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  • File IO with Streams - Best Memory Buffer Size

    - by AJ
    I am writing a small IO library to assist with a larger (hobby) project. A part of this library performs various functions on a file, which is read / written via the FileStream object. On each StreamReader.Read(...) pass, I fire off an event which will be used in the main app to display progress information. The processing that goes on in the loop is vaired, but is not too time consuming (it could just be a simple file copy, for example, or may involve encryption...). My main question is: What is the best memory buffer size to use? Thinking about physical disk layouts, I could pick 2k, which would cover a CD sector size and is a nice multiple of a 512 byte hard disk sector. Higher up the abstraction tree, you could go for a larger buffer which could read an entire FAT cluster at a time. I realise with today's PC's, I could go for a more memory hungry option (a couple of MiB, for example), but then I increase the time between UI updates and the user perceives a less responsive app. As an aside, I'm eventually hoping to provide a similar interface to files hosted on FTP / HTTP servers (over a local network / fastish DSL). What would be the best memory buffer size for those (again, a "best-case" tradeoff between perceived responsiveness vs. performance).

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  • Which way to store this data is effective?

    - by Tattat
    I am writing a game, which need a map, and I want to store the map. The first thing I can think of, is using a 2D-array. But the problem is what data should I store in the 2D-array. The player can tap different place to have different reaction. So, I am thinking store a 2D-array with objects, when player click some position, and I find it in the array, and use the object in that array to execute a cmd. But I have a concern that storing lots of object may use lots of memory. So, I am think storing char/int only. But it seems that not enough for me. I want to store the data like that: { Type:1 Color:Green } No matter what color is, if they are all type 1, the have same reactions in logic, but the visual effect is based on the color. So, it is not easy to store using a prue char/int data, unless I make something like this: 1-5 --> all type 1. 1=color green , 2=color red, 3 = color yellow.... ... 6-10 --> all type 2. 2 = color green, 2 = color red ... ... So, do you have any ideas on how to minimize the ram use, but also easy for me to read... ...thx

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  • Using Custom Generic Collection faster with objects than List

    - by Kaminari
    I'm iterating through a List<> to find a matching element. The problem is that object has only 2 significant values, Name and Link (both strings), but has some other values which I don't want to compare. I'm thinking about using something like HashSet (which is exactly what I'm searching for -- fast) from .NET 3.5 but target framework has to be 2.0. There is something called Power Collections here: http://powercollections.codeplex.com/, should I use that? But maybe there is other way? If not, can you suggest me a suitable custom collection?

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  • Is count(*) really expensive ?

    - by Anil Namde
    I have a page where I have 4 tabs displaying 4 different reports based off different tables. I obtain the row count of each table using a select count(*) from <table> query and display number of rows available in each table on the tabs. As a result, each page postback causes 5 count(*) queries to be executed (4 to get counts and 1 for pagination) and 1 query for getting the report content. Now my question is: are count(*) queries really expensive -- should I keep the row counts (at least those that are displayed on the tab) in the view state of page instead of querying multiple times? How expensive are COUNT(*) queries ?

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  • optimize a string.Format + replace.

    - by acidzombie24
    I have this function. The visual studio profile marked the line with string.Format as hot and were i spend much of my time. How can i write this loop more efficiently? public string EscapeNoPredicate(string sz) { var s = new StringBuilder(sz); s.Replace(sepStr, sepStr + sepStr); foreach (char v in IllegalChars) { string s2 = string.Format("{0}{1:X2}", seperator, (Int16)v); s.Replace(v.ToString(), s2); } return s.ToString(); }

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  • Scalably processing large amount of comlpicated database data in PHP, many times a day.

    - by Eph
    I'm soon to be working on a project that poses a problem for me. It's going to require, at regular intervals throughout the day, processing tens of thousands of records, potentially over a million. Processing is going to involve several (potentially complicated) formulas and the generation of several random factors, writing some new data to a separate table, and updating the original records with some results. This needs to occur for all records, ideally, every three hours. Each new user to the site will be adding between 50 and 500 records that need to be processed in such a fashion, so the number will not be steady. The code hasn't been written, yet, as I'm still in the design process, mostly because of this issue. I know I'm going to need to use cron jobs, but I'm concerned that processing records of this size may cause the site to freeze up, perform slowly, or just piss off my hosting company every three hours. I'd like to know if anyone has any experience or tips on similar subjects? I've never worked at this magnitude before, and for all I know, this will be trivial to the server and not pose much of an issue. As long as ALL records are processed before the next three hour period occurs, I don't care if they aren't processed simultaneously (though, ideally, all records belonging to a specific user should be processed in the same batch), so I've been wondering if I should process in batches every 5 minutes, 15 minutes, hour, whatever works, and how best to approach this (and make it scalable in a way that is fair to all users)?

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  • Why isn't the copy constructor elided here?

    - by Jesse Beder
    (I'm using gcc with -O2.) This seems like a straightforward opportunity to elide the copy constructor, since there are no side-effects to accessing the value of a field in a bar's copy of a foo; but the copy constructor is called, since I get the output meep meep!. #include <iostream> struct foo { foo(): a(5) { } foo(const foo& f): a(f.a) { std::cout << "meep meep!\n"; } int a; }; struct bar { foo F() const { return f; } foo f; }; int main() { bar b; int a = b.F().a; return 0; }

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  • Specific template for the first element.

    - by Kalinin
    I have a template: <xsl:template match="paragraph"> ... </xsl:template> I call it: <xsl:apply-templates select="paragraph"/> For the first element I need to do: <xsl:template match="paragraph[1]"> ... <xsl:apply-templates select="."/><!-- I understand that this does not work --> ... </xsl:template> How to call <xsl:apply-templates select="paragraph"/> (for the first element paragraph) from the template <xsl:template match="paragraph[1]">? So far that I have something like a loop. I solve this problem so (but I do not like it): <xsl:for-each select="paragraph"> <xsl:choose> <xsl:when test="position() = 1"> ... <xsl:apply-templates select="."/> ... </xsl:when> <xsl:otherwise> <xsl:apply-templates select="."/> </xsl:otherwise> </xsl:choose> </xsl:for-each>

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  • Optimizing sparse dot-product in C#

    - by Haggai
    Hello. I'm trying to calculate the dot-product of two very sparse associative arrays. The arrays contain an ID and a value, so the calculation should be done only on those IDs that are common to both arrays, e.g. <(1, 0.5), (3, 0.7), (12, 1.3) * <(2, 0.4), (3, 2.3), (12, 4.7) = 0.7*2.3 + 1.3*4.7 . My implementation (call it dict) currently uses Dictionaries, but it is too slow to my taste. double dot_product(IDictionary<int, double> arr1, IDictionary<int, double> arr2) { double res = 0; double val2; foreach (KeyValuePair<int, double> p in arr1) if (arr2.TryGetValue(p.Key, out val2)) res += p.Value * val2; return res; } The full arrays have about 500,000 entries each, while the sparse ones are only tens to hundreds entries each. I did some experiments with toy versions of dot products. First I tried to multiply just two double arrays to see the ultimate speed I can get (let's call this "flat"). Then I tried to change the implementation of the associative array multiplication using an int[] ID array and a double[] values array, walking together on both ID arrays and multiplying when they are equal (let's call this "double"). I then tried to run all three versions with debug or release, with F5 or Ctrl-F5. The results are as follows: debug F5: dict: 5.29s double: 4.18s (79% of dict) flat: 0.99s (19% of dict, 24% of double) debug ^F5: dict: 5.23s double: 4.19s (80% of dict) flat: 0.98s (19% of dict, 23% of double) release F5: dict: 5.29s double: 3.08s (58% of dict) flat: 0.81s (15% of dict, 26% of double) release ^F5: dict: 4.62s double: 1.22s (26% of dict) flat: 0.29s ( 6% of dict, 24% of double) I don't understand these results. Why isn't the dictionary version optimized in release F5 as do the double and flat versions? Why is it only slightly optimized in the release ^F5 version while the other two are heavily optimized? Also, since converting my code into the "double" scheme would mean lots of work - do you have any suggestions how to optimize the dictionary one? Thanks! Haggai

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  • Will an optimizing compiler remove calls to a method whose result will be multiplied by zero?

    - by Tim R.
    Suppose you have a computationally expensive method, Compute(p), which returns some float, and another method, Falloff(p), which returns another float from zero to one. If you compute Falloff(p) * Compute(p), will Compute(p) still run when Falloff(p) returns zero? Or would you need to write a special case to prevent Compute(p) from running unnecessarily? Theoretically, an optimizing compiler could determine that omitting Compute when Falloff returns zero would have no effect on the program. However, this is kind of hard to test, since if you have Compute output some debug data to determine whether it is running, the compiler would know not to omit it because of that debug info, resulting in sort of a Schrodinger's cat situation. I know the safe solution to this problem is just to add the special case, but I'm just curious.

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  • How do I select a random record efficiently in MySQL?

    - by user198729
    mysql> EXPLAIN SELECT * FROM urls ORDER BY RAND() LIMIT 1; +----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+ | 1 | SIMPLE | urls | ALL | NULL | NULL | NULL | NULL | 62228 | Using temporary; Using filesort | +----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+ The above doesn't qualify as efficient,how should I do it properly?

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  • MySQL, delete and index hint

    - by Manuel Darveau
    I have to delete about 10K rows from a table that has more than 100 million rows based on some criteria. When I execute the query, it takes about 5 minutes. I ran an explain plan (the delete query converted to select * since MySQL does not support explain delete) and found that MySQL uses the wrong index. My question is: is there any way to tell MySQL which index to use during delete? If not, what ca I do? Select to temp table then delete from temp table? Thank you!

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  • Tree iterator, can you optimize this any further?

    - by Ron
    As a follow up to my original question about a small piece of this code I decided to ask a follow up to see if you can do better then what we came up with so far. The code below iterates over a binary tree (left/right = child/next ). I do believe there is room for one less conditional in here (the down boolean). The fastest answer wins! The cnt statement can be multiple statements so lets make sure this appears only once The child() and next() member functions are about 30x as slow as the hasChild() and hasNext() operations. Keep it iterative <-- dropped this requirement as the recursive solution presented was faster. This is C++ code visit order of the nodes must stay as they are in the example below. ( hit parents first then the children then the 'next' nodes). BaseNodePtr is a boost::shared_ptr as thus assignments are slow, avoid any temporary BaseNodePtr variables. Currently this code takes 5897ms to visit 62200000 nodes in a test tree, calling this function 200,000 times. void processTree (BaseNodePtr current, unsigned int & cnt ) { bool down = true; while ( true ) { if ( down ) { while (true) { cnt++; // this can/will be multiple statesments if (!current->hasChild()) break; current = current->child(); } } if ( current->hasNext() ) { down = true; current = current->next(); } else { down = false; current = current->parent(); if (!current) return; // done. } } }

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  • C++ performance, optimizing compiler, empty function in .cpp

    - by Dodo
    I've a very basic class, name it Basic, used in nearly all other files in a bigger project. In some cases, there needs to be debug output, but in release mode, this should not be enabled and be a NOOP. Currently there is a define in the header, which switches a makro on or off, depending on the setting. So this is definetely a NOOP, when switched off. I'm wondering, if I have the following code, if a compiler (MSVS / gcc) is able to optimize out the function call, so that it is again a NOOP. (By doing that, the switch could be in the .cpp and switching will be much faster, compile/link time wise). --Header-- void printDebug(const Basic* p); class Basic { Basic() { simpleSetupCode; // this should be a NOOP in release, // but constructor could be inlined printDebug(this); } }; --Source-- // PRINT_DEBUG defined somewhere else or here #if PRINT_DEBUG void printDebug(const Basic* p) { // Lengthy debug print } #else void printDebug(const Basic* p) {} #endif

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  • [Python] Tips for making a fraction calculator code more optimized (faster and using less memory)

    - by Logic Named Joe
    Hello Everyone, Basicly, what I need for the program to do is to act a as simple fraction calculator (for addition, subtraction, multiplication and division) for the a single line of input, for example: -input: 1/7 + 3/5 -output: 26/35 My initial code: import sys def euclid(numA, numB): while numB != 0: numRem = numA % numB numA = numB numB = numRem return numA for wejscie in sys.stdin: wyjscie = wejscie.split(' ') a, b = [int(x) for x in wyjscie[0].split("/")] c, d = [int(x) for x in wyjscie[2].split("/")] if wyjscie[1] == '+': licz = a * d + b * c mian= b * d nwd = euclid(licz, mian) konA = licz/nwd konB = mian/nwd wynik = str(konA) + '/' + str(konB) print(wynik) elif wyjscie[1] == '-': licz= a * d - b * c mian= b * d nwd = euclid(licz, mian) konA = licz/nwd konB = mian/nwd wynik = str(konA) + '/' + str(konB) print(wynik) elif wyjscie[1] == '*': licz= a * c mian= b * d nwd = euclid(licz, mian) konA = licz/nwd konB = mian/nwd wynik = str(konA) + '/' + str(konB) print(wynik) else: licz= a * d mian= b * c nwd = euclid(licz, mian) konA = licz/nwd konB = mian/nwd wynik = str(konA) + '/' + str(konB) print(wynik) Which I reduced to: import sys def euclid(numA, numB): while numB != 0: numRem = numA % numB numA = numB numB = numRem return numA for wejscie in sys.stdin: wyjscie = wejscie.split(' ') a, b = [int(x) for x in wyjscie[0].split("/")] c, d = [int(x) for x in wyjscie[2].split("/")] if wyjscie[1] == '+': print("/".join([str((a * d + b * c)/euclid(a * d + b * c, b * d)),str((b * d)/euclid(a * d + b * c, b * d))])) elif wyjscie[1] == '-': print("/".join([str((a * d - b * c)/euclid(a * d - b * c, b * d)),str((b * d)/euclid(a * d - b * c, b * d))])) elif wyjscie[1] == '*': print("/".join([str((a * c)/euclid(a * c, b * d)),str((b * d)/euclid(a * c, b * d))])) else: print("/".join([str((a * d)/euclid(a * d, b * c)),str((b * c)/euclid(a * d, b * c))])) Any advice on how to improve this futher is welcome. Edit: one more thing that I forgot to mention - the code can not make use of any libraries apart from sys.

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  • SimpleDB as Denormalized DB

    - by Max
    In an environment where you have a relational database which handles all business transactions is it a good idea to utilise SimpleDB for all data queries to have faster and more lightweight search? So the master data storage would be a relational DB which is "replicated"/"transformed" into SimpleDB to provide very fast read only queries since no JOINS and complicated subselects are needed.

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  • Calculating Growth-Rates by applying log-differences

    - by mropa
    I am trying to transform my data.frame by calculating the log-differences of each column and controlling for the rows id. So basically I like to calculate the growth rates for each id's variable. So here is a random df with an id column, a time period colum p and three variable columns: df <- data.frame (id = c("a","a","a","c","c","d","d","d","d","d"), p = c(1,2,3,1,2,1,2,3,4,5), var1 = rnorm(10, 5), var2 = rnorm(10, 5), var3 = rnorm(10, 5) ) df id p var1 var2 var3 1 a 1 5.375797 4.110324 5.773473 2 a 2 4.574700 6.541862 6.116153 3 a 3 3.029428 4.931924 5.631847 4 c 1 5.375855 4.181034 5.756510 5 c 2 5.067131 6.053009 6.746442 6 d 1 3.846438 4.515268 6.920389 7 d 2 4.910792 5.525340 4.625942 8 d 3 6.410238 5.138040 7.404533 9 d 4 4.637469 3.522542 3.661668 10 d 5 5.519138 4.599829 5.566892 Now I have written a function which does exactly what I want BUT I had to take a detour which is possibly unnecessary and can be removed. However, somehow I am not able to locate the shortcut. Here is the function and the output for the posted data frame: fct.logDiff <- function (df) { df.log <- dlply (df, "code", function(x) data.frame (p = x$p, log(x[, -c(1,2)]))) list.nalog <- llply (df.log, function(x) data.frame (p = x$p, rbind(NA, sapply(x[,-1], diff)))) ldply (list.nalog, data.frame) } fct.logDiff(df) id p var1 var2 var3 1 a 1 NA NA NA 2 a 2 -0.16136569 0.46472004 0.05765945 3 a 3 -0.41216720 -0.28249264 -0.08249587 4 c 1 NA NA NA 5 c 2 -0.05914281 0.36999681 0.15868378 6 d 1 NA NA NA 7 d 2 0.24428771 0.20188025 -0.40279188 8 d 3 0.26646102 -0.07267311 0.47041227 9 d 4 -0.32372771 -0.37748866 -0.70417351 10 d 5 0.17405309 0.26683625 0.41891802 The trouble is due to the added NA-rows. I don't want to collapse the frame and reduce it, which would be automatically done by the diff() function. So I had 10 rows in my original frame and am keeping the same amount of rows after the transformation. In order to keep the same length I had to add some NAs. I have taken a detour by transforming the data.frame into a list, add the NAs, and afterwards transform the list back into a data.frame. That looks tedious. Any ideas to avoid the data.frame-list-data.frame class transformation and optimize the function?

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  • Whether to put method code in a VB.Net data storage class, or put it in a separate class?

    - by Alan K
    TLDR summary: (a) Should I include (lengthy) method code in classes which may spawn multiple objects at runtime, (b) does doing so cause memory usage bloat, (c) if so should I "outsource" the code to a class that is loaded only once and have the class methods call that, or alternatively (d) does the code get loaded only once with the object definition anyway and I'm worrying about nothing? ........ I don't know whether there's a good answer to this but if there is I haven't found it yet by searching in the usual places. In my VB.Net (2010 if it matters) WinForms project I have about a dozen or so class objects in an object model. Some of these are pretty simple and do little more than act as data storage repositories. The ones further up the object model, however, have an increasing number of methods. There can be a significant number of higher level objects in use though the exact number will be runtime dependent so I can't be more precise than that. As I was writing the method code for one of the top level ones I noticed that it was starting to get quite lengthy. Memory optimisation is something of a lost art given how much memory the average PC has these days but I don't want to make my application a resource hog. So my questions for anyone who knows .Net way better than I do (of which there will be many) are: Is the code loaded into memory with each instance of the class that's created? Alternatively is it loaded only once with the definition of the class, and all derived objects just refer to that definition? (I'm not really sure how that could be possible given that, for example, event handlers can be assigned dynamically, but no harm asking.) If the answer to the first one is yes, would it be more efficient to write the code in a "utility" object which is loaded only once and called from the real class' methods? Any thoughts appreciated.

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  • how to avoid temporaries when copying weakly typed object

    - by Truncheon
    Hi. I'm writing a series classes that inherit from a base class using virtual. They are INT, FLOAT and STRING objects that I want to use in a scripting language. I'm trying to implement weak typing, but I don't want STRING objects to return copies of themselves when used in the following way (instead I would prefer to have a reference returned which can be used in copying): a = "hello "; b = "world"; c = a + b; I have written the following code as a mock example: #include <iostream> #include <string> #include <cstdio> #include <cstdlib> std::string dummy("<int object cannot return string reference>"); struct BaseImpl { virtual bool is_string() = 0; virtual int get_int() = 0; virtual std::string get_string_copy() = 0; virtual std::string const& get_string_ref() = 0; }; struct INT : BaseImpl { int value; INT(int i = 0) : value(i) { std::cout << "constructor called\n"; } INT(BaseImpl& that) : value(that.get_int()) { std::cout << "copy constructor called\n"; } bool is_string() { return false; } int get_int() { return value; } std::string get_string_copy() { char buf[33]; sprintf(buf, "%i", value); return buf; } std::string const& get_string_ref() { return dummy; } }; struct STRING : BaseImpl { std::string value; STRING(std::string s = "") : value(s) { std::cout << "constructor called\n"; } STRING(BaseImpl& that) { if (that.is_string()) value = that.get_string_ref(); else value = that.get_string_copy(); std::cout << "copy constructor called\n"; } bool is_string() { return true; } int get_int() { return atoi(value.c_str()); } std::string get_string_copy() { return value; } std::string const& get_string_ref() { return value; } }; struct Base { BaseImpl* impl; Base(BaseImpl* p = 0) : impl(p) {} ~Base() { delete impl; } }; int main() { Base b1(new INT(1)); Base b2(new STRING("Hello world")); Base b3(new INT(*b1.impl)); Base b4(new STRING(*b2.impl)); std::cout << "\n"; std::cout << b1.impl->get_int() << "\n"; std::cout << b2.impl->get_int() << "\n"; std::cout << b3.impl->get_int() << "\n"; std::cout << b4.impl->get_int() << "\n"; std::cout << "\n"; std::cout << b1.impl->get_string_ref() << "\n"; std::cout << b2.impl->get_string_ref() << "\n"; std::cout << b3.impl->get_string_ref() << "\n"; std::cout << b4.impl->get_string_ref() << "\n"; std::cout << "\n"; std::cout << b1.impl->get_string_copy() << "\n"; std::cout << b2.impl->get_string_copy() << "\n"; std::cout << b3.impl->get_string_copy() << "\n"; std::cout << b4.impl->get_string_copy() << "\n"; return 0; } It was necessary to add an if check in the STRING class to determine whether its safe to request a reference instead of a copy: Script code: a = "test"; b = a; c = 1; d = "" + c; /* not safe to request reference by standard */ C++ code: STRING(BaseImpl& that) { if (that.is_string()) value = that.get_string_ref(); else value = that.get_string_copy(); std::cout << "copy constructor called\n"; } If was hoping there's a way of moving that if check into compile time, rather than run time.

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  • Counting context switches per thread

    - by Sarmun
    Is there a way to see how many context switches each thread generates? (both in and out if possible) Either in X/s, or to let it run and give aggregated data after some time. (either on linux or on windows) I have found only tools that give aggregated context-switching number for whole os or per process. My program makes many context switches (50k/s), probably a lot not necessary, but I am not sure where to start optimizing, where do most of those happen.

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  • Merging and splitting overlapping rectangles to produce non-overlapping ones

    - by uj
    I am looking for an algorithm as follows: Given a set of possibly overlapping rectangles (All of which are "not rotated", can be uniformly represented as (left,top,right,bottom) tuplets, etc...), it returns a minimal set of (non-rotated) non-overlapping rectangles, that occupy the same area. It seems simple enough at first glance, but prooves to be tricky (at least to be done efficiently). Are there some known methods for this/ideas/pointers? Methods for not necessarily minimal, but heuristicly small, sets, are interesting as well, so are methods that produce any valid output set at all.

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  • Reduce Processing Time of accessing databse

    - by medma
    hello all, I m making an app which requires remote databse connection. I want the values in picker from database but when I click on button to invoke picker it takes some time to fetch the values and displaying. Is there any way to do it fast? and also is there any way to reduce the time of transition between 2 views? Thanx

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