Rallocates

[LemonParty]

Is it possible to say how many bytes can grow a pointer before it will be reallocated?

[jamie]

Whenever it cannot return a contiguous chunk starting at the same location.

You should always plan on this happening, and it depends on the minimum chuck of allocation.

Last time I checked it was around 1024 but that could be wrong.

P.S.
  that is, only if there isn't any memory following of course.


Jamie

[BrunoK]

MemSize(p) where p la pointer

[Thaddy]

The smallest allocation is 16 -real alloc -. 12 will be reported as used, length is stored. Depends on memory manager, though.
E.g. cmem reports 1.

Code: Pascal  [Select][+][-]

1. {$ifdef fpc}{$mode objfpc}{$endif}
2. //uses cmem;
3. var
4.   mem1,mem2:pointer;
5. begin
6.   mem1 := allocmem(1);
7.   writeln(memsize(mem1));
8. end.

So: no, you can't really predict re-allocations, because they are internals.
What you can do is adapt a memory manager, like cmem - that is easy- and log the reallocations to analyze how your program behaves. You just write to screen or log from the reallocmem function.

Also note that the heap manager is recently completely rewritten and above is tested with trunk from today.

[LV]

In relation to this topic, it might be helpful to examine
https://forum.lazarus.freepascal.org/index.php/topic,70463
https://forum.lazarus.freepascal.org/index.php/topic,69678

[Thaddy]

Suggested mm:

Code: Pascal  [Select][+][-]

1. unit mymem;
2. // works with heaptrc and default memorymanager.
3. // probably not for others.
4. interface
5.  
6. implementation
7. const
8.   signalled = 'realloc:';
9.  
10. var
11.   OldMem,
12.   NewMem:TMemoryManager;
13.  
14.   function MyReAllocMem(var p:pointer;Size:ptruint):Pointer;
15.   begin
16.     MyReAllocMem := OldMem.ReAllocmem(p,size);
17.     writeln(signalled,size,'  ',ptruint(p),' ',memsize(p));
18.   end;
19.  
20. initialization
21.   GetMemoryManager(OldMem);
22.   NewMem:=OldMem;
23.   NewMem.ReAllocMem := @MyReAllocMem;
24.   SetMemoryManager(NewMem);
25. finalization;
26.   SetMemoryManager(OldMem);
27. end.

Redirect writeln in gui apps to log instead.
Helps you analyze impact of reallocations

[runewalsh]

You don’t have any guarantees, any ReallocMem might decide to move. In trunk, if you have a situation like

Code: [Select]

[free 5,000][A 1,000][free 5,000]then any ReallocMem(A, N) will move A to a better place, be it N = 900, or N = 2,000, or even N = 1,000. If ReallocMem(A, 1000) does not decide to move A to a completely different place, it will turn it into

Code: [Select]

[A 1,000][free 10,000]

[Thaddy]

Yes, to my surprise, in trunk even smaller re-allocations to smaller size might trigger a move.
That is usually bad, unless the move is cheaper than alternatives.