Our collector is a stop-the-world GC which means the user’s program is paused until the entire garbage collection process has completed. But how do we find the balance between these when we have no idea how much memory the user’s program needs and how often it allocates? Otherwise with some validation it binary searches the address in a DWARF lookuptable to retrieve the containing module, checks additional lookuptables including some tree traversals to find & output which function & sourcefile the address is in to output, similar for the symbol name & type amongst other things to display as the reader desires. After emptying input or seeing output to emit it updates/resets state & checks whether we’ve reached EOF. For each entry it handles filepaths specially, extracts it into a new ELF symboltable, and/or checks for presence in the hashmap, & possibly formats human output with lots of options. That leads to gnarly code with lots of dark corners where bugs can hide. The size of all of the user code slices added up is the throughput.
Throughput is the total fraction of time spent running user code versus doing garbage collection work. Most of the other code in here deals with the fact that removing a node from a singly linked list is cumbersome. I know that’s kind of a lot of code and pointer shenanigans, but there isn’t much to it once you work through it. There is one remaining corner of the VM that has some unusual requirements around memory. All of the logic lives in one function. We start with the easy ones-strings and native function objects contain no outgoing references so there is nothing to traverse. Like is done everywhere else without the hurdle of function ABIs. In some Assembly languages like x86, a register can be decomposed into multiple smaller subregisters. After initializing counts & allocators it iterates over every codeblock, instruction therein, & the registers each uses to examine where that register was last assigned.
After initializing I/O buffering & internationalization whilst parsing commandline flags objdump iterates over remaining arguments, handling single-arg specially. After possibly reserving the callstack pointer regs, it iterates over those chains to apply them. If it finds a memory store which so happens to align with the new desired callstack value (stackslot referencing?) the callstack pointer will be copied from that register. Then we adjust the threshold to some value larger than that. If the key string object’s mark bit is not set, then it is a white object that is moments from being swept away. So whenever we reach a black object, we go ahead and clear the bit now in anticipation of the next run. We want our GC to run frequently enough to minimize latency but infrequently enough to maintain decent throughput. Of course, it came at the slight expense of potentially running out of memory and crashing if the user’s program ran long enough. This particular set of semantics comes up frequently enough that it has a name: a weak reference.
Selling fewer loaves of bread a day is bad, and making any particular customer sit and wait while you clean all the dishes is too. But we can’t wait until after the sweep phase is done because by then the objects-and their mark bits-are no longer around to check. It no longer makes sense to wait until you "have to", to run the GC, so we need a more subtle timing strategy. So, clearly, we want to run the collector really frequently. So, clearly, we want to run the collector really infrequently. When that happens, we need to ensure our collector doesn’t get stuck in an infinite loop as it continually re-adds the same series of objects to the gray stack. It’s a measure of how "chunky" the collector is. Memory offsets are stored in the AST for structs whilst parsing them, computed from this sizing & alignment info, to be looked up when one of it’s fields are accessed. But with all the options available, it can be hard to choose the right one for your space. That in turn triggers a GC, which fails to mark the new constant object and thus sweeps it right before we have a chance to add it to the table.
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