BTW, this modernness doesnt stop here, we do have other specialization also available for map like std::swap(C++17), std::erase_if(C++20) & bunch of comparison operators. My program creates these giant map that takes up about 1 GB of RAM and then does some processing and the map is destroyed when it goes out of scope. However, you can, you can sponsor my open-source work on GitHub, Parsing time stamps faster with SIMD instructions, Science and Technology links (June 25 2023), Citogenesis in science and the importance of real problems, Science and Technology links (June 11 2023), I wrote a blog post showing that each Integer in Java stored in an array uses 20 bytes, https://github.com/facebook/folly/blob/master/folly/docs/FBVector.md, http://daringfireball.net/projects/markdown/syntax.
How C++ and alike maps are actually stored in memory? What should be chosen as country of visit if I take travel insurance for Asian Countries. C++ Class inheriting from template class without knowing the type? If load_factor <= 1, it indicates that bucket_count() >= the items in the map, then bucket_count() is the size of memory usage. How can I force the STL memory cache to clear? If you do a lot of allocation/de-allocation of large numbers of small items, the cost of the heap can a problem. It must add an extra book keeping space( trading space with time) in addition to the user data to add the hash table fast access time, If possible, give me a fn of N, no of elements, say for, type myStruct struct {. Doesn't bucket_size() only return amount of elements in this bucket? Anyways, whether it is a Binary Search Tree or a Red Black Tree it is 2N more pointers (for left&right of the tree) + the pointer already in the struct = 3N pointers. WebSo for a rough memory usage could look like this: unsigned n = mymap.bucket_count(); float m = mymap.max_load_factor(); if (m > 1.0) { return n * m; } else { return n; } If Id like to respond to your criticism of academic work. I was asking for a specific problem where the original designer worked so hard to find & design & prove a certain tree strucutre is better to use than RBT or BST, then come those developers implementing the theoritical idea by adding a map to the original design tree; that's even worse than before we get back to all the problems that motivated the original design (of why not using BST or RBT and design this), + doubling the space of older methods the original design were trying to optimize. How do I do something straightforward like get the value associated with at looked-up key and assign it to a variable? If you just store integer/integers as key/values, then you have some big overhead (e.g. The downside of this approach is memory allocation & deallocation, which sounds bad in terms of performance. We can also insert data in std::map using operator [] i.e. WebIf you want to get the accurate memory usage, you may need to count all the buckets to see how many elements in it: size_t count = 0; for (unsigned i = 0; i <
Memory, memory, always memory If you used Solution 1 The unordered_map structure is designed to hold large numbers of objects in a way that makes adds, deletes, lookups, and orderless traverses efficient. ELEMENT_OVERHEAD Not suitable for value type that does not default constructible and assignable(in layman term, doesnt have default constructor & copy/move constructor). What syntax could be used to implement both an exponentiation operator and XOR? Yesterday i tried to use std::unordered_map and this code confused me how much memory it used. This can be raised through the IndexType class template parameter, check the API for details. It uses 4 bytes to store each 4 byte elements. Connect and share knowledge within a single location that is structured and easy to search. The overhead is usually fixed per allocation, usually at least one (32-bit or 64-bit) word. Namely asking for large blocks, and not giving it back. How I can detect memory leaks of C++ application in Linux (Ubuntu OS)? Why a template specialization cannot change the return type? If you want to get more detail infomation about how malloc work, please see the chepter 3 in glibc manual. Published Aug 3, 2020 + Follow std::map and its siblings ( std::multimap, std::unordered_map / multimap) used to be my favourite containers when I was doing competitive programming. core dump while accessing the data from STL map, How to call a class member function via function pointer in map [C++], http://www.boost.org/libs/pool/doc/index.html. Will hardware/implementation affect the time/space complexity of algorithms? Why did only Pinchas (knew how to) respond? To the above figure you should also add the overhead of memory management structures used for storing the map's elements. This makes sense because the extract is all about avoiding unnecessary copies and allocations. A map with 150 million nodes soaked up ~ 15GB, which implies the 8 byte L, 8 byte R, 8 byte int key, and 8 byte datum, totaling 32 bytes, soaked up about 2/3rds of the map's memory for internal nodes, leaving 1/3rd for leaves. To learn more, see our tips on writing great answers. How can I track memory allocation of C++ standard library calls? How could the Intel 4004 address 640 bytes if it was only 4-bit? Use MathJax to format equations. Non-anarchists often say the existence of prisons deters violent crime. Is it normal behavior of unordered_map? Regarding std::vector, one can use shrink_to_fit (something I did not do here). WebBy default the map can only hold up to 2 32 - 1 values, that is 4 294 967 295 values. At what point will the memory be given back to the OS and is there any way to tell it that it will no longer be needed? in what chunks they will allocate it?
Standard library header - cppreference.com Keys are sorted by using the comparison function . Yes, the numbers I offer are optimistic. rev2023.7.3.43523. The limitations of Joe Blows super-set class will have long been forgotten until they manifest as bugs when someone uses the class in a manner which only Joe knew would not be supported. What if the key value type doesnt have a default operator< ? I do expect the memory usage to be linear, for large enough sets (I am guessing that 1024 is large enough). in fact, i found below things, which may be useful to you: tests. Asking for help, clarification, or responding to other answers. Thus size () is usually between 0.4375*bucket_count () and 0.875*bucket_count (). To learn more, see our tips on writing great answers. Why do most languages use the same token for `EndIf`, `EndWhile`, `EndFunction` and `EndStructure`? I am having a much harder time than I should trying to deallocate the memory used by an STL map. how to give credit for a picture I modified from a scientific article? Hi, if(it != mapOfWords.end()) How C++ and alike maps are actually stored in memory? double the space because of pointers), but if you store some very big objects the overhead is mostly neglectable. So why would I use STL? Plot multiple lines along with converging dotted line. To learn more, see our tips on writing great answers. If it is stored as Red-Black trees as I understand from your answer, then it uses (2N-1)*[size of(myStruct)+2*pointerSize] that's too much memory even if we omit the 2N null/nil pointers for the leaves? it takes logarithmic time. Results will vary depending on your compiler, standard library, the size of the container, and so forth You should run your own tests Still, here are some numbers I got on my Mac: (My Linux box gives slightly different numbers but the conclusion is the same.). All results are for the STL implementation I had ready at hand (g++5.3 on In the above example, we used the second one, which accepts a key and then finds & extracts the map node that matches the key parameter. C++ remains one of the most popular languages today. I feel like I must be doing something stupid as this seems so simple. article is good but it misses some important stuff that would make it much better: Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. in Latin? data types. Thus you only need to take container's size() and then increment it once per each non-used bucket. WebAccepted answer. Each entry in std::map
is std::pair therefore through iterator, A[1][2]=3; // change: 3 instead of 1, at the right side of the = At first I was puzzled with your results for unordered_set and deque, but it makes sense. Getting more memory from the OS usually involves a system call (slow), so the program run-time library works hard to avoid this as much as possible. How can i estimate memory usage of std::map? C++, Development, Linux, Compiling an optimized version of ArangoDB The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user. Checkout following articles for complete examples, He is a techno-optimist and a free-speech advocate. std::map is a sorted associative container that contains key-value pairs with unique keys. Note: These names are only for exposition, they are not part of the interface. I'm using G++ 3.4.6. https://www.quora.com/What-is-a-map-data-structure-How-does-it-store-data I have tried what was explained in Diomidis Spinellis's answer and expanded his answer over here which could be helpful to others. It's true that there are a lot of nullptr's, and a smaller (but not simpler) data structure could be used. I havent noticed memory utilization go up over our legacy code but then our legacy code had many leaks and corner case that never worked properly. What is "sum of fields size"? 2) you can see the malloc status with the function 'malloc_stats()' or 'mallinfo()', which are defined in His research is focused on software performance and data engineering. How do I distinguish between chords going 'up' and chords going 'down' when writing a harmony? std::map & User defined class objects as keys. Of course, the biggest problem I see with stl containers is that people either use the wrong one for their job (and performance requirements is part of that), or they fail to use them (and implement their own). I mean it's kind of "not best choice" to use a map to help in traversing a tree, this way I'm allocating a space for 2 trees, and I'm not sure the running time requirements will follow my tree kind that I worked so hard to think of, or the kind of tree the C++ implementation used??? I am expanding on his answer by adding few lines of code. If I can write an write an equivalent class in the same time, hard to justify the time to study (and periodically re-studying) the STL. What does skinner mean in the context of Blade Runner 2049. How Much Memory Does an STL Container Use? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. But it is still not nearly as economical as you might expect. This is quite useful from a performance point of view when the insertion sequence of items is somewhat predictable. Note also that the general-purpose heap manager has a compute cost on allocation and de-allocation. If load_factor > 1, then bucket_count() * load_factor indicates the max item in the map. MathJax reference. View all posts by Daniel Lemire. Should I sell stocks that are performing well or poorly first? The code we write is written first for human consumption & only secondarily for the computer to understand. John Sonmez. You theme in this post is exactly relevant. I meant in general before I knew how they're implemented in C++, any hash table trade space with time; ie have to use extra space either thru buckets/extra empty slots/. unless the data specifies previously known values of a collision free hash function. How can I find out how much memory is physically installed in Windows? The technical storage or access that is used exclusively for anonymous statistical purposes. Do you like it? std::map and Comparators The thing here to note is what happens when there are duplicates! By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. Outputs (On my ARM Cortex A-9 iMX6Solo-X processor running Linux [4.9.175] and compiler: arm-fslc-linux-gnueabi-gcc (GCC) 7.3.0): Considering std::map, I am interested in size of ELEMENT_OVERHEAD as it grows linearly with the number of elements present in the map. Some (or most, I am not sure about this) memory allocators require more memory from the OS than needed at that point in time for efficiency. Browse other questions tagged, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site. Would a passenger on an airliner in an emergency be forced to evacuate? if element is added or not. This is typically a binary tree, such as a Red-Black Tree. To create code blocks or other preformatted text, indent by four spaces: To create not a block, but an inline code span, use backticks: For more help see http://daringfireball.net/projects/markdown/syntax. } Actually, I dont think the organizer was joking he was being serious but the audience thought he was joking. The flash use is stated as program storage space . The organizer joked that one should not be surprised to use 32 bytes per 32-bit integer in Java. Solution 1 If you want to get a rough size, I think bucket_count() and max_load_factor() is enough, which gives the current count of buckets and the load I hope this question has informed/reminded of the fact that a map is implemented as a tree, unordered map as a hash table, and rang a bell to software developers to check how language library data structures are really implemented in memory to be aware of the consequences when using them. They are clean, well tested and well documented. map Powered by Octopress, vector => 0 bytes allocd at end, total: 0 bytes mallocd, 0 malloc(s), 0 free(s), map => 0 bytes allocd at end, total: 0 bytes mallocd, 0 malloc(s), 0 free(s), set => 0 bytes allocd at end, total: 0 bytes mallocd, 0 malloc(s), 0 free(s), unordered_map => 0 bytes allocd at end, total: 0 bytes mallocd, 0 malloc(s), 0 free(s), unordered_set => 0 bytes allocd at end, total: 0 bytes mallocd, 0 malloc(s), 0 free(s), deque => 576 bytes allocd at end, total: 576 bytes mallocd, 2 malloc(s), 0 free(s), vector => 8 bytes allocd at end, total: 8 bytes mallocd, 1 malloc(s), 0 free(s), map => 48 bytes allocd at end, total: 48 bytes mallocd, 1 malloc(s), 0 free(s), set => 40 bytes allocd at end, total: 40 bytes mallocd, 1 malloc(s), 0 free(s), unordered_map => 40 bytes allocd at end, total: 40 bytes mallocd, 2 malloc(s), 0 free(s), unordered_set => 32 bytes allocd at end, total: 32 bytes mallocd, 2 malloc(s), 0 free(s), vector => 16 bytes allocd at end, total: 24 bytes mallocd, 2 malloc(s), 1 free(s), map => 96 bytes allocd at end, total: 96 bytes mallocd, 2 malloc(s), 0 free(s), set => 80 bytes allocd at end, total: 80 bytes mallocd, 2 malloc(s), 0 free(s), unordered_map => 88 bytes allocd at end, total: 104 bytes mallocd, 4 malloc(s), 1 free(s), unordered_set => 72 bytes allocd at end, total: 88 bytes mallocd, 4 malloc(s), 1 free(s), vector => 32 bytes allocd at end, total: 56 bytes mallocd, 3 malloc(s), 2 free(s), map => 192 bytes allocd at end, total: 192 bytes mallocd, 4 malloc(s), 0 free(s), set => 160 bytes allocd at end, total: 160 bytes mallocd, 4 malloc(s), 0 free(s), unordered_map => 136 bytes allocd at end, total: 152 bytes mallocd, 6 malloc(s), 1 free(s), unordered_set => 104 bytes allocd at end, total: 120 bytes mallocd, 6 malloc(s), 1 free(s), vector => 64 bytes allocd at end, total: 120 bytes mallocd, 4 malloc(s), 3 free(s), map => 384 bytes allocd at end, total: 384 bytes mallocd, 8 malloc(s), 0 free(s), set => 320 bytes allocd at end, total: 320 bytes mallocd, 8 malloc(s), 0 free(s), unordered_map => 280 bytes allocd at end, total: 336 bytes mallocd, 11 malloc(s), 2 free(s), unordered_set => 216 bytes allocd at end, total: 272 bytes mallocd, 11 malloc(s), 2 free(s), vector => 128 bytes allocd at end, total: 248 bytes mallocd, 5 malloc(s), 4 free(s), map => 768 bytes allocd at end, total: 768 bytes mallocd, 16 malloc(s), 0 free(s), set => 640 bytes allocd at end, total: 640 bytes mallocd, 16 malloc(s), 0 free(s), unordered_map => 568 bytes allocd at end, total: 712 bytes mallocd, 20 malloc(s), 3 free(s), unordered_set => 440 bytes allocd at end, total: 584 bytes mallocd, 20 malloc(s), 3 free(s), vector => 256 bytes allocd at end, total: 504 bytes mallocd, 6 malloc(s), 5 free(s), map => 1536 bytes allocd at end, total: 1536 bytes mallocd, 32 malloc(s), 0 free(s), set => 1280 bytes allocd at end, total: 1280 bytes mallocd, 32 malloc(s), 0 free(s), unordered_map => 1144 bytes allocd at end, total: 1472 bytes mallocd, 37 malloc(s), 4 free(s), unordered_set => 888 bytes allocd at end, total: 1216 bytes mallocd, 37 malloc(s), 4 free(s), vector => 512 bytes allocd at end, total: 1016 bytes mallocd, 7 malloc(s), 6 free(s), map => 3072 bytes allocd at end, total: 3072 bytes mallocd, 64 malloc(s), 0 free(s), set => 2560 bytes allocd at end, total: 2560 bytes mallocd, 64 malloc(s), 0 free(s), unordered_map => 2312 bytes allocd at end, total: 3016 bytes mallocd, 70 malloc(s), 5 free(s), unordered_set => 1800 bytes allocd at end, total: 2504 bytes mallocd, 70 malloc(s), 5 free(s), deque => 1088 bytes allocd at end, total: 1088 bytes mallocd, 3 malloc(s), 0 free(s), vector => 1024 bytes allocd at end, total: 2040 bytes mallocd, 8 malloc(s), 7 free(s), map => 6144 bytes allocd at end, total: 6144 bytes mallocd, 128 malloc(s), 0 free(s), set => 5120 bytes allocd at end, total: 5120 bytes mallocd, 128 malloc(s), 0 free(s), unordered_map => 4664 bytes allocd at end, total: 6144 bytes mallocd, 135 malloc(s), 6 free(s), unordered_set => 3640 bytes allocd at end, total: 5120 bytes mallocd, 135 malloc(s), 6 free(s), deque => 1600 bytes allocd at end, total: 1600 bytes mallocd, 4 malloc(s), 0 free(s), vector => 2048 bytes allocd at end, total: 4088 bytes mallocd, 9 malloc(s), 8 free(s), map => 12288 bytes allocd at end, total: 12288 bytes mallocd, 256 malloc(s), 0 free(s), set => 10240 bytes allocd at end, total: 10240 bytes mallocd, 256 malloc(s), 0 free(s), unordered_map => 9416 bytes allocd at end, total: 12488 bytes mallocd, 264 malloc(s), 7 free(s), unordered_set => 7368 bytes allocd at end, total: 10440 bytes mallocd, 264 malloc(s), 7 free(s), deque => 2624 bytes allocd at end, total: 2624 bytes mallocd, 6 malloc(s), 0 free(s), vector => 4096 bytes allocd at end, total: 8184 bytes mallocd, 10 malloc(s), 9 free(s), map => 24576 bytes allocd at end, total: 24576 bytes mallocd, 512 malloc(s), 0 free(s), set => 20480 bytes allocd at end, total: 20480 bytes mallocd, 512 malloc(s), 0 free(s), unordered_map => 18872 bytes allocd at end, total: 25216 bytes mallocd, 521 malloc(s), 8 free(s), unordered_set => 14776 bytes allocd at end, total: 21120 bytes mallocd, 521 malloc(s), 8 free(s), deque => 4752 bytes allocd at end, total: 4816 bytes mallocd, 11 malloc(s), 1 free(s), Compiling an optimized version of ArangoDB. However, the testmap() function doesn't work the same way. The unordered_map structure is designed to hold large numbers of objects in a way that makes adds, deletes, lookups, and orderless traverses efficient. It looks as though you are counting memory allocated without the overhead of heap structures which could possibly add up to quite a lot.
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