[fpc-pascal] Problem with heaptrc

[Michael Van Canneyt]

On Sun, 9 Nov 2014, Xiangrong Fang wrote:

> 2014-11-09 22:14 GMT+08:00 Xiangrong Fang <xrfang at gmail.com>:
>       2014-11-09 22:05 GMT+08:00 Tomas Hajny <XHajT03 at hajny.biz>:
>       On 9 Nov 14, at 21:58, Xiangrong Fang wrote:
> 
> Are you sure that it really is endless (i.e. did you let to run for a
> sufficiently long time)? If you increase the amount of allocated
> blocks (which is what happens in case of increasing your constant),
> traversing through all the blocks (especially if they are not freed
> at the end) may indeed take fairly long time...
> 
> 
> ​You are right Tomas, it finished, generated a 300M+(!) trace file, looks like this:
> 
> ​Heap dump by heaptrc unit
> 8137678 memory blocks allocated : 258591549/258591552
> 5226767 memory blocks freed     : 142155109/142155112
> 2910911 unfreed memory blocks : 116436440
> True heap size : 951812096
> True free heap : 392917184
> Should be : 462779048
> Call trace for block $00007F57C456D680 size 40
>   $000000000040071F line 29 of demo.lpr
>   $0000000000400180
> 
> The last 3 likes repeated until the end of 300M log data!
> 
> Why when ITEM_COUNT is small, there is no memory leak, but when it is large, there are so many leaks?

I didn't check in detail, but looking at the code, it seems likely that you overwrite 
memory somewhere in your tcache class. As soon as that happens somewhere, memory is 
corrupted and is unlikely to be free corrected.
The greater the number of items, the greater the probability of memory corruption.

Code like this:

    ci := PCacheItem((buf + SizeOf(Pointer) * Random(FDepth))^);
     for i := 0 to FDepth - 1 do begin
       slot := buf + SizeOf(Pointer) * i;
       if Pointer(slot^) = nil then begin
         Inc(N);
         Pointer(slot^) := GetMem(SizeOf(TCacheItem));
         ci := PCacheItem(slot^);

makes me frown, because the first assignment to ci is useless, so highly suspect.

Michael.