Mail Archives: djgpp/1997/03/19/18:10:02
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"A.Appleyard" <A DOT APPLEYARD AT fs2 DOT mt DOT umist DOT ac DOT uk> wrote:
>Re my messages about malloc(), here is a version of malloc() free() realloc()
>which I made by altering djgpp\src\libc\ansi\stdlib\malloc.c; I have tested it
>quickly and it seems to work. See the comments `/*******' herinunder.
I taked a look to your code and to the code in libc, and:
1) Seems to be right.
2) Seems to be better.
You ask in your code:
> /****** If size>=pagesize, the standard form gets (size/pagesize + 1) */
> /****** pages here. What is the extra unused page for? */
The extra page is for realloc, see the following lines taked from the
libc version of realloc:
onb = 1 << (i + 3);
if (onb < pagesz)
onb -= sizeof (*op) + RSLOP;
else
onb += pagesz - sizeof (*op) - RSLOP;
So here if size>=pagesize realloc asumes that we have an extra page in
the block. It helps realloc but eats a lot of memory if you are allocating
blocks of size 4K or similar.
And now another thing, I don't know what a hell the ANSI says but the
realloc implementation isn't very good for my code, here is the relevant
code (I taked it from the A. APPLEYARD code because is much more clear):
if (N+OW<=oldsize)
if (N+OW>oldsize/2)
return Old;
free(Old);
if (!(New=malloc(N)))
return 0;
From my point of view the free must be moved AFTER the call to malloc
because in this way if malloc fails the old block isn't moved to the free
pool of memory.
We don't gain anything freeing the block first because we checked that
malloc won't reuse this block.
And finally another one: The algorithm used by malloc is fast but is
easy to get the memory fragmented, for example if we allocate a lot of
blocks of 100Kb and then we free these blocks and we try to allocate one
of 200Kb ... malloc will fail even when all the memory is free just because
the free memory is in the 128Kb bucket and not in the 256Kb one.
I don't know if that was discused some days ago or was another topic
about malloc but I guess that we need some kind of optimizer to be called
under these situations to reorder the heap and liberate the unused memory
so then morecore can allocate the requested block.
I attached to this mail a modified version of the A. Appleyard malloc to
reuse buckets when there is no more memory. The code is unoptimized and was
tested only with the file included in the zip.
I included 2 examples:
1) Just allocates as many 1Mb blocks as posible and then frees all the
blocks, after that just tries to allocate 2 Mb, imposible with the actual
malloc, but this malloc joins 2 1Mb buckets to create one of 2 Mb.
2) The program allocate blocks as large as posible, then liberate the blocks
and finally tries to allocate a block of size = 1/2 of the smaller previously
allocated. This time malloc will split a big bucket into smaller ones.
Sorry if I'm talking about allready solved things but I discovered this
problem testing my editor and was very annoying see how 24 Mb of memory
wasn't enough to handle a 9 Mb clipboard that grown incrementally.
SET
--------------- 0 --------------------------------
Salvador Eduardo Tropea (SET).
Address: Curapaligue 2124, Caseros, 3 de Febrero
Buenos Aires, (1678), ARGENTINA
TE: +(541) 759 0013
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