I don't get the GC. (heapSizeFactor followup)

[FeepingCreature]

A follow-up to 
https://forum.dlang.org/thread/befrzndhowlwnvlqcoxx@forum.dlang.org

I don't get it.

Okay, this is how I understand the conservative GC to work:

- do allocations
- allocations increment `usedSmallPages`/`usedLargePages`
- at some time, we cross 
`smallCollectThreshold`/`largeCollectThreshold`
   - then we do a fullcollect, ie. mark, sweep
   - then set the new threshold to `usedSmallPages * 
heapSizeFactor`, which defaults to 2
     - plus some smooth-decay magic that means threshold doesn't 
go down super fast
   - unlock and resume

Rinse and repeat.

We have a service. It's pretty `std.json` intensive, it handles a 
lot of networking on startup, and it has about 30 threads. When 
we start it up with everything at default settings, it uses 3.3GB.

This would indicate, given `heapSizeFactor=2` by default, that 
the high-water mark of used pages is 1.6GB.

We've tried setting `heapSizeFactor` to 0.25. This is not quite 
equivalent to running the GC on every allocation 
(`smallCollectThreshold < usedSmallPages`), but AIUI it first 
runs the GC every time it would allocate a new pool. It's pretty 
aggressive; something like 70% of our startup performance goes to 
GC. But, after startup, the service sits at RSS 961MB.

Here's the confusing part. RSS 961MB is an upper limit on the 
live memory. If this is correct, the `smallCollectThreshold` 
after startup should be at most 1.6GB? Right? So the untuned GC 
should not let the process grow above 1.6GB? Right? But it's 
3.3GB, more than twice that.

I mean, wrong, because we might get unlucky and run our GC when 
we have a lot of temporary memory live. But we'd need to have 
twice as much temporary memory truly live during startup as we do 
after startup, right? And if I watch the RSS during startup with 
`heapSizeFactor=0.25`, I don't see it above 961MB *ever*. And it 
can't be that net queues run emptier during startup with 
`heapSizeFactor=0.25` than without, because the process is a lot 
slower with `heapSizeFactor=0.25` than without! It should clear 
queues faster with it off!

So what is the GC doing?!