kern/61691: very bad performance of realloc()/brk()

[Radim Kolar]

> You seem to be confused as to what responsibilities the programmer
> has when it comes to practicing sensible memory management.
I have run some benchmarks for you. There are doing 32 times realloc() in
1 MB chunks. I don't think that this is example of bad programming
practice. The major problem is that realloc() copies data while Linux
doesn't.

FreeBSD 5.2
===========
It looks that brk() syscall is quite slow in FreeBSD when comparing
against Linux. FreeBSD calls 32 times brk() and 10 times mmap+munmap (for page directory).

(hsn at ttyv0):~/forkbomb% time ./forkbomb -l 32 -i 256 -M --quit            12:45
Safety alarm at 300 sec. enabled.
Actions: alloc 32 MB (step 1024 kB) and touch it.
Forkbomb 1.2 started.
./forkbomb -l 32 -i 256 -M --quit  4.58s user 5.28s system 74% cpu 13.159 total

linux2.4+glibc2.3.2
===================
Linux does 1 times mmap + 31 times mremap syscall

Safety alarm at 300 sec. enabled.
Actions: alloc 32 MB (step 1024 kB) and touch it.
Forkbomb 1.2 started.
./forkbomb -l 32 -i 256 -M --quit  0.00s user 0.30s system 107% cpu 0.280 total

Well my pr-report/wish is: optimize realloc() function (which is about 3 pages
long) to avoid copying data while brk() is sufficient , because the reallocated block is
last block.