What is heap size in linux

What is the «standard» size of the stack and the heap in a C program?

I have read that the «standard» and initial stack size on Linux is ~8MB and Windows is ~1MB. But how does the heap allocation work? Does the OS set a «virtual» size to the process heap like it does for the stack with committed and reserved memory?

2 Answers 2

Classically, the layout of a program has the ‘text’ (or ‘code’) segment at the low end of memory, followed by the fixed data (‘data’ and ‘bss’ segments), followed by a gap, with the stack growing downwards from high memory. The gap in the middle becomes the heap, which grows from the end of the data towards the stack.

Things are more complex with threaded programs, shared libraries loaded, shared memory, etc.

The initial stack size is o/s dependent. The initial heap size is logically zero, but tends to grow almost immediately (as the program and the shared libraries are loaded).

And What is the limit of the heap size?. It would be the same as the stack? I found this file that contains the max size for the stack in some architectures link

@user1030615 no the heap is far bigger than the stack. It’s generally «what’s left over» when all the other sections get their memory.

So this brings me another question, What is the size of «what’s left over» I mean the size of the total memory layout.

On a 32-bit machine, the total address space is nominally 4 GB. Out of that, you have the code, data, and stack segments — and anything left over can be used for heap. Also, memory mapped files, shared memory segments, etc eat into your 4GB of virtual address space. On a 64-bit machine, you have many times as much total available virtual memory, but approximately the same rules apply — the main difference being you won’t run out of addressing capacity for the logical address space but may run out of physical address space (and even virtual address space).

There is no general «standard size». Individual operating systems will have a default size, but usually they can be altered with appropriate parameters in the program image or on the command line.

C executes in a vast range of systems from tiny microprocessors with only a few hundred bytes of available memory to gigantic processor arrays with hundreds of gigabytes.

In your larger systems (including most Windows and Linux environments) the stack and heap will be assigned to segments that can be expanded, so that physical memory for maximum sizes does not need to be pre-reserved. Many micros, though, lack memory mapping hardware and the sizes must be pre-reserved (though sometimes the stack and heap are made to grow towards each other so that there’s only one common limit).

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How to find Java Heap Size and Memory Used (Linux)?

To find the Java heap size and the amount of memory used on a Linux system, you can use the jstat command and specify the process ID (PID) of the Java process.

Here is an example of how to do this:

This will display the Java heap size and the amount of memory used, as well as other information about the garbage collector.

Alternatively, you can use the jmap command to get a detailed memory map of the Java process. Here is an example:

This will display the size and usage of the different memory regions in the Java heap, as well as the number of objects and the amount of memory used in each region.

You can find the PID of the Java process by using the ps command and looking for the process that is running the Java executable. For example:

This will list all processes that have «java» in their command line, along with the PID of each process.

You can also use the top command to find the PID of the Java process. The top command will display a list of all running processes, along with the PID, the CPU and memory usage, and the command line of each process. You can use the top command to find the Java process and its PID.

To get the total Java heap size, you can use the -XX:+PrintFlagsFinal option with the java command and search for the MaxHeapSize flag. For example:

java -XX:+PrintFlagsFinal -version | grep MaxHeapSize

This will print the value of the MaxHeapSize flag, which is the maximum heap size that can be used by the Java process.

Keep in mind that these commands will only work if the Java process was started with the jstat , jmap , or java commands, and if you have permission to run these commands on the system.

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How is heap memory allocated?

Heap Allocation: The memory is allocated during the execution of instructions written by programmers. Note that the name heap has nothing to do with the heap data structure. It is called heap because it is a pile of memory space available to programmers to allocated and de-allocate.

How is memory allocated in Linux?

Linux provides a variety of APIs for memory allocation. You can allocate small chunks using kmalloc or kmem_cache_alloc families, large virtually contiguous areas using vmalloc and its derivatives, or you can directly request pages from the page allocator with alloc_pages .

What is heap size in Linux?

The heap usually is as large as the addressable virtual memory on your architecture. You should check your systems current limits with the ulimit -a command and seek this line max memory size (kbytes, -m) 3008828 , this line on my OpenSuse 11.4 x86_64 with ~3.5 GiB of ram says I have roughly 3GB of ram per process.

What is the command to check heap memory in Linux?

Step four: Verify heap space change

1. Open a terminal window.
2. Enter the following command: ps -ef | grep java | grep Xmx.
3. Review the command output.

What are different memory zone in Linux?

The Linux kernel divides memory into memory zones. On a mainframe, three zones are used: DMA , Normal , and Movable . Memory in the Movable zone cannot be used for arbitrary kernel allocations, but only for memory buffers that can easily be moved by the kernel, such as user memory allocations and page cache memory.

What is virtual memory in Linux?

What is virtual memory? Linux supports virtual memory, that is, using a disk as an extension of RAM so that the effective size of usable memory grows correspondingly. The kernel will write the contents of a currently unused block of memory to the hard disk so that the memory can be used for another purpose.

What is maximum heap size for a process?

The default startup heap size is 1.5 GB. This value must be a number between 1.5 GB and the maximum amount of memory allowed by your operating system and JVM version. Consider the following examples: If you have a Windows system with a 32-bit JVM, then a process can have a maximum heap size of 2 GB.

Is memory allocated on stack or on heap?

Why does Java use heap for memory allocation?

What are the differences between heap and stack memory?

When should I allocate on the heap?

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What are the bounds of the heap?

I’m afraid the question is not suitable as it is. If you want more details on how to get heap from the OS, go ahead and ask that. If you want to know how to introspect your languages runtime on how it organizes the heap, go ahead and ask that. But as it stands, the question is too broad.

@brooks94 As the question is opened for bounty you shouldn’t have accepted my answer well before the bounty period completes. This may get lack of attention from other members who could address it in with different solutions.

2 Answers 2

I assume you are trying to write your own heap allocator here, and from the tags assume you are doing it in Linux.

SunEric has given you a useful indication of what memory you might be able to use, however, the memory you can use is the memory that the operating system gives you. IE to get memory into your process, you will need to call the operating system to map virtual memory into the process space (and some physical memory behind it). malloc() abstracts this for you, and implements ‘the heap’ in C. It can get its memory two ways:

1. Using the brk system call (mapped to the C library brk or sbrk )
2. Using mmap with MAP_ANON (or more precisely the underlying system call mmap2 ).

brk is the classical way of allocating memory for the heap, and normally when we talk about ‘the heap’, we mean memory allocated this way (though brk can be used to allocate memory other than for the heap, and heap items may live elsewhere — see below). Here is a great answer to how brk allocation works, upon which I am unable to improve. What location the memory uses is really a result of arithmetic. The heap follows the BSS of the program when loaded — i.e. the BSS’s value is grown as the heap expands, so the start is really determined by the OS and the dynamic loader. The end of the heap is thus determined by this and the size of the heap, (i.e. how large you’ve grown it to).

mmap is less clear cut. It takes an addr parameter:

If addr is NULL , then the kernel chooses the address at which to create the mapping; this is the most portable method of creating a new mapping. If addr is not NULL , then the kernel takes it as a hint about where to place the mapping; on Linux, the mapping will be created at a nearby page boundary. The address of the new mapping is returned as the result of the call.

So if you use mmap to get space for particular heap items (as malloc may do particularly for large objects), either the OS choses its location, with or without a hint. If you use MAP_FIXED it will give you exactly that location or failed. In this sense, your heap (or items within it) could be anywhere the OS will let you map memory.

You asked whether there is a portable way to find out where the heap begins and ends. Portable implies a language, and I’ll assume C. In respect of the brk type heap, yes there is (well reasonably portable). man end gives:

• etext : This is the first address past the end of the text segment (the program code).
• edata : This is the first address past the end of the initialized data segment.
• end : This is the first address past the end of the uninitialized data segment (also known as the BSS segment).

As the heap runs from the end of the BSS at load time to the top of the BSS at run time, one approach would be to take the value of end at load as the start as the bottom of the heap and the value of end when evaluating as the end of the heap. This would miss the fact that libc itself and the shared libraries may allocate things before main() is called. So a more conservative approach would be to say it is the area between edata and end , though this might strictly speaking include things not on the heap.

If you didn’t mean in C, you need to use a similar technique. Take the ‘program break’ (i.e. the top of memory space) and subtract the lowest address you gave for your heap.

If you want to see the memory allocation for the heap for an arbitrary process:

$ cat /proc/$$/maps | fgrep heap 01fe6000-02894000 rw-p 00000000 00:00 0 [heap] 

Replace $$ by the PID of the process you want to examine.

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