CODEWITHSUNDEEP
memory-managementlinux-kernellinux-device-drivervirtual-memory
feeling_lonely
feeling_lonely

Reputation: 6853

Where is the heap?

I understand that in Linux the mm_struct describes the memory layout of a process. I also understand that the start_brk and brk mark the start and end of the heap section of a process respectively.

Now, this is my problem: I have a process, for which I wrote the source code, that allocates 5.25 GB of heap memory using malloc. However, when I examine the process's mm_sruct using a kernel module I find the value of is equal to 135168. And this is different from what I expected: I expected brk - start_brk to be equal slight above 5.25 GB.

So, what is going on here?

Thanks.

Upvotes: 0

Views: 571

Answers (1)

hcs
hcs

Reputation: 1534

I notice the following in the manpage for malloc(3):

Normally, malloc() allocates memory from the heap, and adjusts the size of the heap as required, using sbrk(2). When allocating blocks of memory larger than MMAP_THRESHOLD bytes, the glibc malloc() implementation allocates the memory as a private anonymous mapping using mmap(2). MMAP_THRESHOLD is 128 kB by default, but is adjustable using mallopt(3). Allocations performed using mmap(2) are unaffected by the RLIMIT_DATA resource limit (see getrlimit(2)).

So it sounds like mmap is used instead of the heap.

Upvotes: 2

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