delphi - 为什么释放内存后我的程序的内存使用量没有恢复正常？

Question

考虑下一个示例应用程序

program TestMemory;


{$APPTYPE CONSOLE}

uses
  PsAPI,
  Windows,
  SysUtils;

function GetUsedMemoryFastMem: cardinal;
var
    st: TMemoryManagerState;
    sb: TSmallBlockTypeState;
begin
    GetMemoryManagerState(st);
    result := st.TotalAllocatedMediumBlockSize + st.TotalAllocatedLargeBlockSize;
    for sb in st.SmallBlockTypeStates do
    begin
        result := result + sb.UseableBlockSize * sb.AllocatedBlockCount;
    end;
end;

function GetUsedMemoryWindows: longint;
var
  ProcessMemoryCounters: TProcessMemoryCounters;
begin
  Result:=0;
  ProcessMemoryCounters.cb := SizeOf(TProcessMemoryCounters);
  if GetProcessMemoryInfo(GetCurrentProcess(), @ProcessMemoryCounters, ProcessMemoryCounters.cb) then
   Result:= ProcessMemoryCounters.WorkingSetSize
  else
   RaiseLastOSError;
end;

procedure Test;
const
  Size = 1024*1024;
var
  P : Pointer;
begin
  GetMem(P,Size);

      Writeln('Inside');
      Writeln('FastMem '+FormatFloat('#,', GetUsedMemoryFastMem));
      Writeln('Windows '+FormatFloat('#,', GetUsedMemoryWindows));
      Writeln('');

  FreeMem(P);
end;

begin
      Writeln('Before');
      Writeln('FastMem '+FormatFloat('#,', GetUsedMemoryFastMem));
      Writeln('Windows '+FormatFloat('#,', GetUsedMemoryWindows));
      Writeln('');

      Test;

      Writeln('After');
      Writeln('FastMem '+FormatFloat('#,', GetUsedMemoryFastMem));
      Writeln('Windows '+FormatFloat('#,', GetUsedMemoryWindows));
      Writeln('');
      Readln;
end.

应用程序返回的结果是

Before
FastMem 1.844
Windows 3.633.152

Inside
FastMem 1.050.612
Windows 3.637.248

After
FastMem 2.036
Windows 3.633.152

我想知道为什么Before和After的内存使用结果不同:

Answer 1

任何内存管理器(包括 FastMM)都会产生一些开销，否则 Delphi 可能只使用 Windows 内存管理。

您观察到的差异是开销:

• FastMM 用于跟踪内存使用情况的结构，
• FastMM 尚未返回到 Windows 内存管理以优化 future 类似内存分配的内存。