collection And made harder by garbage collection Usually we simply forbid dynamic allocation Usually we simply forbid dynamic allocation Does that mean no Python in the next Boeing plane – probably! Does that mean no Python in the next Boeing plane – probably!