I am working on a Pattern searching program that continuously allocates and deallocates from the heap. The program is intended to be very conscious about memory usage so when I'm done with memory it needs to be freed to the OS. With that being said I have done extensive memory leak detection and found all of the leaks that appear on the CRT dump log, with some help from the Visual Leak detector from Visual Studio and WinDbg for stack tracing.
The major problem is that the more heap activity that occurs in the processing of a file, the more I am left with this chunk of memory that is not freed to the OS, but neither is it showing as a leak after program terminate. I grab the program's resource usage using these lines of code:
PROCESS_MEMORY_COUNTERS pmc;
GetProcessMemoryInfo(GetCurrentProcess(), &pmc, sizeof(pmc));
PListType physMemUsedByMe = pmc.WorkingSetSize;
return physMemUsedByMe/1000000.0f;
This returns the amount of memory in MB the program is using and I make this call right before the program exits. I can have zero memory leaks but up to 500 MB of this left-over data. Not sure if this stuff is somehow sitting on the stack or is this heap fragmentation? I do use STL maps and vectors but I clean them up properly if I use pointers.
I have been doing detective work for quite awhile and I am really not coming up with answers using forums like this so I wanted to ask my question specifically. Any help goes a long way. It doesn't matter if i'ts just a guess, I just need some brainstorming outside of my own head LOL. Thank you!
Just because you free memory does not mean that it is immediately returned to the operating system. Your standard library likely has a pile of memory that it has obtained from the OS but which is not current in use in your program. This is not leaked memory, and it is usually not a problem in practice.
Related
In C++ programs, I have sometimes had problems with "weak" memory leaks. By that, I mean that some objects accumulate resources, but, eventually, these objects are destroyed properly and their memory is released, so that these leaks do not show up using the traditional memory debugging tools like valgrind or address sanitizers.
A typical example would be a poorly-written cache that keeps all the cached results from the beginning of the program. It grows forever, but its memory is reclaimed at the end of the program, when the cache is destroyed.
How can one debug this ? Are there tools available to see where are the largest objects allocated by the program ? To dump the current state of allocated memory (including call stack) ? To see which objects are growing ? I'm using Linux, but I am interested in other platforms as well.
If other platforms are an option, I would recommend Visual Studio on Windows.
It has powerful profiling options, including one for memory usage.
https://learn.microsoft.com/en-us/visualstudio/profiling/memory-usage
While debugging you can take a snapshot to see where memory is being used.
You can also take memory usage snapshots at different times and compare them.
You can use a profiler like e.g. Intel VTune (this is available for Linux as well) to trace memory consumption of your application. In VTune, you can see the memory consumption over time, and select a time window to see where memory was allocated during that window.
It still will be difficult to detect such problems if your application allocates and deallocates a lot of memory correctly, and only a small fraction is deallocated too late. In that case you need to check a lot of allocations/deallocations before you can find the bad one(s).
I want to verify the memory stability of a C++ application I wrote and compiled for Linux.
It is a network application that responds to remote clients connectings in a rate of 10-20 connections per second.
On long run, memory was rising to 50MB, eventhough the app was making calls to delete...
Investigation shows that Linux does not immediately free memory. So here are my questions :
How can force Linux to free memory I actually freed? At least I want to do this once to verify memory stability.
Otherwise, is there any reliable memory indicator that can report memory my app is actually holding?
What you are seeing is most likely not a memory leak at all. Operating systems and malloc/new heaps both do very complex accounting of memory these days. This is, in general, a very good thing. Chances are any attempt on your part to force the OS to free the memory will only hurt both your application performance and overall system performance.
To illustrate:
The Heap reserves several areas of virtual memory for use. None of it is actually committed (backed by physical memory) until malloc'd.
You allocate memory. The Heap grows accordingly. You see this in task manager.
You allocate more memory on the Heap. It grows more.
You free memory allocated in Step 2. The Heap cannot shrink, however, because the memory in #3 is still allocated, and Heaps are unable to compact memory (it would invalidate your pointers).
You malloc/new more stuff. This may get tacked on after memory allocated in step #3, because it cannot fit in the area left open by free'ing #2, or because it would be inefficient for the Heap manager to scour the heap for the block left open by #2. (depends on the Heap implementation and the chunk size of memory being allocated/free'd)
So is that memory at step #2 now dead to the world? Not necessarily. For one thing, it will probably get reused eventually, once it becomes efficient to do so. In cases where it isn't reused, the Operating System itself may be able to use the CPU's Virtual Memory features (the TLB) to "remap" the unused memory right out from under your application, and assign it to another application -- on the fly. The Heap is aware of this and usually manages things in a way to help improve the OS's ability to remap pages.
These are valuable memory management techniques that have the unmitigated side effect of rendering fine-grained memory-leak detection via Process Explorer mostly useless. If you want to detect small memory leaks in the heap, then you'll need to use runtime heap leak-detection tools. Since you mentioned that you're able to build on Windows as well, I will note that Microsoft's CRT has adequate leak-checking tools built-in. Instructions for use found here:
http://msdn.microsoft.com/en-us/library/974tc9t1(v=vs.100).aspx
There are also open-source replacements for malloc available for use with GCC/Clang toolchains, though I have no direct experience with them. I think on Linux Valgrind is the preferred and more reliable method for leak-detection anyway. (and in my experience easier to use than MSVCRT Debug).
I would suggest using valgrind with memcheck tool or any other profiling tool for memory leaks
from Valgrind's page:
Memcheck
detects memory-management problems, and is aimed primarily at
C and C++ programs. When a program is run under Memcheck's
supervision, all reads and writes of memory are checked, and calls to
malloc/new/free/delete are intercepted. As a result, Memcheck can
detect if your program:
Accesses memory it shouldn't (areas not yet allocated, areas that have been freed, areas past the end of heap blocks, inaccessible areas
of the stack).
Uses uninitialised values in dangerous ways.
Leaks memory.
Does bad frees of heap blocks (double frees, mismatched frees).
Passes overlapping source and destination memory blocks to memcpy() and related functions.
Memcheck reports these errors as soon as they occur, giving the source
line number at which it occurred, and also a stack trace of the
functions called to reach that line. Memcheck tracks addressability at
the byte-level, and initialisation of values at the bit-level. As a
result, it can detect the use of single uninitialised bits, and does
not report spurious errors on bitfield operations. Memcheck runs
programs about 10--30x slower than normal. Cachegrind
Massif
Massif is a heap profiler. It performs detailed heap profiling by
taking regular snapshots of a program's heap. It produces a graph
showing heap usage over time, including information about which parts
of the program are responsible for the most memory allocations. The
graph is supplemented by a text or HTML file that includes more
information for determining where the most memory is being allocated.
Massif runs programs about 20x slower than normal.
Using valgrind is as simple as running application with desired switches and give it as an input of valgrind:
valgrind --tool=memcheck ./myapplication -f foo -b bar
I very much doubt that anything beyond wrapping malloc and free [or new and delete ] with another function can actually get you anything other than very rough estimates.
One of the problems is that the memory that is freed can only be released if there is a long contiguous chunk of memory. What typically happens is that there are "little bits" of memory that are used all over the heap, and you can't find a large chunk that can be freed.
It's highly unlikely that you will be able to fix this in any simple way.
And by the way, your application is probably going to need those 50MB later on when you have more load again, so it's just wasted effort to free it.
(If the memory that you are not using is needed for something else, it will get swapped out, and pages that aren't touched for a long time are prime candidates, so if the system runs low on memory for some other tasks, it will still reuse the RAM in your machine for that space, so it's not sitting there wasted - it's just you can't use 'ps' or some such to figure out how much ram your program uses!)
As suggested in a comment: You can also write your own memory allocator, using mmap() to create a "chunk" to dole out portions from. If you have a section of code that does a lot of memory allocations, and then ALL of those will definitely be freed later, to allocate all those from a separate lump of memory, and when it's all been freed, you can put the mmap'd region back into a "free mmap list", and when the list is sufficiently large, free up some of the mmap allocations [this is in an attempt to avoid calling mmap LOTS of times, and then munmap again a few millisconds later]. However, if you EVER let one of those memory allocations "escape" out of your fenced in area, your application will probably crash (or worse, not crash, but use memory belonging to some other part of the application, and you get a very strange result somewhere, such as one user gets to see the network content supposed to be for another user!)
Use valgrind to find memory leaks : valgrind ./your_application
It will list where you allocated memory and did not free it.
I don't think it's a linux problem, but in your application. If you monitor the memory usage with « top » you won't get very precise usages. Try using massif (a tool of valgrind) : valgrind --tool=massif ./your_application to know the real memory usage.
As a more general rule to avoid leaks in C++ : use smart pointers instead of normal pointers.
Also in many situations, you can use RAII (http://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initialization) instead of allocating memory with "new".
It is not typical for an OS to release memory when you call free or delete. This memory goes back to the heap manager in the runtime library.
If you want to actually release memory, you can use brk. But that opens up a very large can of memory-management worms. If you directly call brk, you had better not call malloc. For C++, you can override new to use brk directly.
Not an easy task.
The latest dlmalloc() has a concept called an mspace (others call it a region). You can call malloc() and free() against an mspace. Or you can delete the mspace to free all memory allocated from the mspace at once. Deleting an mspace will free memory from the process.
If you create an mspace with a connection, allocate all memory for the connection from that mspace, and delete the mspace when the connection closes, you would have no process growth.
If you have a pointer in one mspace pointing to memory in another mspace, and you delete the second mspace, then as the language lawyers say "the results are undefined".
I have a C++ application that has some minimal leaks, and I would like to fix them. I am using AppVerifier to dump the leaked objects, and I can get the addresses and first few bytes of the allocated memory.
Unfortunately, those first bytes and raw address is not enough to pinpoint the allocation stack trace, is there a method to get complete allocation data dump, and find the stack that's allocating the memory?
I could put _CrtSetBreakAlloc via the leak number, but unfortunatelly it's a threaded application and those numbers float up and down.
Does anyone have a suggestion what I could try?
With the gflags utility you can enable storing call stack information (gflags +ust). However, your applications will now run slower and take more memory.
Side-remark: To be honest, I never got all those Microsoft utilities (leak-tracing in the C-RunTime, Gflags, UMDH, AppVerifier, LeakDiag) to do exactly what I wanted. In the end, I simply wrote my own memory allocator in which I can add whatever tracing I want (call stack, red zone marking, delayed freeing, consistency checking, ...).
You could try using UMDH to track memory leaks. You first have to use GFlags to turn on storing call stack tracing whenever memory is allocated. The docs on UMDH state how to use it.
But recently I've finally tried out visual leak detector, and it works fabulous on my monstrous, big app.
http://vld.codeplex.com
_CrtDumpMemoryLeaks(); if you didn't know, is a function that dumps all the memory leaks in a program. Mine currently displays that I have a 3632062 byte memory leak (it's not being deallocated).
I was wondering:
Is there any way to cause Visual C++ Express to break when a certain amount of bytes has been allocated? That way I could break when 3632062 bytes have been allocated, then read the stack trace to see where I allocated it.
This is currently the only method I can think of for finding where the memory is being allocated so I can fix it. I've been searching my code a lot but I can't find anywhere where I would need to allocate 3632062 bytes (since the only file I load is 2767136 bytes..) although I am certain the leak is related to the file I'm operating on.
Any ideas for finding the source of the memory leak? I'm using Native C++, Visual C++ 2010
You could do this using _CrtSetAllocHook to track the total memory usage.
UMDH will give you a list of allocated blocks in all heaps. This might be what you want, since breaking on hitting a given total alloc threshold will not tell you where all of the blocks were allocated.
I have previously used this simple memory leak detector with good success for finding memory leaks.
I have a graphics program where I am creating and destroying the same objects over and over again. All in all, there are 140 objects. They get deleted and newed such that the number never increases 140. This is a requirement as it is a stress test, that is I cannot have a memory pool or dummy objects. Now I am fairly certain there aren't any memory leaks. I am also using a memory leak detector which is not reporting any leaks.
The problem is that the memory footprint of the program keeps increasing (albeit quite slowly, slower than the rate at which the objects are being destroyed/created). So my question then is whether an increasing memory footprint is a solid sign for memory leaks or can it sometimes be deceiving?
EDIT: I am using new/delete to create/destroy the objects
It does seem possible that this behavior could come from a situation in which there is no leak.
Is there any chance that your heap is getting fragmented?
Say you make lots of allocations of size n. You free them all, which makes your C library insert those buffers into a free list. Some other code path then makes allocations smaller than n, so those blocks in the free list get chunked up into smaller units. Then the next iteration of the loop does another batch of allocations of size n, and the free list no longer contains contiguous memory at that size, and malloc has to ask the kernel for more memory. Eventually those "smaller-than-n" allocations get freed as would your "n-sized" ones, but if you run enough iterations where the fragmentation exists, I could see the process gradually increasing its memory footprint.
One way to avoid this might be to allocate all your objects once, and not keep allocating/freeing them. Since you're using C++ this might necessitate placement new or something similar. Since you are using Windows, I might also mention that Win32 supports having multiple heaps in a process, so if your objects come from a different heap than other allocations you may avoid this.
It depends if you're under a CLR (or a virtual machine with garbage collector) or your still in the old mode (like C++, MFC ect...)
When you have a GC around - you can't really tell, only if you test it long enough. GC can decide not to clean your objects for now... (there is a way to force it)
In the native applications, yes, a footprint increase might mean a leak.
there are some tools (very good tools) for c++ that find these leaks (google devpartner or boundschecker)
I guess there are some tools for c# and Java as well.
If your application's process footprint increases beyond a reasonable limit, which depends on your application and what it does, and continues to increase until eventually you (will) run out of virtual memory, you definitely have a memory leak.
Try memory allocation tests included in CRT: http://msdn.microsoft.com/en-us/library/e5ewb1h3%28VS.80%29.aspx
They help A LOT.
But I've noticed that apps do tend to vary their memory consumption a little if you look at some factors. Windows 7 might also create extra padding in memory allocation to fix bugs: http://msdn.microsoft.com/en-us/library/dd744764%28VS.85%29.aspx
I strongly suggest to try Visual Studio 2015 (the Community Edition is free). It comes with Diagnostic Tools that helps you analyze Memory Usage; it allows you to take snapshots and view the heap