std::vector memory overheadstd::vector memory overhead

I believe that option B takes 1 32 bit pointer (assuming 32 bit here) score:1 In n is known at compile-time, then you should choose std::array as: std::array<int, n> data; //n is compile-time constant and if n is not known at compile-time, OR the array might grow at runtime, then go for std::vector: std::vector<int> data (n); //n may be known at runtime Unlike OpenCL buffers, SYCL buffers must be accessed using SYCL accessors. Joe Blows super-set class less so. Hope you made sure to turn optimizations on :) Would be good to know compiler & OS too. A default constructed vector has no size and never had any, so it should have no dynamic allocation. Hello, sorry if this is the wrong place, let me know please. Memory overhead using `std::vector` and `gmp` Hello, sorry if this is the wrong place, let me know please. a subreddit for c++ questions and answers. I think C++ focuses more on safety and making the language more like JavaScript/C# even though the language fundamentally lacks the foundation to be one. 21 June 2017 by Phillip Johnston Last updated 15 December 2021, I was working on an article comparing C++ containers to builtin arrays, which quickly grew out of hand due to the required prerequisite information. That avoids dependency on Boost or alternatively a complicated home-brewed implementation, while still reaping the benefits of exception-free but still guaranteed correct normal case code (only exception when one forgets to check, as is . One of the benefits of C++ is the built-in STL containers offering the standard data structures like vector, list, map, set. The problem in the real (non academic) world with implementing ones own super efficient version, is that people in the private sector switch jobs frequently, and it is often the case that one has to take over code written by someone else. What's the purpose of using braces (i.e. If you declare a std::vector in a specific scope, when that scope is no longer valid the std::vector memory will be destructed and freed automatically. what would happened if I changed the C++ Dynamic Shared Library on Linux while my executable program using on it, understanding the operator<<() from std::cout, std::function/bind like type-erasure without Standard C++ library, is it valid to copy a float array to an array of uint8_t using memcpy, Recursion vs bitmasking for getting all combinations of vector elements, Qt Beginner: QPainter widget not rendering. 589). And what about dynamic memory used internally by the vector? I did some research but have no way of telling if this is normal, and I don't want to use dynamuc C type arrays, since it's so discouraged. For small elements, this collection has a very big memory overhead. But what does default mean? Find centralized, trusted content and collaborate around the technologies you use most. I was telling the audience that using close to 32 bits per 32-bit value stored in a container was pretty good sometimes. Scheduling an event for everyone 1ms using Windows and C++ chrono - am I asking for too much? You could take a look at Google sparsehash (http://code.google.com/p/sparsehash/). Typically operator[] will have practically no discernible overhead over C-array subscripting in an optimized build. size() Vs empty() in vector - why empty() is preferred? What's your STL implementation? . Two methods can access specific elements: the at() function and the [] operator. Make your choice! Thanks for contributing an answer to Stack Overflow! If the size is known at compile-time, opt for std::array<>, which is a lightweight, safe wrapper of C-style arrays which introduces zero overhead. Actually, I dont think the organizer was joking he was being serious but the audience thought he was joking. I would assume 4 bytes for the pointer, 4 bytes for the size and 4 bytes for the capacity. These member functions are quite similar, with one critical distinction: emplace_back() allows for constructor arguments to be forwarded so that the new element can be constructed in place. Algorithm for ordering strings to and from disk efficiently using minimal internal memory resources, Using Eigen types with STL containers and std::vector, Using operator new and operator delete with a custom memory pool/allocator, Unexpectedly high memory usage, using a std::stack and std::map, Understanding a memory leak while using boost::asio and boost::thread, Using optimistic locks in C++ and memory order. Also, ID software largely switched to vectors when they found the overhead of searching through a vector was much less than the overhead of using linked lists in general. From the project home page: An extremely memory-efficient hash_map implementation. Hi, size up front, I'll use something like: to get an already sized vector, rather than using push_back. Hi, I log on to your blog daily. Although cl.exe officially supports only C++11, the helper function std::make_unique is already available. C++11 offers four different smart pointers. Efficiency shouldn't be an issue, since you have throwing and non-throwing member functions to access the contained elements, so you have a choice whether to favor safety or performance. In terms of storage the vector data is (usually) placed in dynamically allocated memory thus requiring some minor overhead; conversely C-arrays and std::array use automatic storage relative to the declared location and thus do not have any overhead. std::unique_ptr needs, by default, no additional memory. From my (admittedly poor) understanding, the C++ specification of an unordered_set and its behaviour with respect to object storage pretty much requires a hash table with separate chaining in the buckets for an unknown general class. I was wondering if I already know the type std::variant stores and attempt to access it using std::get(v) or even std::get_if(v), will there be an overhead compared to accessing a union with data.particular_member? Function pointer (to other kernel) as kernel arg in CUDA. Using the familiar [] operator will not provide any bounds checking, providing for faster access while increasing the risk of a segmentation fault. Making statements based on opinion; back them up with references or personal experience. I use C++ when I need bare-metal performance. You can also use the resize() function to manually increase or decrease the size of your std::vector at will. The current number of elements stored in the std::vector is not necessarily the same as the amount of memory that is allocated. 2 bits/entry overhead! how much work is done by calling vector.size()? Temporary policy: Generative AI (e.g., ChatGPT) is banned, A function to populate an array with maps, std::vector > vs std::vector, Program killed: using vector of set of vector. You dont need to worry about remembering the correct arguments for memcpy, use the equals operator (=): Or declare a new std::vector and use the vector you want copied as a constructor argument: std::vector provides interfaces for accessing the underlying data: The first element of the vector can be accessed using front(). See these: C++ benchmark std::vector VS std::list VS std::deque. How to asynchronously copy memory from the host to the device using thrust and CUDA streams, store and retrieve matrices in memory using xptr, Using memcpy to copy part of an array, and other memory manipulation tools. If you know the size in advance (especially at compile time), and don't need the dynamic re-sizing abilities of std::vector, then using something simpler is fine. In opposition to the std::unique_ptr, the std::shared_ptr has a little overhead. What are the main purposes of using std::forward and which problems it solves? (To be precise, there is an additional reference counter for the std::weak_ptr). Allocation time is required tor the initial storage allocation, as well as any resizing that is required. In a modern compiler, in release mode, with optimisations enabled, there is no overhead in using operator [] compared to raw pointers: the call is completely inlined and resolves to just a pointer access. std::make_shared make one memory allocation out of them. Instead of specifying the format we are all used to: We can use this format to iterate through all elements of a vector: Heres a trivial example which prints every element using the new for loop construct: Using std::vector does come with minor overhead costs (which, to me, are worth the benefits). Which field is more rigorous, mathematics or philosophy? How can I manually (on paper) calculate a Bitcoin public key from a private key? Performance penalty for large C++ dll's with autogenerated C code. Not the answer you're looking for? What does a potential PhD Supervisor / Professor expect when they ask you to read a certain paper? I want to draw a few interesting conclusions from the table. Often, you are aware of how many elements you need to store in your std::vector (at least as a maximum). Read file line by line using ifstream in C++. If you are performing multiple accesses through the same vector or pointer in the same section of code without causing the vector to perhaps need reallocating, then the cost of this extra dereference may be shared over the multiple accesses and may well be negligible. How to create a recursive method to compare two values? (This is severe performance problem for me) Kind regards, Mitsuru Nishikawa Tags: CC++ Development Tools How can I improve this solution? Inserting elements into the middle of an array is simplified into a function call. I have just provided a cursory overview of the std::vector container. The general purpose heap code has to be thread-safe, which adds an additional cost. The organizer joked that one should not be surprised to use 32 bytes per 32-bit integer in Java. generates the following code for me on g++ (some guff trimmed): Note how the pointer and vector version produce exactly the same code, with only the array version "winning". Another aspect, some would argue a more important one, is the performance cost of passing shared_ptr by value to functions. Modulo Multiplication Function: Multiplying two integers under a modulus. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. Make your choice! However, that something should preferably be std::array if you have C++11, or something like boost::scoped_array otherwise. Instead of a vector, I could use a std::map, however the overhead of ~32 bytes per node also becomes very expensive for such light weight pairs. C++ Containers library std::vector 1) std::vector is a sequence container that encapsulates dynamic size arrays. How big are the vectors? In C++, at least, you can rewrite the problematic code so it is fast and memory conscious. Precisely, where exclusive ownership by unique_ptr is needed because an std::vector can be copied, or where the memory usage is a more pressing concern because an std::vector has a small memory overhead, or when we are dealing with a library that requires passing a pointer. The Overflow #186: Do large language models know what theyre talking about? What is the space complexity of a vector of empty vectors? But will that require us to rewrite STL at some point? This makes sense mostly for large tables where you expect a lot of lookups. When the pattern of usage is not random, you can use allocation pools (few/large allocations from the general-purpose heap) and free-lists to make huge reductions in memory and CPU usage. I expected to have a memory overhead, but I'm experiencing a X3 in memory. I dont know the answer to that question, but I do know (from working at companies who have reimplemented the STL to be more efficient) that something that violates the guarantees in the standard discussed all over the place (stackoverflow answers, etc) in some corner cases is NOT a nice programming environment. If all you do is program in C++ all day, you might take STL for granted, but more recent languages like Go or JavaScript do not have anything close to STL built-in. Making statements based on opinion; back them up with references or personal experience. I just want to briefly note: since std::vector is a container class, any operations meant to be used on containers will work: I will be discussing container operations further in a future article. Memory overhead std::unique_ptr std::unique_ptr needs, by default, no additional memory. difference: you're profiling, which means that you're executing It is very efficient. US Port of Entry would be LAX and destination is Boston. std::set? This email address is being protected from spambots. Instead of a vector, I could use a std::map, however the overhead of ~32 bytes per node also becomes very expensive for such light weight pairs. The fact that every C++ compiler comes with a decent set of STL containers is just very convenient. And what are you profiling with? For instance, at one point of the algorithm I know it should use 5GB and be of size 24 (yes, Im working with huge numbers). Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, G Prvulovic, Reinhold Drge, Abernitzke, Frank Grimm, Sakib, Broeserl, Antnio Pina, Sergey Agafyin, , Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Daniel Hufschlger, Alessandro Pezzato, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky, Leo Goodstadt, John Wiederhirn, Yacob Cohen-Arazi, Florian Tischler, Robin Furness, Michael Young, Holger Detering, Bernd Mhlhaus, Matthieu Bolt, Stephen Kelley, Kyle Dean, Tusar Palauri, Dmitry Farberov, Juan Dent, George Liao, Daniel Ceperley, Jon T Hess, Stephen Totten, Wolfgang Ftterer, Matthias Grn, Phillip Diekmann, Ben Atakora, Ann Shatoff, Rob North, and David Poole. vector makes sense. Is there any overhead using this-> for accessing a member? File Locking in C++ For simultaneous Read and Write Lock. He is a techno-optimist and a free-speech advocate. classes. You should try to avoid C-style-arrays in C++ whenever possible. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Migrating OpenCL FPGA Designs to SYCL* FPGA Design Flow Modify Your Design Flags, Attributes, Directives, and Extensions Histogram Design Example Walkthrough Additional Information Document Revision History for Migrating OpenCL FPGA Designs to SYCL* 2) std::pmr::vector is an alias template that uses a polymorphic allocator. It is very efficient. Scan this QR code to download the app now. Also vector has 0 overhead only if you know its size in advance and you do reserve. std::vector v; takes up sizeof(v) space. The size of the std::vector or a std::deque can increase without an expensive allocation of new memory. In your strategy on a 64-bit machine, you need an 8-byte pointer to that byte. The functions std::make_shared (line 16) and std::make_unique(line 18) are quite handy. The STL provides containers which usually suffice for every need. Note that the inner loop is always from 0 to 999, independently of the value of n; what is different is just the memory address. Transitivity of the Acquire-Release Semantic, Thread Synchronization with Condition Variables or Tasks, For the Proofreaders and the Curious People, Thread-Safe Initialization of a Singleton (370526 hits), C++ Core Guidelines: Passing Smart Pointers (328804 hits), C++ Core Guidelines: Be Aware of the Traps of Condition Variables (314725 hits), C++17 - Avoid Copying with std::string_view (268688 hits), Optimization matters. For fast code the only viable option is a passive statistical profiler (e.g. Copyright 2023 www.appsloveworld.com. Here is a compilation of my standard seminars. What are pros and cons of using uint64_t as an universal address of memory instead of void*? In the original strategy, each byte occupies a byte in memory. memory string we should copy data from a std::stringstreamto a std::stringand then to the shared-memory string. In order to provide type . Theres also std::forward_list, which has pointer less and thus consumes less, as expected (16b on my Linux box). One question about the threadSafeInterface.cpp example. Initialize 256 keyed array elements to badKeyHandler, fill in KeyedCollectionHandler objects for good key values. Performance results from unoptimized builds are intrinsically not appropriate for an article focussed on performance. I sincerely doubt that this is related to any. I'll use std::array (if I have access to C++11), or even Now that we know a little bit about std::vector, lets see how we work with one. With MSVC 7.1 this: gives me 16 (bytes). vector . (Ep. I share your feeling. The only overheads you'd experience realistically are memory allocations and safety checks for boundaries. The problem with the level of abstraction offered by C++ is that you can be completly unaware of how much memory you are using. You can tune this capacity on an embedded system to limit the max vector size on your system. std::make_unique is available since C++14; the other smart pointer functionality since C++11. Your vector often ends up occupying more space than a builtin array, as memory can be allocated to handle future growth. (This was fixed in the C++11 language update with the addition of the shrink_to_fit method.) (I've seen another related question on SO, but regarding [] vs at() -- but apparently even [] is too slow for me?! Relatively small vectors (a few thousand elements at most, often only a few dozen), with mostly random access. Connect and share knowledge within a single location that is structured and easy to search. take a greater percent of time than if it were inlined. std::vector is worth adding to your toolbox because it provides extra features over builtin arrays. I wrote a small test. Having a 128x128 sized std::vector of 4K Bytes took more than 50MB. This is unlikely to happen: vector/array is an extremely thin layer around a plain old array. In practice, you can expect 16B overhead per heap allocation (64b glibc malloc, but most do the same AFAIK). Inside the block, there is a place . See here on SO. In particular, std::make_shared is very interesting. A pure array access is an (almost) direct memory read, whereas operator[] is a member method of vector<>. There are more memory allocations necessary for the creation of a std::shared_ptr. std::vector is not magic, so the class still suffers from the runtime costs associated with dynamic memory allocation. https://github.com/greg7mdp/sparsepp. As mentioned above, std::array is a templated class that represents fixed-size arrays. (I've been working in C++ for over 25 Depending on the processor/architecture I've observed even 10x slowdowns because of cache trashing. Are you looking to make an array of them, and want to know how much space you save by making it an array of pointers? std::array provides many benefits over built-in arrays, such as preventing automatic decay into a pointer, maintaining the array size, providing bounds checking, and allowing the use of C ++ container operations. If you wanted to you could even define KCH as collection of objects, each containing a hashmap, so each KCH[i] can implement a convenient interface for working with std::pair objects for that key. Sequential or random access? Currently are 227guests and no members online. What is the overhead cost of an empty vector? Therefore, I'm interested in comparing the raw memory allocation and the smart pointers. True, my code spends all of its time reading and writing in vectors (and a few vectors too), but still, I'd like to know if there's supposed to be some significant overhead of operator[] compared to C-style arrays? In the other direction, towards simplicity at the possible cost of efficiency, one can use a `std::vector` as value carrier for a very simple Optional class. struct Point that has 2 doubles overhead would be smaller. The Overflow #186: Do large language models know what theyre talking about? Associative containers are used when you have pairs of key/value and you want to find a value given its key. Luckily the std::vector class provides us a few functions for managing memory: Reallocations are costly in terms of performance. It might vary by implementation, so run it and find out how much it takes for you. Initialize all keyedArray elements to dummyHashMap, then fill in with different hash maps for valid keys. Because std::vector contains enough memory to hold N copies of a type T. Whereas std::variant contains only enough memory to hold the largest type because there can only be one "active" type at a time. Using generic std::function objects with member functions in one class, Already defined in .obj - no double inclusions. C++ unordered_map using a custom class type as the key. . Asking for help, clarification, or responding to other answers. In this example, the accelerator board can provide a bandwidth of 25600 megabytes per second (MB/s). You are also relying on tested code and eliminating extra boilerplate code that is no longer required. Meanwhile a hash table might allocate enough space to store 2-4 times as many entries as it actually contains. If you have an existing object (or dont mind a temporary object being created), then you would use push_back(). The std::vector class simplifies operations which are cumbersome with builtin arrays. All rights reserved. I compiled with option -pg and used gprof. On the diagram above, you can see that all elements of the vector are next to each other in the memory block. I have an std::vector<mpz_class> of size not known in advance, so I keep pushing elements to it. Syntax: vector<data_type> v Declaring a vector: Some ways of declaring a vector are provided below Every C++ developer knows that std::string represents a sequence of characters in memory. Remarks Use of an std::vector requires the inclusion of the <vector> header using #include <vector>. It's easy to replace hash_map or unordered_map by sparse_hash_map or dense_hash_map in C++ code. To subscribe to this RSS feed, copy and paste this URL into your RSS reader.