I am currently developing application for STM32F407 using STM32CubeMx and Keil uVision. I know that dynamic memory allocation in embedded systems is mostly discouraged, but from spot to spot on internet I can find some arguments in favor of it.
Due to my inventors soul I wanted to try to do it, but do it safely. Let's assume I'm creating a dynamically allocated fifo for incoming UART messages, holding structs composed of the msg itself and its' length. However I wouldn't like to consume all the heap size doing so, therefore I want to check how much of it I have left: Me new (?) idea is to try temporarily allocating some big chunk of memory (say 100 char) - if it's successful, I accept the incoming msg, if not - it means that I'm running out of heap and ignore the msg (or accept it and dequeue the oldest). After checking I of course free the temp memory.
A few questions arise in my mind:
First of all, does it make sens at all? Do you think, basic on your experience, that it could be usefull and safe?
I couldn't find precise info about what exactly shares RAM in ES (I know about heap, stack and volatile vars) so my question is: providing that answer to 1. isn't "hell no go home", what size of the temp memory checker would you pick for the mentioned controller?
About the micro itself - it has 192kB RAM, however in the Drivers\CMSIS\Device\ST\STM32F4xx\Source\Templates\arm\startup_stm32f407xx.s file only 512B+1024B are allocated for heap and stack - isn't that very little, leaving the whooping, remaining 190kB for volatile vars? Would augmenting the heap size to, say 50kB be sensible? If yes, do I do it directly in this file or it's a better practice to do it somewhere else?
Probably for some of you "safe dynamic memory" and "embedded" in one post is both schocking and dazzling, but keep in mind that this is experimenting and exploring new horizons :) Thanks and greetings.
Keil uVision describes only the IDE. If you are using KEil MDK-ARM which implies ARM's RealView compiler then you can get accurate heap information using the __heapstats() function.
__heapstats() is a little strange in that rather than simply returning a value it outputs heap information to a formatted output stream facilitated by a function pointer and file descriptor passed to it. The output function must have an fprintf() like interface. You can use fprintf() of course, but that requires that you have correctly retargetted the stdio
For example the following:
typedef int (*__heapprt)(void *, char const *, ...);
__heapstats( (__heapprt)fprintf, stdout ) ;
outputs for example:
4180 bytes in 1 free blocks (avge size 4180)
1 blocks 2^11+1 to 2^12
Unfortunately that does not really achieve what you need since it outputs text. You could however implement your own function to capture the data in memory and parse the result. You may only need to capture the first decimal digit characters and discard anything else, except that the amount of free memory and the largest allocatable block are not necessarily the same thing of course. Fragmentation is indicated by the number or free blocks and their average size. You can perhaps guarantee to be able to allocate at least an average sized block.
The issue with dynamic allocation in embedded systems are to do with handling memory exhaustion and, in real-time systems, the non-deterministic timing of both allocation and deallocation using the default malloc/free implementations. In your case you might be better off using a fixed-block allocator. You can implement such an allocator by creating a static array of memory blocks (or by dynamically allocating them from the heap at start-up), and placing a pointer to each block on a queue or linked list or stack structure. To allocate you simply remove a pointer from the queue/list/stack, and to free you place a pointer back. When the available blocks structure is empty, memory is exhausted. It is entirely deterministic, and because it is your implementation can be easily monitored for performance and capacity.
With respect to question 3. You are expected to adjust the heap and system stack size to suit your application. Most tools I have used have a linker script that automatically allocates all available memory not statically allocated, allocated to a stack or reserved for other purposes to the heap. However MDK-ARM does not do that in the default linker scripts but rather allocates a fixed size heap.
You can use the linker map file summary to determine how much space is unused and manually expand the heap. I usually do that leaving a small amount of unused space to account for maintenance when the amount of statically allocated data may increase. At some point however; you end up running out of memory, and the arcane error messages from the linker may not make it obvious that your heap is just too big. It is possible to override the default linker script and provide your own, and no doubt possible then to automatically size the heap - though I have never taken the trouble to try it.
Okay I have tested my idea with dynamic heap free space checking and it worked well (although I didn't perform long-run tests), however #Clifford answer and this article convinced me to abandon the idea of dynamic allocation. Eventually I implemented my own, static heap with pages (2d array), occupied pages indicator (0-1 array of size of number of pages) and fifo of structs consisting of pointer to the msg on my static heap (actually just the index of the array) and length of message (to determine how many contiguous pages it occupies). 95% of msg I receive should take up only one page, 5% - 2 or 3 pages, so fragmentation is still possible, but at least I keep a tight rein on it and it affects only the part of memory assigned to this module of the code (in other words: the fragmentation doesn't leak to other parts of the code). So far it has worked without any problems and for sure is faster because the lookup time is O(n*m), n - number of pages, m - the longest page possible, but taking into consideration the laws of probability it goes down to O(n). Moreover n is always a lot smaller the number of all allocation units in memory, so way less to look for.
Related
I am currently working on an embedded firmware development which uses FreeRTOS running on an STM32F777II microcontroller. Resource wise, I have around 10 tasks (total sum of stack size will be under 40 KByte) at the same priority, around 4 queues of 1KByte each, 4 binary semaphores. I know this would be an incomplete question without posting the actual code, but I really do not have any specific portion in my firmware that I think will be worth sharing related to my issue. I have a ton of business logic in my code which I cannot fully share as well.
I have a struct which consists of multiple char and int arrays of a specific length. 4 of the tasks uses these structures each. Each structure consumes around 15KByte of space and is defined in the global space of the FreeRTOS environment, not local to a task. The structs are allocated statically only and not dynamically on runtime. And since I initialize few members of the struct when declaring, so they go to the .data section only if I am not mistaken. Until now, there had been absolutely no problem whatsoever in my code and it worked 100% without any issue at all. Now I recently had a requirement where I had to add the same stuct to 2 more tasks. So, I added this 15KByte stuct to one of my tasks, basically just allocated and initialized and did not do any processing in any of the tasks. Observed no problems, nothing, no data corruption, nothing. Now when I allocated one more struct variable of the same type only, what I observe is data corruption in a lot of other places in my project. Some of the queues stopped working correctly and showed garbage data when read. Some of the other buffers also showed data corruption. I am really not sure why just one more variable allocation of this struct is triggering a lot of data corruption at other places in my project. If I remove this one allocation, everything goes back to normal. My MCU has 512KB of RAM and as per the IDE's build analyzer feature, it showed below 40% RAM usage, so what is triggering this issue, any suggestions to try? Could be because of some overlapping of .data or .bss sections or something? I did not observe any stack overflows or hard faults in the system during this.
For a quick resolution,
I randomly just disabled the D-cache by commenting out the function:
SCB_EnableDCache();
and voila, everything started to function correctly as it should without any instances of data corruption.
For long run and correct resolution:
Looks like there are some latent issues with my coding. I need to review the memory use, and regions of memory with different properties. Look at the buses, review any DMA usage, and MPU memory settings. Also, review the correct usage of volatile memory directives, thread-safe operation, and cache-coherency issues. Also, Use memory fencing and cache flushing as appropriate.
More details: Level 1 cache on STM32F7 Series and STM32H7 Series
I'm creating a list of elements inside a task in the following way:
l = (dllist*)pvPortMalloc(sizeof(dllist));
dllist is 32 byte big.
My embedded system has 60kB SRAM so I expected my 200 element list can be handled easily by the system. I found out that after allocating space for 8 elements the system is crashing on the 9th malloc function call (256byte+).
If possible, where can I change the heap size inside freeRTOS?
Can I somehow request the current status of heap size?
I couldn't find this information in the documentation so I hope somebody can provide some insight in this matter.
Thanks in advance!
(Yes - FreeRTOS pvPortMalloc() returns void*.)
If you have 60K of SRAM, and configTOTAL_HEAP_SIZE is large, then it is unlikely you are going to run out of heap after allocating 256 bytes unless you had hardly any heap remaining before hand. Many FreeRTOS demos will just keep creating objects until all the heap is used, so if your application is based on one of those, then you would be low on heap before your code executed. You may have also done something like use up loads of heap space by creating tasks with huge stacks.
heap_4 and heap_5 will combine adjacent blocks, which will minimise fragmentation as far as practical, but I don't think that will be your problem - especially as you don't mention freeing anything anywhere.
Unless you are using heap_3.c (which just makes the standard C library malloc and free thread safe) you can call xPortGetFreeHeapSize() to see how much free heap you have. You may also have xPortGetMinimumEverFreeHeapSize() available to query how close you have ever come to running out of heap. More information: http://www.freertos.org/a00111.html
You could also define a malloc() failed hook (http://www.freertos.org/a00016.html) to get instant notification of pvPortMalloc() returning NULL.
For the standard allocators you will find a config option in FreeRTOSConfig.h .
However:
It is very well possible you run out of memory already, depending on the allocator used. IIRC there is one that does not free() any blocks (free() is just a dummy). So any block returned will be lost. This is still useful if you only allocate memory e.g. at startup, but then work with what you've got.
Other allocators might just not merge adjacent blocks once returned, increasing fragmentation much faster than a full-grown allocator.
Also, you might loose memory to fragmentation. Depending on your alloc/free pattern, you quickly might end up with a heap looking like swiss cheese: Many holes between allocated blocks. So while there is still enough free memory, no single block is big enough for the size required.
If you only allocate blocks that size there, you might be better of using your own allocator or a pool (blocks of fixed size). Thaqt would be statically allocated (e.g. array) and chained as a linked list during startup. Alloc/free would then just be push/pop on a stack (or put/get on a queue). That would also be very fast and have complexity O(1) (interrupt-safe if properly written).
Note that normal malloc()/free() are not interrupt-safe.
Finally: Do not cast void *. (Well, that's actually what standard malloc() returns and I expect that FreeRTOS-variant does the same).
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Possible Duplicate:
How do malloc() and free() work?
I have encountered a weird problem and I'm really not sure why it doesn't work.
I have the following code in Xcode:
void *ptr = malloc(1024 * 1024 * 100);
memset(ptr, 0, 1024 * 1024 * 100);
free (ptr); //trace this line
ptr = malloc (1024 * 1024 * 100);
memset(ptr, 0, 1024 * 1024 * 100);
free (ptr); //trace this line
I put a breakpoint on each of the free() line, and when I traced the program, free didn't really free up the 100mb. However, if I change the number from 100 to 500 (allocate 500mb twice), memset 500mb, free() works fine. Why?
free can never fail(it does not have a return value) unless you call it with a improper address, which gives you undefined behavior.
You do not have to bother whether free actually frees memory or not you just have to ensure that you call free on the correct address after you are done with dynamic memory usage, rest the compiler should take care for you.
This is one of those things that you should just believe on your compiler to handle correctly.
Also, free just marks the memory being deallocated free(as name says) for reuse. It does not zero out or initialize the memory being deallocated.
When you pass a block of memory to free, that memory does not necessarily get returned to the operating system right away. In fact, based on the wording in the C standard, some argue that the memory can't be returned to the OS until the program exits.
The wording in question is (C99, ยง7.20.3.2/2): "The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation." Their argument is that when/if a block of memory is allocated and then freed, it should be available for allocation again -- but if it's returned to the OS, some other process might take it, so it's no longer available for further allocation, as the standard requires. Personally, I don't find that argument completely convincing (I think "allocated by another process" is still allocation), but such is life.
Most libraries allocate large chunks of memory from the OS, and then sub-allocate pieces of those large chunks to the program. When memory is freed by the program, the put that block of memory on an "available" list for further allocation. Most also (at least at times) walk through the list of free blocks, merging free blocks that are adjacent addresses.
Many also follow some heuristics about what memory to keep after it's been freed. First, the keep an entire block as long as any of the memory in that block remains in use. If, however, all the memory in a block has been freed, they look at its size, and (often) at how much free memory they have available. If the amount available and/or size of the free block exceeds some threshold, they'll usually release it back to the OS.
Rather than having fixed thresholds, some try to tailor their behavior to the environment by (for example) basing their thresholds on percentages of available memory instead of fixed sizes. Without that, programs written (say) ten years ago when available memory was typically a lot smaller would often do quite a bit of "thrashing" -- repeatedly allocating and releasing the same (or similar) size blocks to/from the OS.
free() does not have to immediately unmap and return to the OS the pages backing up previously but no longer allocated buffers. It may keep them around so you can allocate memory quickly again. When the program finishes, the pages will be unmapped and returned to the OS.
As others already said, free() doesn't have to return memory to the OS. But I reject an idea that you should never care whether the memory is returned. There should be a good reason to care, but there are valid reasons.
If you do want to return memory to OS, use a platform-specific way which provides this guarantee:
mmap with MAP_ANONYMOUS on systems supporting it (there are many, but MAP_ANONYMOUS is not POSIX): mmap instead of malloc, munmap instead of free.
VirtualAlloc and VirtualFree on Windows.
[Shoul I add something here for other systems? Feel free to suggest.]
These ways of allocating memory work with big memory units (system page size or more).
I am developing a GUI-heavy C++ application on a Freescale MX51-based board Linux 2.6.35. I would like to perform heap profiling.
Unfortunately, all heap profiling tools I have found have either been too intrusive or ostensibly non-working on ARM. Specific tools I've tried:
Valgrind Massif: unworkable on my platform due to the platform's feeble CPU. The 80% CPU time overhead introduced by Massif causes a range of problems in my application that cannot be compensated for.
gperftools (formerly Google Performance Tools) tcmalloc: All features of this rather un-intrusive, library-based libc malloc() replacement work on my target except for the heap profiler. To rephrase, the thread caching allocator works but the profiler does not. I'll explain the failure mode of the profiler below for anyone curious.
Can anyone suggest a set of replacement tools for performing C++ heap profiling on ARM platforms? Ideal output would ultimately be a directed allocation graph, similar to what gperftools' tcmalloc outputs. Low resource utilization is a must- my platform is highly resource constrained.
Failure mode of gperftools' tcmalloc explained:
I'm providing this information only for those that are curious; I do not expect a response. I'm seeing something similar to gperftools' issue #407 below, except on ARM rather than x86.
Specifically, I always get the message "Hooked allocator frame not found, returning empty trace." I spent some time debugging the issue and it appears that, when dynamically linking the tcmalloc library, frame pointers at the boundary between my application and the dynamic library are null- the stack cannot be walked "above" the call into the dynamic library.
gperftools issue #407: https://github.com/gperftools/gperftools/issues/410
stackoverflow user seeing similar problems on ARM: Missing frames on shared libraries on ARM
Heaps. Many ways to do them, but I've only run across 3 main types that matter in embedded land:
Linked list heaps. Each alloc is tracked in a "used" list. Once freed, they are dropped into a "free" list. On freeing, adjacent blocks of free memory are "joined" into larger pieces. Allocs can be any size. Each alloc and free is a O(N) op as it has to traverse the free list to give you a piece of memory plus break the free block into a size close to what you asked for while leaving the remaining block in the free list. Because of the increasing overhead per alloc, this system cannot be used by itself on smaller systems. This also tends to cause memory fragmentation over time if steps aren't taken to minimize it.
Fixed size (unit) heaps. You break your heap into equal size (smaller) parts. This wastes memory a bit, depending on how big the chunks are (and how many different sized, fixed allocator heaps you create), but alloc and free are both O(1) time operations. No searching, no joining. This style is often combined with the first one for "small object allocations" as the engines I've worked with have 95% of their allocations below a set size (say 256 bytes). This way, you use the unit heap for small allocs for huge speed and only minimal memory loss, while using the list heap for larger allocs. No external fragmentation of memory either.
Relocatable memory heaps. You don't give out pointers to memory, but handles. That way, behind the scenes, you can change memory pointers when needed to remove fragmentation or whatever. High overhead. High pain the the #$$ quotient as it's easy to abuse and get dangling pointer all over. Also added overhead for each memory dereference. But wanted to mention it.
There's some basic patterns. You can find all sorts of libs out in the wild that use them and also have built in statistics for number of allocs, fragmentation, and other useful stats. It's also not the hard to roll your own really, though I'd not recommend it for anything outside of satisfying curiosity as debugging without a working malloc is painful indeed. Adding thread support is pretty straightforward as well, but again, downloading a ready made solution is the better choice.
The above info applies to all platforms, ARM or otherwise, though most of my experience has been on low level ARM stuff so the above info is battle tested for your platform. Hope this helps!
Im interested in understanding how a computer allocates variables for physical memory vs files in virtual memory ( such as on a hard drive ), in terms of how does the computer determine know where to put data. It almost seems random in both memory storage types, but its not because it simply can't put data at a memory address or sector (any location) of a hard drive that's occupied or allocated for another process already. When I was studying how Norton's speed disk ( a program that de-fragments files on hard drives ) on my old W95 system, I noticed from the program's representation of hard drive's data ( a color coded visual map of different data types, e.g. swap files were always first at the top.), consisting of many files spread out all over the hard drive with empty unused areas. In addition some of these areas, I saw what looked like a mix of data and empty space showed a spotty pattern. I want to think its random for that to happen. Like wise, when I was studying the memory addresses of a simple program I wrote in C, I noticed that each version of my program after recompiling it after changes - showed different addresses for segments and offsets. I was expecting the computer to use the same address when I recompiled it. Sometimes the same address would be used, other times it was different. Again, I want to think its random also for memory locations to be chosen by programs. I thought that memory allocation or file writing was based on the first empty space available, written in a contiguous manner.
So my question is, I want to know how and what is it in the logic works of a common computer, that decides where it writes its data in such a arbitrary manner for either type of location (physical RAM or Dynamic )? What area of computer science (if not assembly language) would I need to study that would explains this, almost random behavior?
Thanks in Advance
Something broader and directly from computer science would be a linked list. http://en.wikipedia.org/wiki/Linked_list
Imagine if you had a linked list and simply added items to the end, these items might live linearly in memory or disk or whatever somewhere. But as you remove some items in the middle of the list by having say item number 7 point at item number 9 eliminating item number 8. As with memory allocation for allocs or virtual memory or hard drive sector allocation, etc how fast you fragment your storage has to do with the algorithm you use for allocating the next item.
file systems can/do use a link list type scheme to keep track of what sectors are tied to a single file. it is fast and easy to use the link list but deal with fragmentation. A much slower method would be to have no fragmentation but be constantly copying/moving files around to keep them on linear sectors.
malloc() allocation schemes and MMU allocation schemes also fall under this category. Basically any time you take something, slice it up into fractions and put a virtual interface in front of those fractions to give the appearance to the programmer/user that they are linear. Malloc() (not counting the virtual memory via the MMU) is the other way around allocating a number of linear chunks of those fractions to meed the alloc need, and having an alloc/free scheme that attempts to keep as many large chunks available, just in case, a bad malloc system is one where you have half of your memory free but the maximum malloc that works without an out of memory error is a malloc of a small fraction of that memory, say you have a gig free and can only allocate 4096 bytes.
You should look at virtual memory and TLB (translation lookaside buffer) or paging.
It is not trivial to implement virtual memory and paging. The performance of your whole system depends on it. If it's not done properly your system will thrash.
It is early morning here so Wikipedia will have to do for now: https://en.m.wikipedia.org/wiki/Translation_lookaside_buffer
EDIT:
Those coloured spots you saw in your defrag were chunks on your HDD. Each chunk is of some specified size. Depending on how fragmented your HDD is, you might have portions of your HDD that look like this:
*-*-***-***-*
where * means full, and - means empty
This (above) could be part of one application/file or multiple files; I will assume one file is split across those to simplify my example. At the end of each * there is a pointer to the next location where the next * chunk is (this is called a linked list). The more fragmented your HDD is (or memory) the more of these pointers to next chunk you will have. This in turn uses more space for next pointers instead of using space for data and the result is more overhead when reading that data. If this is a file on disk, you will have multiple seeks (which are bad because they're slow) if your data is not grouped together (locality principle). When you use defrag, it moves and groups all chunks together (as best as it can).
*-*-***-***-*
becomes
*********----
The OS decides paging and virtual memory addressing (and such). TLB is a hardware (a cache) that aids this process (it maps physical memory to virtual memory addresses for fast look up). The CPU communicates with the TLB via MMU
To answer your questions
You should study operating systems.
Yes the locations where to place your files on HDD are decided by the OS. If you deleted a file and download it again, there is no guarantee it will be placed in the same location-most likely not.
A nice summary of all these components and principles I mentioned here work: Click Here. It's a ppt with slides from a Real Time Operating Systems book (if I'm not mistaken the same exact one I used)