• d-bugmail puremagic.com (39/40) Nov 30 2018 https://issues.dlang.org/show_bug.cgi?id=19455

d-bugmail puremagic.com writes:

https://issues.dlang.org/show_bug.cgi?id=19455

          Issue ID: 19455
           Summary: GC wastes too much memory
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: druntime
          Assignee: nobody puremagic.com
          Reporter: r.sagitario gmx.de

The GC allocates in chunks that are a power of 2. This causes a lot of memory
wasted:

module waste;
import core.memory;

struct S
{
        char[129] data;
}

void main()
{
        GC.disable();
        foreach (_; 0 .. 1000000)
                new S;
}

compile and run with

waste.exe --DRT-gcopt=profile:1

        Number of collections:  2
        Total GC prep time:  0 milliseconds
        Total mark time:  0 milliseconds
        Total sweep time:  8 milliseconds
        Total page recovery time:  3 milliseconds
        Max Pause Time:  0 milliseconds
        Grand total GC time:  12 milliseconds
GC summary:  247 MB,    2 GC   12 ms, Pauses    0 ms <    0 ms

=> even though only 129 MB are allocated, the GC needs 247 MB. With a struct
size of 128, it needs 145 MB.

--