About Dynamic Memory Allocation in C [duplicate] - c

Question

I was trying to figure out how much memory I can malloc to maximum extent on my machine
(1 Gb RAM 160 Gb HD Windows platform).
I read that the maximum memory malloc can allocate is limited to physical memory (on heap).
Also when a program exceeds consumption of memory to a certain level, the computer stops working because other applications do not get enough memory that they require.
So to confirm, I wrote a small program in C:
int main(){
int *p;
while(1){
p=(int *)malloc(4);
if(!p)break;
}
}
I was hoping that there would be a time when memory allocation would fail and the loop would break, but my computer hung as it was an infinite loop.
I waited for about an hour and finally I had to force shut down my computer.
Some questions:
Does malloc allocate memory from HD also?
What was the reason for above behaviour?
Why didn't loop break at any point of time?
Why wasn't there any allocation failure?

Answer 1

I read that the maximum memory malloc can allocate is limited to physical memory (on heap).
Wrong: most computers/OSs support virtual memory, backed by disk space.
Some questions: does malloc allocate memory from HDD also?
malloc asks the OS, which in turn may well use some disk space.
What was the reason for above behavior? Why didn't the loop break at any time?
Why wasn't there any allocation failure?
You just asked for too little at a time: the loop would have broken eventually (well after your machine slowed to a crawl due to the large excess of virtual vs physical memory and the consequent super-frequent disk access, an issue known as "thrashing") but it exhausted your patience well before then. Try getting e.g. a megabyte at a time instead.
When a program exceeds consumption of memory to a certain level, the
computer stops working because other applications do not get enough
memory that they require.
A total stop is unlikely, but when an operation that normally would take a few microseconds ends up taking (e.g.) tens of milliseconds, those four orders of magnitude may certainly make it feel as if the computer had basically stopped, and what would normally take a minute could take a week.

Answer 2

I know this thread is old, but for anyone willing to give it a try oneself, use this code snipped
#include <stdlib.h>
int main() {
int *p;
while(1) {
int inc=1024*1024*sizeof(char);
p=(int*) calloc(1,inc);
if(!p) break;
}
}
run
$ gcc memtest.c
$ ./a.out
upon running, this code fills up ones RAM until killed by the kernel. Using calloc instead of malloc to prevent "lazy evaluation". Ideas taken from this thread:
Malloc Memory Questions
This code quickly filled my RAM (4Gb) and then in about 2 minutes my 20Gb swap partition before it died. 64bit Linux of course.

Answer 3

/proc/sys/vm/overcommit_memory controls the maximum on Linux
On Ubuntu 19.04 for example, we can easily see that malloc is implemented with mmap(MAP_ANONYMOUS by using strace.
Then man proc then describes how /proc/sys/vm/overcommit_memory controls the maximum allocation:
This file contains the kernel virtual memory accounting mode. Values are:
0: heuristic overcommit (this is the default)
1: always overcommit, never check
2: always check, never overcommit
In mode 0, calls of mmap(2) with MAP_NORESERVE are not checked, and the default check is very weak, leading to the risk of getting a process "OOM-killed".
In mode 1, the kernel pretends there is always enough memory, until memory actually runs out. One use case for this mode is scientific computing applications that em‐ ploy large sparse arrays. In Linux kernel versions before 2.6.0, any nonzero value implies mode 1.
In mode 2 (available since Linux 2.6), the total virtual address space that can be allocated (CommitLimit in /proc/meminfo) is calculated as
CommitLimit = (total_RAM - total_huge_TLB) * overcommit_ratio / 100 + total_swap
where:
total_RAM is the total amount of RAM on the system;
total_huge_TLB is the amount of memory set aside for huge pages;
overcommit_ratio is the value in /proc/sys/vm/overcommit_ratio; and
total_swap is the amount of swap space.
For example, on a system with 16GB of physical RAM, 16GB of swap, no space dedicated to huge pages, and an overcommit_ratio of 50, this formula yields a Com‐ mitLimit of 24GB.
Since Linux 3.14, if the value in /proc/sys/vm/overcommit_kbytes is nonzero, then CommitLimit is instead calculated as:
CommitLimit = overcommit_kbytes + total_swap
See also the description of /proc/sys/vm/admiin_reserve_kbytes and /proc/sys/vm/user_reserve_kbytes.
Documentation/vm/overcommit-accounting.rst in the 5.2.1 kernel tree also gives some information, although lol a bit less:
The Linux kernel supports the following overcommit handling modes
0 Heuristic overcommit handling. Obvious overcommits of address
space are refused. Used for a typical system. It ensures a
seriously wild allocation fails while allowing overcommit to
reduce swap usage. root is allowed to allocate slightly more
memory in this mode. This is the default.
1 Always overcommit. Appropriate for some scientific
applications. Classic example is code using sparse arrays and
just relying on the virtual memory consisting almost entirely
of zero pages.
2 Don't overcommit. The total address space commit for the
system is not permitted to exceed swap + a configurable amount
(default is 50%) of physical RAM. Depending on the amount you
use, in most situations this means a process will not be
killed while accessing pages but will receive errors on memory
allocation as appropriate.
Useful for applications that want to guarantee their memory
allocations will be available in the future without having to
initialize every page.
Minimal experiment
We can easily see the maximum allowed value with:
main.c
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <string.h>
#include <unistd.h>
int main(int argc, char **argv) {
char *chars;
size_t nbytes;
/* Decide how many ints to allocate. */
if (argc < 2) {
nbytes = 2;
} else {
nbytes = strtoull(argv[1], NULL, 0);
}
/* Allocate the bytes. */
chars = mmap(
NULL,
nbytes,
PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_ANONYMOUS,
-1,
0
);
/* This can happen for example if we ask for too much memory. */
if (chars == MAP_FAILED) {
perror("mmap");
exit(EXIT_FAILURE);
}
/* Free the allocated memory. */
munmap(chars, nbytes);
return EXIT_SUCCESS;
}
GitHub upstream.
Compile and run to allocate 1GiB and 1TiB:
gcc -ggdb3 -O0 -std=c99 -Wall -Wextra -pedantic -o main.out main.c
./main.out 0x40000000
./main.out 0x10000000000
We can then play around with the allocation value to see what the system allows.
I can't find a precise documentation for 0 (the default), but on my 32GiB RAM machine it does not allow the 1TiB allocation:
mmap: Cannot allocate memory
If I enable unlimited overcommit however:
echo 1 | sudo tee /proc/sys/vm/overcommit_memory
then the 1TiB allocation works fine.
Mode 2 is well documented, but I'm lazy to carry out precise calculations to verify it. But I will just point out that in practice we are allowed to allocate about:
overcommit_ratio / 100
of total RAM, and overcommit_ratio is 50 by default, so we can allocate about half of total RAM.
VSZ vs RSS and the out-of-memory killer
So far, we have just allocated virtual memory.
However, at some point of course, if you use enough of those pages, Linux will have to start killing some processes.
I have illustrated that in detail at: What is RSS and VSZ in Linux memory management

Answer 4

Try this
#include <stdlib.h>
#include <stdio.h>
main() {
int Mb = 0;
while (malloc(1<<20)) ++Mb;
printf("Allocated %d Mb total\n", Mb);
}
Include stdlib and stdio for it.
This extract is taken from deep c secrets.

Answer 5

malloc does its own memory management, managing small memory blocks itself, but ultimately it uses the Win32 Heap functions to allocate memory. You can think of malloc as a "memory reseller".
The windows memory subsystem comprises physical memory (RAM) and virtual memory (HD). When physical memory becomes scarce, some of the pages can be copied from physical memory to virtual memory on the hard drive. Windows does this transparently.
By default, Virtual Memory is enabled and will consume the available space on the HD. So, your test will continue running until it has either allocated the full amount of virtual memory for the process (2GB on 32-bit windows) or filled the hard disk.

Answer 6

As per C90 standard guarantees that you can get at least one object 32 kBytes in size, and this may be static, dynamic, or automatic memory. C99 guarantees at least 64 kBytes. For any higher limit, refer your compiler's documentation.
Also, malloc's argument is a size_t and the range of that type is [0,SIZE_MAX], so the maximum you can request is SIZE_MAX, which value varies upon implementation and is defined in <limits.h>.

Answer 7

I don't actually know why that failed, but one thing to note is that `malloc(4)" may not actually give you 4 bytes, so this technique is not really an accurate way to find your maximum heap size.
I found this out from my question here.
For instance, when you declare 4 bytes of memory, the space directly before your memory could contain the integer 4, as an indication to the kernel of how much memory you asked for.

Answer 8

Does malloc allocate memory from HD also?
Implementation of malloc() depends on libc implementation and operating system (OS). Typically malloc() doesn't always request RAM from the OS but returns a pointer to previously allocated memory block "owned" by libc.
In case of POSIX compatible systems, this libc controlled memory area is usually increased using syscall brk(). That doesn't allow releasing any memory between two still existing allocations which causes the process to look still using all the RAM after allocating areas A, B, C in sequence and releasing B. This is because areas A and C around the area B are still in use so the memory allocated from the OS cannot be returned.
Many modern malloc() implementations have some kind of heuristic where small allocations use the memory area reserved via brk() and "big" allocations use anonymous virtual memory blocks reserved via mmap() using MAP_ANONYMOUS flag. This allows immediately returning these big allocations when free() is later called. Typically the runtime performance of mmap() is slightly slower than using previously reserved memory which is the reason malloc() implements this heuristic.
Both brk() and mmap() allocate virtual memory from the OS. And virtual memory can be always backed up by swap which may be stored in any storage that the OS supports, including HDD.
In case you run Windows, the syscalls have different names but the underlying behavior is probably about the same.
What was the reason for above behaviour?
Since your example code never touched the memory, I'd guess you're seeing behavior where OS implements copy-on-write for virtual RAM and the memory is mapped to shared page with whole page filled with zeroes by default. Modern operating systems do this because many programs allocate more RAM than they actually need and using shared zero page by default for all memory allocations avoids needing to use real RAM for these allocations.
If you want to test how OS handles your loop and actually reserve true storage, you need to write something to the memory you allocated. For x86 compatible hardware you only need to write one byte per each 4096 byte segment because page size is 4096 and the hardware cannot implement copy-on-write behavior for smaller segments; once one byte is modified, the whole 4096 byte segment called page must be reserved for your process. I'm not aware of any modern CPU that would support smaller than 4096 byte pages. Modern Intel CPUs support 2 MB and 1 GB pages in addition to 4096 byte pages but the 1 GB pages are rarely used because the overhead of using 2 MB pages is small enough for any sensible RAM amounts. 1 GB pages might make sense if your system has hundreds of terabytes of RAM.
So basically your program only tested reserving virtual memory without ever using said virtual memory. Your OS probably has special optimization for this which avoids needing more than 4 KB of RAM to support this.
Unless your objective is to try to measure the overhead caused by your malloc() implementation, you should avoid trying to allocate memory block smaller than 16-32 bytes. For mmap() allocations the minimum possible overhead is 8 bytes per allocation on x86-64 hardware due the data needed to return the memory to the operating system so it really doesn't make sense for malloc() to use mmap() syscall for a single 4 byte allocation.
The overhead is needed to keep track of memory allocations because the memory is freed using void free(void*) so memory allocation routines must keep track of the allocated memory segment size somewhere. Many malloc() implementations also need additional metadata and if they need to keep track of any memory addresses, those need 8 bytes per address.
If you truly want to search for the limits of your system, you should probably do binary search for the limit where malloc() fails. In practice, you try to allocate ..., 1KB, 2KB, 4KB, 8KB, ..., 32 GB which then fails and you know that the real world limit is between 16 GB and 32 GB. You can then split this size in half and figure out the exact limit with additional testing. If you do this kind of search, it may be easier to always release any successful allocation and reserve the test block with a single malloc() call. That should also avoid accidentally accounting for malloc() overhead so much because you need only one allocation at any time at max.
Update: As pointed out by Peter Cordes in the comments, your malloc() implementation may be writing bookkeeping data about your allocations in the reserved RAM which causes real memory to be used and that can cause system to start swapping so heavily that you cannot recover it in any sensible timescale without shutting down the computer. In case you're running Linux and have enabled "Magic SysRq" keys, you could just press Alt+SysRq+f to kill the offending process taking all the RAM and system would run just fine again. It is possible to write malloc() implementation that doesn't usually touch the RAM allocated via brk() and I assumed you would be using one. (This kind of implementation would allocate memory in 2^n sized segments and all similarly sized segments are reserved in the same range of addresses. When free() is later called, the malloc() implementation knows the size of the allocation from the address and bookkeeping about free memory segments are kept in separate bitmap in single location.) In case of Linux, malloc() implementation touching the reserved pages for internal bookkeeping is called dirtying the memory, which prevents sharing memory pages because of copy-on-write handling.
Why didn't loop break at any point of time?
If your OS implements the special behavior described above and you're running 64-bit system, you're not going to run out of virtual memory in any sensible timescale so your loop seems infinite.
Why wasn't there any allocation failure?
You didn't actually use the memory so you're allocating virtual memory only. You're basically increasing the maximum pointer value allowed for your process but since you never access the memory, the OS never bothers the reserve any physical memory for your process.
In case you're running Linux and want the system to enforce virtual memory usage to match actually available memory, you have to write 2 to kernel setting /proc/sys/vm/overcommit_memory and maybe adjust overcommit_ratio, too. See https://unix.stackexchange.com/q/441364/20336 for details about memory overcommit on Linux. As far as I know, Windows implements overcommit, too, but I don't know how to adjust its behavior.

Answer 9

when first time you allocate any size to *p, every next time you leave that memory to be unreferenced. That means
at a time your program is allocating memory of 4 bytes only
. then how can you thing you have used entire RAM, that's why SWAP device( temporary space on HDD) is out of discussion. I know an memory management algorithm in which when no one program is referencing to memory block, that block is eligible to allocate for programs memory request. That's why you are just keeping busy to RAM Driver and that's why it can't give chance to service other programs. Also this a dangling reference problem.
Ans : You can at most allocate the memory of your RAM size. Because no program has access to swap device.
I hope your all questions has got satisfactory answers.