How to tune my Linux to malloc(200000) on 512Mb ROM

If i run program compiled
from C: main() {malloc(200000)} or fortran CALL MALLOC(200000)
This call on my system
for malloc(100000)>0 but for malloc(200000)<0.
How could I nune bdflush or VM parameters on 2.6.8 linux kernel
to have both pozitive result from malloc(100000) and malloc(200000)?
Cold Anybody help me?
Thank You in advance.