[PATCH] malloc on top to consider already available memory

[Corinna Vinschen]

Hi Petter, hi guys,

On Feb 12 16:23, Petter Österlund wrote:
> 
> Hi,
> 
> This is my fist post. I suggest a patch for malloc considering already
> available memory available on the top instead of always extending top
> with the allocation request size when there is not enough space, which may
> cause near double memory requirements in some cases. For an MCU with
> small memory it may be quite crucial and for anyone it is a bit unexpected
> behavior.
> 
> An example of the problem; Do malloc(30000); free(); malloc(40000);
> free();  then without this patch then peak memory will become 70KB
> but with the patch it will be only 40KB as second extend will only be
> for 10KB extra  not 40K.
> 
> Newer versions of Doug Lea malloc, starting with version 2.7.0,
> already contains similar fix since several years ago and it would be
> nice if this fix could be included in Newlib also.

Can you point to this patch?

> The patch below first try and extend top with the diff of what should
> be needed, if that fails to extend top large enough for some reason
> the behavior falls back on old behavior (a failure may happen if there
> are intervening sbrk() or if out of memory).
> 
> br Petter
> 
> $ diff -up newlib-2.3.0.20160104/newlib/libc/stdlib/mallocr.c
> newlib-patch/newlib/libc/stdlib/mallocr.c
> --- newlib-2.3.0.20160104/newlib/libc/stdlib/mallocr.c  2016-02-12
> 15:32:05.380345800 +0100
> +++ newlib-patch/newlib/libc/stdlib/mallocr.c   2016-02-12
> 15:36:17.184029400 +0100
> @@ -2572,7 +2572,20 @@ Void_t* mALLOc(RARG bytes) RDECL size_t
>  #endif
> 
>      /* Try to extend */
> -    malloc_extend_top(RCALL nb);
> +    if (bytes > chunksize(top)) {
> +      /* extend including whats available on top */
> +      INTERNAL_SIZE_T nb_diff  = request2size(long_sub_size_t(bytes,
> chunksize(top)));
> +      malloc_extend_top(RCALL nb_diff);
> +      remainder_size = long_sub_size_t(chunksize(top), nb);
> +      if (chunksize(top) < nb || remainder_size < (long)MINSIZE)
> +      {
> +        /* top not large enough; failure or intervening sbrk() */
> +        malloc_trim(RCALL 0);
> +        malloc_extend_top(RCALL nb);
> +      }
> +    }
> +    else
> +      malloc_extend_top(RCALL nb);
>      remainder_size = long_sub_size_t(chunksize(top), nb);
>      if (chunksize(top) < nb || remainder_size < (long)MINSIZE)
>      {

Can anybody here using the newlib malloc give this a whirl?  Cygwin
is using its own malloc, so I can't test this easily.


Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer
Red Hat
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