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).
Related
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++ process running in Solaris which creates 3 threads to do some tasks.
These threads execute in loops and it runs as long as the process is running.
But, I see that the memory usage of the process grows continuously and the process core dumps once the memory usage exceeds 4GB.
Can someone give me some pointers on what could be the issue behind memory usage growth?
What can I do to prevent process from core dumping because of memory exhaustion?
Will thread restart help?
Any pointers welcome.
No, restarting a thread would not help.
It seems like you have a memory leak in your application.
In my experience there are two types of memory leaks:
real memory leaks that you can see when the application exits
'false' memory leaks, like a big list that increases during the lifetime of your application but which is correctly cleaned up at the end
For the first type, there are tools which can report the memory that has not been freed by your application when it exits. I don't know about Solaris but there are numerous tools under Windows which can do that. For Unix, I think that Valgrind does this.
For the second type, there are also tools under Windows that can take snapshots of the memory of your application. Simply take two snapshots with an interval of a few minutes or hours (depending on your application) and let them compare by the tool. There are probably simlar tools like this on Solaris.
Using these tools will probably require your application to take much more memory, since the tool needs to store the call stack of every memory allocation. Because of this it will also run much slower. However, you will only see this effect when you are actively using this tool, so there is no effect in real-life production code.
So, just look for this kind of tools under Solaris. I quickly Googled for it and found this link: http://prefetch.net/blog/index.php/2006/02/19/finding-memory-leaks-on-solaris-systems/. This could be a starting point.
EDIT: Some additional information: are you looking at the right kind of memory? Even if you only allocated 3GB in total, the total virtual address space may still reach 4GB because of memory fragmentation. Unfortunately, there is nothing you can do about this (except using another memory allocation strategy).
It seems that memory leak occurs in my code, so I try to locate the place in my code which causes the memory leak.
In the post
Can't obtain accurate information of available memory in the heap
I was told that OS may allocate large memory when a small memory is request to reduce the system call.
Is it correct in Windows?
What's relevant here, after seeing your other question, is not what happens when you allocate memory. What matters is what happens when you release it. In particular a 1 KB allocation will never be released back to the OS, it is too small. It gets added to a list of free blocks, ready to be used by the next allocation of (about) the same size.
You cannot reliably detect memory leaks with VirtualQuery().
If you use Visual Studio then use its built-in leak detection feature. There are plenty of other tools.
On most systems (including most recent compilers on Windows), the heap manager will allocate relatively large "chunks" of memory from the OS, then divide that up into pieces for use by the program. That allocation from the OS will typically be at least tens of kilobytes.
Those large chunks of memory will be returned to the OS when the program ends execution. It can happen sooner than that, but end of execution is the most common.
Each of those large chunks will be tracked by the OS as a single allocation (even though the heap manager will then break it up into smaller pieces for use by your code). Any that have been released back to the OS will show up as free memory blocks.
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
An application I am working with is exhibiting the following behaviour:
During a particular high-memory operation, the memory usage of the process under Task Manager (Mem Usage stat) reaches a peak of approximately 2.5GB (Note: A registry key has been set to allow this, as usually there is a maximum of 2GB for a process under 32-bit Windows)
After the operation is complete, the process size slowly starts decreasing at a rate of 1MB per second.
I am trying to figure out the easiest way to quickly determine who is freeing this memory, and where it is being free'd.
I am having trouble attaching a memory profiler to my code, and I don't particularly want to override the new/delete operators to track the allocations/deallocations (IOW, I want to do this without re-compiling my code).
Can anyone offer any useful suggestions of how I could do this via the Visual Studio debugger?
Update
I should also mention that it's a multi-threaded application, so pausing the application and analysing the call stack through the debugger is not the most desirable option. I considered freezing different threads one at a time to see if the memory stops reducing, but I'm fairly certain this will cause the application to crash.
Ahh! You're looking at the wrong counter!
Mem Usage doesn't tell you that memory is being freed. Only that the working set is being purged! This could mean some other application needs memory, or the VMM decided to mark some of your process's pages as Stand By for some other process to quickly use. It does not mean that VirtualFree, HeapFree or any other free function is being called.
Look at the commit size (VM Size, Private Bytes, etc).
But if you still want to know when memory is being decommitted or freed or what-have-you, then break on some free calls. E.g. (for Visual C++)
{,,kernel32.dll}HeapFree
or
{,,msvcr80.dll}free
etc.
Or just a regular function breakpoint on the above. Just make sure it resolves the address.
cdb/WinDbg let you do it via
bp kernel32!HeapFree
bp msvcrt!free
etc.
Names may vary depending on which CRT version you use and how you link against it (via /MT or /MD and its variants)
You might find this article useful:
http://www.gamasutra.com/view/feature/1430/monitoring_your_pcs_memory_usage_.php?print=1
basically what I had in mind was hooking the low level allocation functions.
A couple different ideas:
The C runtime has a set of memory debugging functions; you'd need to recompile though. You could get a snapshot at computation completion and later, and use _CrtMemDifference to see what changed.
Or, you can attach to the process in your debugger, and cause it to dump a core before and after the memory freeing. Using NTSD, you can see what heaps are around, and the sizes of things. (You'll need a lot of disk space, and a fair amount of patience.) There's a setting (I think you get it through gflags, but I don't remember) that causes it to save a piece of the call stack as part of the dump; using that you can figure out what kind of object is being deallocated. Unfortunately, it only stores 4 or 5 stack frames, so you'll likely have to do something more clever as the next step to figure out where it's being freed. Either look at the code ("oh yeah, there's only one place where that can happen") or put in breakpoints on those destructors, or add tracing to the allocations and deallocations.
If your memory manager wipes free'd data to a known value (usually something like 0xfeeefeee), you can set a data breakpoint on a particular instance of something you're interested in. When it gets free'd, the breakpoint will trigger when the memory gets wiped.
I recommend you to check UMDH tool that comes with Debugging Tools For Windows (You can find usage and samples in the debugging tools help). You can snap shot running process's heap allocations with stack trace and compare them.
You could try Memory Validator to monitor the allocations and deallocations. Memory Validator has a couple of features that will help you identify where data is being deallocated:
Hotspots view. This can show you a tree of all allocations and deallocations or just all allocations or just all deallocations. It presents the data as a percentage of memory activity (based on amount of memory (de)allocated at a given location).
Analysis view. You can perform queries asking for data in a given address range. You can restrict these queries to any of alloc, realloc, dealloc behaviours.
Objects view. You can view allocations by type and see the maximum number of objects of each type (plus lots of other stats). Right click on a type to get a context menu, choose show all deallocations - will show deallocation locations for that type on Analysis tab.
I think the Hotspots view may give you the insight you need.