Messages in this thread | | | Subject | Re: Relation between free() and remove_vm_struct() | From | Arjan van de Ven <> | Date | Thu, 17 Aug 2006 08:59:25 +0200 |
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On Thu, 2006-08-17 at 12:29 +0530, Abu M. Muttalib wrote: > Hi, > > In an application I am freeing some memory address, earlier reserved with > malloc. > > I have put prints in remove_vm_struct() function in ./mm/mmap.c. For few > calls to free(), there is no corresponding call to remove_vm_struct(). I am > not able to understand why the user space call to free() is not propagated > to kernel, where in the remove_vm_strcut() function should get called.
Hi, there is 2 parts to this question
first of all, glibc malloc doesn't always use mmap for it's allocations, it's a split between the brk() area and mmap() depending on the size of the allocation. (>= 128Kb uses mmap, smaller uses brk(). brk using allocations will not end up in remove_vm_struct at all)
second of all, glibc delays freeing of some memory (in the brk() area) to optimize for cases of frequent malloc/free operations, so that it doesn't have to go to the kernel all the time (and a free would imply a cross cpu TLB invalidate which is *expensive*, so batching those up is a really good thing for performance)
I hope this answer helps you... it's probably worth reading the malloc/malloc.c code in the glibc code tree, this behavior is documented there...
Greetings, Arjan van de Ven
-- if you want to mail me at work (you don't), use arjan (at) linux.intel.com
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