Garbage collection

[James Gray]

On Saturday, 27 June 2020 at 14:12:09 UTC, kinke wrote:
> On Saturday, 27 June 2020 at 10:08:15 UTC, James Gray wrote:
>> have run into a memory leak
>
> Something seems really off indeed. I've run this on Win64 with 
> DMD (2.092) and LDC (1.22), without any extra cmdline options:
>
> -----
> import core.memory;
> import core.stdc.stdio;
> import std.range;
> import std.format;
>
> auto f(R)(R r) { return format("%s", r); }
>
> int toMB(ulong size) { return cast(int) (size / 1048576.0 + 
> 0.5); }
>
> void printGCStats()
> {
>     const stats = GC.stats;
>     const used = toMB(stats.usedSize);
>     const free = toMB(stats.freeSize);
>     const total = toMB(stats.usedSize + stats.freeSize);
>     printf("  GC stats: %dM used, %dM free, %dM total\n", used, 
> free, total);
> }
>
> void main()
> {
>     printGCStats();
>
>     while (true)
>     {
>         puts("Starting");
>         string str = f(iota(100_000_000));
>         printf("  string size: %dM\n", toMB(str.length));
>         str = null;
>         GC.collect();
>         printGCStats();
>     }
> }
> -----
>
> Output with DMD (no change with the precise GC via 
> `--DRT-gcopt=gc:precise`):
> -----
>   GC stats: 0M used, 1M free, 1M total
> Starting
>   string size: 943M
>   GC stats: 1168M used, 1139M free, 2306M total
> Starting
>   string size: 943M
>   GC stats: 1168M used, 2456M free, 3623M total
> Starting
>   string size: 943M
>   GC stats: 1168M used, 2456M free, 3623M total
> Starting
>   string size: 943M
>   GC stats: 1168M used, 2456M free, 3623M total
> Starting
>   string size: 943M
>   GC stats: 1168M used, 2456M free, 3623M total
> -----
>
> With LDC:
> -----
>   GC stats: 0M used, 1M free, 1M total
> Starting
>   string size: 943M
>   GC stats: 1168M used, 1139M free, 2306M total
> Starting
>   string size: 943M
>   GC stats: 2335M used, 1288M free, 3623M total
> Starting
>   string size: 943M
>   GC stats: 2335M used, 2605M free, 4940M total
> Starting
>   string size: 943M
>   GC stats: 2335M used, 2605M free, 4940M total
> Starting
>   string size: 943M
>   GC stats: 2335M used, 2605M free, 4940M total
> -----
>
> Note that I explicitly clear the `str` slice before 
> GC.collect(), so that the stack shouldn't contain any refs to 
> the fat string anymore.

Thank you for doing this. I hope my example doesn't obscure what 
you show here.
(I borrowed some of your code).

I have produced something which essentially reproduces my problem.

-----------
import std.range;
import std.algorithm;
import std.format;
import std.stdio;
import core.thread;
import core.memory;


struct Node {
  Node* next;
  Node* prev;
  ulong val;
}

Node* insertAfter(Node* cur, ulong val) {
  Node* node = new Node;
  if (cur != null) {
   node.next = cur.next;
   node.prev = cur;
   cur.next = node;
   node.next.prev = node;
  }
  else {
   node.next = node;
   node.prev = node;
  }
  node.val = val;
  return node;
}

int toMB(ulong size) { return cast(int) (size / 1048576.0 + 0.5); 
}

void printGCStats()
{
  const stats = GC.stats;
  const used = toMB(stats.usedSize);
  const free = toMB(stats.freeSize);
  const total = toMB(stats.usedSize + stats.freeSize);
  writef("  GC stats: %dM used, %dM free, %dM total\n", used, 
free, total);
}


void main()
{
  while(true)
  {
   printGCStats();
   writeln("Starting");
   Node* dll;
   dll = iota(200000000).fold!((c,x)=>insertAfter(c,x))(dll);
   writef("Last element %s\n", dll.val);
   dll = null;
   writeln("Done");
   GC.collect();
   GC.minimize();
   Thread.sleep( dur!("msecs")( 10000 ) );
  }
}
----------
With DMD this produces:

   GC stats: 0M used, 0M free, 0M total
Starting
Last element 199999999
Done
   GC stats: 6104M used, 51M free, 6155M total
Starting
Last element 199999999
Done
   GC stats: 12207M used, 28M free, 12235M total

With LDC2 this produces:

   GC stats: 0M used, 0M free, 0M total
Starting
Last element 199999999
Done
   GC stats: 6104M used, 51M free, 6155M total
Starting
Last element 199999999
Done
   GC stats: 12207M used, 28M free, 12235M total