Is there a kernel function which returns amount of kernel memory available(Not vmalloc related).
First, let me say that if you're going to make any policy decisions (should I proceed with this operation?) based on this information, STOP. As WGW pointed out, there are unavoidable races here; memory can be used up between when you check and when you use it. Just test for errors on your memory allocations and have an appropriate failure path. Moreover, if you request memory when there isn't enough free memory, often the kernel can obtain more free memory by cleaning up various cache memory, swapping to disk, freeing slabs, etc. And kernel memory fragmentation can fail large (multiple page) allocations when not made through vmalloc even with plenty of memory free.
That said, there are APIs for querying kernel memory availability. You should note that the kernel has multiple memory pools, so even if one of these API says you have no free RAM, it could be that it's available in the memory pool you are interested in.
First, we have si_meminfo. This is the call that provides availability data for /proc/meminfo, among other things, and reports on the current state of the buddy page allocator. Note that cached and buffer ram can be converted to free ram very quickly.
global_page_state(NR_SLAB_RECLAIMABLE) can also be used to get counts of how much slab memory can be quickly reclaimed. If you request an allocation, this memory can and will be freed on demand.
The SLUB allocator (used for kalloc() and the like, among others) also provides statistics for its internal memory pools that can also reflect free memory within each memory pool. This may not be available with the same API depending on which allocator is selected in your configuration - please do not use this data except for debugging. The relevant code (implementing /proc/slabinfo) can be found in mm/slub.c
What kind of use is the available memory for you? Worst case you run in a race condition with checking available memory:
You get the available memory. It`s enough.
Multitasking, a.k.a. the scheduler of the kernel, stops your process and continues with another one which allocates a bunch of the available memory.
The scheduler continues with your process.
Your allocations fails though step 1 showed enough available memory.
Related
Here's my question: Does calling free or delete ever release memory back to the "system". By system I mean, does it ever reduce the data segment of the process?
Let's consider the memory allocator on Linux, i.e ptmalloc.
From what I know (please correct me if I am wrong), ptmalloc maintains a free list of memory blocks and when a request for memory allocation comes, it tries to allocate a memory block from this free list (I know, the allocator is much more complex than that but I am just putting it in simple words). If, however, it fails, it gets the memory from the system using say sbrk or brk system calls. When a memory is free'd, that block is placed in the free list.
Now consider this scenario, on peak load, a lot of objects have been allocated on heap. Now when the load decreases, the objects are free'd. So my question is: Once the object is free'd will the allocator do some calculations to find whether it should just keep this object in the free list or depending upon the current size of the free list it may decide to give that memory back to the system i.e decrease the data segment of the process using sbrk or brk?
Documentation of glibc tells me that if the allocation request is much larger than page size, it will be allocated using mmap and will be directly released back to the system once free'd. Cool. But let's say I never ask for allocation of size greater than say 50 bytes and I ask a lot of such 50 byte objects on peak load on the system. Then what?
From what I know (correct me please), a memory allocated with malloc will never be released back to the system ever until the process ends i.e. the allocator will simply keep it in the free list if I free it. But the question that is troubling me is then, if I use a tool to see the memory usage of my process (I am using pmap on Linux, what do you guys use?), it should always show the memory used at peak load (as the memory is never given back to the system, except when allocated using mmap)? That is memory used by the process should never ever decrease(except the stack memory)? Is it?
I know I am missing something, so please shed some light on all this.
Experts, please clear my concepts regarding this. I will be grateful. I hope I was able to explain my question.
There isn't much overhead for malloc, so you are unlikely to achieve any run-time savings. There is, however, a good reason to implement an allocator on top of malloc, and that is to be able to trace memory leaks. For example, you can free all memory allocated by the program when it exits, and then check to see if your memory allocator calls balance (i.e. same number of calls to allocate/deallocate).
For your specific implementation, there is no reason to free() since the malloc won't release to system memory and so it will only release memory back to your own allocator.
Another reason for using a custom allocator is that you may be allocating many objects of the same size (i.e you have some data structure that you are allocating a lot). You may want to maintain a separate free list for this type of object, and free/allocate only from this special list. The advantage of this is that it will avoid memory fragmentation.
No.
It's actually a bad strategy for a number of reasons, so it doesn't happen --except-- as you note, there can be an exception for large allocations that can be directly made in pages.
It increases internal fragmentation and therefore can actually waste memory. (You can only return aligned pages to the OS, so pulling aligned pages out of a block will usually create two guaranteed-to-be-small blocks --smaller than a page, anyway-- to either side of the block. If this happens a lot you end up with the same total amount of usefully-allocated memory plus lots of useless small blocks.)
A kernel call is required, and kernel calls are slow, so it would slow down the program. It's much faster to just throw the block back into the heap.
Almost every program will either converge on a steady-state memory footprint or it will have an increasing footprint until exit. (Or, until near-exit.) Therefore, all the extra processing needed by a page-return mechanism would be completely wasted.
It is entirely implementation dependent. On Windows VC++ programs can return memory back to the system if the corresponding memory pages contain only free'd blocks.
I think that you have all the information you need to answer your own question. pmap shows the memory that is currenly being used by the process. So, if you call pmap before the process achieves peak memory, then no it will not show peak memory. if you call pmap just before the process exits, then it will show peak memory for a process that does not use mmap. If the process uses mmap, then if you call pmap at the point where maximum memory is being used, it will show peak memory usage, but this point may not be at the end of the process (it could occur anywhere).
This applies only to your current system (i.e. based on the documentation you have provided for free and mmap and malloc) but as the previous poster has stated, behavior of these is implmentation dependent.
This varies a bit from implementation to implementation.
Think of your memory as a massive long block, when you allocate to it you take a bit out of your memory (labeled '1' below):
111
If I allocate more more memory with malloc it gets some from the system:
1112222
If I now free '1':
___2222
It won't be returned to the system, because two is in front of it (and memory is given as a continous block). However if the end of the memory is freed, then that memory is returned to the system. If I freed '2' instead of '1'. I would get:
111
the bit where '2' was would be returned to the system.
The main benefit of freeing memory is that that bit can then be reallocated, as opposed to getting more memory from the system. e.g:
33_2222
I believe that the memory allocator in glibc can return memory back to the system, but whether it will or not depends on your memory allocation patterns.
Let's say you do something like this:
void *pointers[10000];
for(i = 0; i < 10000; i++)
pointers[i] = malloc(1024);
for(i = 0; i < 9999; i++)
free(pointers[i]);
The only part of the heap that can be safely returned to the system is the "wilderness chunk", which is at the end of the heap. This can be returned to the system using another sbrk system call, and the glibc memory allocator will do that when the size of this last chunk exceeds some threshold.
The above program would make 10000 small allocations, but only free the first 9999 of them. The last one should (assuming nothing else has called malloc, which is unlikely) be sitting right at the end of the heap. This would prevent the allocator from returning any memory to the system at all.
If you were to free the remaining allocation, glibc's malloc implementation should be able to return most of the pages allocated back to the system.
If you're allocating and freeing small chunks of memory, a few of which are long-lived, you could end up in a situation where you have a large chunk of memory allocated from the system, but you're only using a tiny fraction of it.
Here are some "advantages" to never releasing memory back to the system:
Having already used a lot of memory makes it very likely you will do so again, and
when you release memory the OS has to do quite a bit of paperwork
when you need it again, your memory allocator has to re-initialise all its data structures in the region it just received
Freed memory that isn't needed gets paged out to disk where it doesn't actually make that much difference
Often, even if you free 90% of your memory, fragmentation means that very few pages can actually be released, so the effort required to look for empty pages isn't terribly well spent
Many memory managers can perform TRIM operations where they return entirely unused blocks of memory to the OS. However, as several posts here have mentioned, it's entirely implementation dependent.
But lets say I never ask for allocation of size greater than say 50 bytes and I ask a lot of such 50 byte objects on peak load on the system. Then what ?
This depends on your allocation pattern. Do you free ALL of the small allocations? If so and if the memory manager has handling for a small block allocations, then this may be possible. However, if you allocate many small items and then only free all but a few scattered items, you may fragment memory and make it impossible to TRIM blocks since each block will have only a few straggling allocations. In this case, you may want to use a different allocation scheme for the temporary allocations and the persistant ones so you can return the temporary allocations back to the OS.
How does actually malloc get the present free memory space available in the microcontroller.
Does it keep a list of areas unallocated continously in the runtime?
How does it get information of a previous malloc assignment memory allocation if there are two malloc statements in the code
How can one know which memory is free and which one is not at runtime. At compilation time we can know which all locations in RAM is assigned by the compiler for the variable. Does malloc uses this information to do this.
As the commentators said above, there are multiple implementations of malloc and the algorithm may vastly vary for each of these implementations. This is a vast and complicated area and you should read up on the memory management to get a complete idea on the topic.
In simple words, all the malloc implementations are backed up by the kernel's memory management schemes. The kernel see the whole system memory as pages for fixed size (4k, 8k etc) and all the allocations and frees are done on the pages. There will be a memory management subsystem exists for all the kernel implementations and which does the accounting of whole memory allocations and frees happening on the system. When you call a malloc, it will eventually reaches this memory management subsystem, and looks for the next available free page from the pool and allocates for the requesting process. Before giving the page to the requester, he will make sure to mark it as used and same way when you free up the memory it will add it back to the free pool and unmark used. There exists so many implementations on how the kernel does all these effectively (read up on memory manager implementations in linux)
In common implementations, there exists a minimal memory manager functionality in the userspace itself. The user space process itself maintains a free pool and when a malloc requests memory, before breaking in to kernel, it will look in its own free pool if memory is available. If available it will mark it up and satisfies the request without the help of kernel. Similarly, when you free up the memory, the freed up chunk of memory will not immediately go back to kernel's free pool instead it will stay with the process's free pool so that next malloc can use this.
As I said in the beginning, this is a huge and complicated topic and you can find a lot of documentations available in the internet about this.
I created a linked list with 1,000,000 items which took 16M memory. Then I removed and freed half of them. I thought that the memory usage would decrease, but it didn't.
Why was that?
I'm checking the Memory usage via Activity Monitor On Mac OS X 10.8.2.
In case you want to check my code, here it is.
Generally speaking, free doesn't release memory back to the OS. It's still allocated to the process, so the OS reports it as allocated. From the POV of your program, it's available to satisfy an new allocations you make.
Be aware that since you freed every other node, your memory is almost certainly now very fragmented. This free memory is in small chunks with allocated memory in between them, so can only be used to satisfy small allocations. If you make a larger allocation, the process will go to the OS for more memory.
Since the process gets memory from the OS one page at a time, even if it wanted to it can't release such fragmented memory back to the OS. You're using part of every page.
i need to have reliable measurement of allocated memory in a linux process. I've been looking into mallinfo but i've read that it is deprecated. What is the state of the art alternative for this sort of statistics?
basically i'm interested in at least two numbers:
number (and size) of allocated memory blocks/pages from the kernel by any malloc or whatever implementation uses the C library of choice
(optional but still important) number of allocated memory by userspace code (via malloc, new, etc.) minus the deallocated memory by it (via free, delete, etc.)
one possibility i have is to override malloc calls with LD_PRELOAD, but it might introduce an unwanted overhead at runtime, also it might not interact properly with other libraries i'm using that also rely on LD_PRELOAD aop-ness.
another possibility i've read is with rusage.
To be clear, this is NOT for debugging purposes, the memory usage is intrinsic feature of the application (similar to Mathematica or Matlab that display the amount of memory used, only that more precise at the block-level)
For this purpose - a "memory usage" introspection feature within an application - the most appropriate interface is malloc_hook(3). These are GNU extensions that allow you to hook every malloc(), realloc() and free() call, maintaining your statistics.
To see how much memory is mapped by your application from the kernel's point of view, you can read and collate the information in the /proc/self/smaps pseudofile. This also lets you see how much of each allocation is resident, swapped, shared/private, clean/dirty etc.
/proc/PID/status contains a few useful pieces of information (try running cat /proc/$$/status for example).
VmPeak is the largest your process's virtual memory space ever became during its execution. This includes all pages mapped into your process, including executable pages, mmap'ed files, stack, and heap.
VmSize is the current size of your process's virtual memory space.
VmRSS is the Resident Set Size of your process; i.e., how much of it is taking up physical RAM right now. (A typical process will have lots of stuff mapped that it never uses, like most of the C library. If no processes need a page, eventually it will be evicted and become non-resident. RSS measures the pages that remain resident and are mapped into your process.)
VmHWM is the High Water Mark of VmRSS; i.e. the highest that number has been during the lifetime of the process.
VmData is the size of your process's "data" segment; i.e., roughly its heap usage. Note that small blocks on which you have done malloc and then free will still be in use from the kernel's point of view; large blocks will actually be returned to the kernel when freed. (If memory serves, "large" means greater than 128k for current glibc.) This is probably the closest to what you are looking for.
These measurements are probably better than trying to track malloc and free, since they indicate what is "really going on" from a system-wide point of view. Just because you have called free() on some memory, that does not mean it has been returned to the system for other processes to use.
I have made a program in c and wanted to see, how much memory it uses and noticed, that the memory usage grows while normally using it (at launch time it uses about 250k and now it's at 1.5mb). afaik, I freed all the unused memory and after some time hours, the app uses less memory. Could it be possible, that the freed memory just goes from the 'active' memory to the 'wired' or something, so it's released when free space is needed?
btw. my machine runs on mac os x, if this is important.
How do you determine the memory usage? Have you tried using valgrind to locate potential memory leaks? It's really easy. Just start your application with valgrind, run it, and look at the well-structured output.
If you're looking at the memory usage from the OS, you are likely to see this behavior. Freed memory is not automatically returned to the OS, but normally stays with the process, and can be malloced later. What you see is usually the high-water mark of memory use.
As Konrad Rudolph suggested, use something that examines the memory from inside the process to look for memory links.
The C library does not usually return "small" allocations to the OS. Instead it keeps the memory around for the next time you use malloc.
However, many C libraries will release large blocks, so you could try doing a malloc of several megabytes and then freeing it.
On OSX you should be able to use MallocDebug.app if you have installed the Developer Tools from OSX (as you might have trouble finding a port of valgrind for OSX).
/Developer/Applications/PerformanceTools/MallocDebug.app
I agree with what everyone has already said, but I do want to add just a few clarifying remarks specific to os x:
First, the operating system actually allocates memory using vm_allocate which allocates entire pages at a time. Because there is a cost associated with this, like others have stated, the C library does not just deallocate the page when you return memory via free(3). Specifically, if there are other allocations within the memory page, it will not be released. Currently memory pages are 4096 bytes in mac os x. The number of bytes in a page can be determined programatically with sysctl(2) or, more easily, with getpagesize(2). You can use this information to optimize your memory usage.
Secondly, user-space applications do not wire memory. Generally the kernel wires memory for critical data structures. Wired memory is basically memory that can never be swapped out and will never generate a page fault. If, for some reason, a page fault is generated in a wired memory page, the kernel will panic and your computer will crash. If your application is increasing your computer's wired memory by a noticeable amount, it is a very bad sign. It generally means that your application is doing something that significantly grows kernel data structures, like allocating and not reaping hundreds of threads of child processes. (of course, this is a general statement... in some cases, this growth is expected, like when developing a virtual host or something like that).
In addition to what the others have already written:
malloc() allocates bigger chunks from the OS and spits it out in smaller pieces as you malloc() it. When free()ing, the piece first goes into a free-list, for quick reuse by another malloc if the size fits. It may at this time be merged with another free item, to form bigger free blocks, to avoid fragmentation (a whole bunch of different algorithms exist there, from freeLists to binary-sized-fragments to hashing and what not else).
When freed pieces arrive so that multiple fragments can be joined, free() usually does this, but sometimes, fragments remain, depending on size and orderof malloc() and free(). Also, only when a big such free block has been created will it be (sometimes) returned to the OS as a block. But usually, malloc() keeps things in its pocket, dependig on the allocated/free ratio (many heuristics and sometimes compile or flag options are often available).
Notice, that there is not ONE malloc/free algotrithm. There is a whole bunch of different implementations (and literature). Highly system, OS and library dependent.