Hybird GC issues in lua 5.4

[云风 Cloud Wu <cloudwu@...>]

We switched to lua 5.4 in our new online game project , and tried
generational GC.

1. Major cycle is too slow to accept. （More than 1 second stop-the-world)
2. Minor cycle is good, it's fast and it reduces the rate of memory
growth and the peak memory usage.

We tried a hybird mode between incremental gc and generational gc.

1. Stop the automatic collector. Memory allocate driven collector is
not suitable in our case.
2. We use a timer to call gc step manually. Every time we call lua_gc
with LUA_GCSTEP, we measure the time the collector cost, and do not
let the time too long. That is to say, we use a time-driven collector
rather than allocate-driven collector.
3. We use generational mode at start, run some minor/young cycles, and
when the total memory increased to a threshold value, we switch it to
incremental  mode, and switch back after a full cycle (It goes through
a number of steps).

They works fine except enterinc(). When switching from generational to
incremental , it stop-the-world about 100ms~400ms / 1GB memory.

I guess the reason is `whitelist(g, g->allgc)`. It traverses all the
object. Is it possible to improved this in future ? I think it's not
very diffcult, we can add a stage something like GCSenterinc before
GCSpause.

Another issue is about the memory layout of TString.

I think using a pointer to hold the string data would be better the
putting the data into a continuous TString structure.

Although an additonal pointer would waste 4/8 bytes memory, but
TString object can be a small fixed size object. It's more
cache-friendly to GC , and many general purpose allocator could manage
small objects more efficiently.




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