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  • Database indexes and their Big-O notation

    - by miket2e
    I'm trying to understand the performance of database indexes in terms of Big-O notation. Without knowing much about it, I would guess that: Querying on a primary key or unique index will give you a O(1) lookup time. Querying on a non-unique index will also give a O(1) time, albeit maybe the '1' is slower than for the unique index (?) Querying on a column without an index will give a O(N) lookup time (full table scan). Is this generally correct ? Will querying on a primary key ever give worse performance than O(1) ? My specific concern is for SQLite, but I'd be interested in knowing to what extent this varies between different databases too.

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  • which is better, creating a materialized view or a new table?

    - by Carson
    I have some demanding mysql queries that are needed to grap same up-to-date datasets from 5-7 mysql tables. I am thinking of creating a table or materialized view to gather all demanding columns from other tables, so as to increase performance. If I create that table, I may need to do extra insert / update / delete operation each time other tables updated. if I create materialized view, I am worrying if the performance can be greatly improved. Because data from other tables are changing very frequently. Most likely, the view may need to be created first everytime before selecting it. Any ideas? e.g. how to cache? other extra measures I can do?

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  • Is there a module that implements an efficient array type in Erlang?

    - by dsmith
    I have been looking for an array type with the following characteristics in Erlang. append(vector(), term()) O(1) nth(Idx, vector()) O(1) set(Idx, vector(), term()) O(1) insert(Idx, vector(), term()) O(N) remove(Idx, vector()) O(N) I normally use a tuple for this purpose, but the performance characteristics are not what I would want for large N. My testing shows the following performance characteristics... erlang:append_element/2 O(N). erlang:setelement/3 O(N). I have started on a module based on the clojure.lang.PersistentVector implementation, but if it's already been done I won't reinvent the wheel.

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  • What Use are Threads Outside of Parallel Problems on MultiCore Systesm?

    - by Robert S. Barnes
    Threads make the design, implementation and debugging of a program significantly more difficult. Yet many people seem to think that every task in a program that can be threaded should be threaded, even on a single core system. I can understand threading something like an MPEG2 decoder that's going to run on a multicore cpu ( which I've done ), but what can justify the significant development costs threading entails when you're talking about a single core system or even a multicore system if your task doesn't gain significant performance from a parallel implementation? Or more succinctly, what kinds of non-performance related problems justify threading? Edit Well I just ran across one instance that's not CPU limited but threads make a big difference: TCP, HTTP and the Multi-Threading Sweet Spot Multiple threads are pretty useful when trying to max out your bandwidth to another peer over a high latency network connection. Non-blocking I/O would use significantly less local CPU resources, but would be much more difficult to design and implement.

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  • Inline function and calling cost in C

    - by Eonil
    I'm making a vector/matrix library. (GCC, ARM NEON, iPhone) typedef struct{ float v[4]; } Vector; typedef struct{ Vector v[4]; } Matrix; I passed struct data as pointer to avoid performance degrade from data copying when calling function. So I thought designed function like this: void makeTranslation(const Vector* factor, Matrix* restrict result); But, if function is inline, is there any reason to pass values as pointer for performance? Do those variables copied too? How about register and caches? inline Matrix makeTranslation(Vector factor) __attribute__ ((always_inline)); How do you think about calling costs of each cases?

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  • How does c# type safety affect the garbage collection?

    - by Indeera
    I'm dealing with code that handles large buffers ( 100MB) and manipulation of these is done in unsafe blocks. I'd like to refactor these to avoid unsafe code. I'm wondering about the likely memory performance gains (positive/negative/neutral) before I embark on that. I assert that if the compiler can verify types, it could possibly generate better code and that could also mean good GC performance. Is this a valid assertion? What is your experience? Thanks.

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  • Icons in Silverlight: Images vs. Vectors

    - by Shnitzel
    I like using the vector drawing feature of Expression Blend to create icons. That way I can change colors easily on my icons without having to resort to an image editor. But my question is... Say I have a treeview control that has an icon next to each tree element and say I have hundreds of elements. Do you think using images is faster - performance wise than using vector icons? B/c I'd rather use vectors but I'm wondering about performance concerns.

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  • Is a program compiled with -g gcc flag slower than the same program compiled without -g?

    - by e271p314
    I'm compiling a program with -O3 for performance and -g for debug symbols (in case of crash I can use the core dump). One thing bothers me a lot, does the -g option results in a performance penalty? When I look on the output of the compilation with and without -g, I see that the output without -g is 80% smaller than the output of the compilation with -g. If the extra space goes for the debug symbols, I don't care about it (I guess) since this part is not used during runtime. But if for each instruction in the compilation output without -g I need to do 4 more instructions in the compilation output with -g than I certainly prefer to stop using -g option even at the cost of not being able to process core dumps. How to know the size of the debug symbols section inside the program and in general does compilation with -g creates a program which runs slower than the same code compiled without -g?

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  • Model of hql query firing at back end by hql engine?

    - by Maddy.Shik
    I want to understand how hibernate execute hql query internally or in other models how hql query engine works. Please suggest some good links for same? One of reason for reading is following problem. Class Branch { //lazy loaded @joincolumn(name="company_id") Company company; } Since company is heavy object so it is lazy loaded. now i have hql query "from Branch as branch where branch.Company.id=:companyId" my concern is that if for firing above query, hql engine has to retrieve company object then its a performance hit and i would prefer to add one more property in Branch class i.e. companyId. So in this case hql query would be "from Branch as branch where branch.companyId=:companyId" If hql engine first generate sql from hql followed by firing of sql query itself, then there should be no performance issue. Please let me know if problem is not understandable.

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  • Real pagination vs Next and Previous buttons

    - by Pablo
    By real pagination i mean something like this when in page 3: <<Previous 1 | 2 | {3} | 4 | 5 |...| 15 | Next>> By Next and Previous buttons i mean something like this when in page 3: <<previous Next>> Performance wise im sure the Previous and Next Buttons are better since unlike the real pagination it doesn't require over-querying the database. By over-querying the database i mean getting more information from the database than what you will need to display on the page. My theory is that the Previous and Next Buttons can drastically increase a site performance as it only requires the exact information you will need to display on a page, please correct me if im wrong on this. so, do users really have preference when it comes to this two options? is it just a Developer preference and its convenience? Which one do you prefer? why? *Note: Previous and Next Buttons are usually labeled Newer and older.

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  • iphone app photo upload to server from app threads

    - by user290380
    I have an app that needs to upload a least 5 photos to a server using API call available with the server. For that I am planning to use threads which will take care of photo upload and the main process can go on with the navigation of views etc. What I cant decide is whether it is OK to spawn five separate threads in iphone or use a single thread that will do the upload. In the later cases obviously it will become quite slow. Basically an HTTP POST request will be made to the server with the NSMutableURLRequest object using NSCOnnection. More threads mean more complexity and sync issues, but I can try to write code as neat as possible if it means better performance than a single thread which is simple but is a real stopper if performance is considered. Anybody with any experience in this kinda app. ??

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  • Combining FileStream and MemoryStream to avoid disk accesses/paging while receiving gigabytes of data?

    - by w128
    I'm receiving a file as a stream of byte[] data packets (total size isn't known in advance) that I need to store somewhere before processing it immediately after it's been received (I can't do the processing on the fly). Total received file size can vary from as small as 10 KB to over 4 GB. One option for storing the received data is to use a MemoryStream, i.e. a sequence of MemoryStream.Write(bufferReceived, 0, count) calls to store the received packets. This is very simple, but obviously will result in out of memory exception for large files. An alternative option is to use a FileStream, i.e. FileStream.Write(bufferReceived, 0, count). This way, no out of memory exceptions will occur, but what I'm unsure about is bad performance due to disk writes (which I don't want to occur as long as plenty of memory is still available) - I'd like to avoid disk access as much as possible, but I don't know of a way to control this. I did some testing and most of the time, there seems to be little performance difference between say 10 000 consecutive calls of MemoryStream.Write() vs FileStream.Write(), but a lot seems to depend on buffer size and the total amount of data in question (i.e the number of writes). Obviously, MemoryStream size reallocation is also a factor. Does it make sense to use a combination of MemoryStream and FileStream, i.e. write to memory stream by default, but once the total amount of data received is over e.g. 500 MB, write it to FileStream; then, read in chunks from both streams for processing the received data (first process 500 MB from the MemoryStream, dispose it, then read from FileStream)? Another solution is to use a custom memory stream implementation that doesn't require continuous address space for internal array allocation (i.e. a linked list of memory streams); this way, at least on 64-bit environments, out of memory exceptions should no longer be an issue. Con: extra work, more room for mistakes. So how do FileStream vs MemoryStream read/writes behave in terms of disk access and memory caching, i.e. data size/performance balance. I would expect that as long as enough RAM is available, FileStream would internally read/write from memory (cache) anyway, and virtual memory would take care of the rest. But I don't know how often FileStream will explicitly access a disk when being written to. Any help would be appreciated.

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  • What is the optimal number of threads for performing IO operations in java?

    - by marc
    In Goetz's "Java Concurrency in Practice", in a footnote on page 101, he writes "For computational problems like this that do not I/O and access no shared data, Ncpu or Ncpu+1 threads yield optimal throughput; more threads do not help, and may in fact degrade performance..." My question is, when performing I/O operations such as file writing, file reading, file deleting, etc, are there guidelines for the number of threads to use to achieve maximum performance? I understand this will be just a guide number, since disk speeds and a host of other factors play into this. Still, I'm wondering: can 20 threads write 1000 separate files to disk faster than 4 threads can on a 4-cpu machine?

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  • Archiving Database Tables using Java

    - by HonorGod
    My application demands archiving database tables between sybase and db2 and vice-a-versa and within(db2 to db2 and sybase to sybase) using java. I am trying to understand the best strategies around in terms performance, implementation, ease of use and scalability. Here is my current process - source and destination tables with the acceptable parameters (from java) are defined within xml. the application reads the source and destination configurations and execute them sequentially. destination is sometime optional when source is just deleting data from a specific table or when the source is just calling a stored procedure. dataset between source and destination is extremely large (in millions) From top of my head, it looks like I can define dependencies between multiple source and destination combination and have them execute in parallel in multiple treads. But will this improve any performance(i hope it will)? Are there any open-source frameworks for data archiving using java? Any other thoughts on the implements side will be really helpful. Thanks

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  • MS Access vs SQL Server and others ? Is it worth taking a db server when less than 2 Gb and only 20

    - by asksuperuser
    After my experiment with MSAccess vs MySQL which shows MS Access hugely overperforming Mysql odbc insert by a factor 1000% before I would do the same experiment with SQL Server I searched for some other's people and found this one: http://blog.nkadesign.com/2009/access-vs-sql-server-some-stats-part-1/ which says "As a side note, in this particular test, Access offers much better raw performance than SQL Server. In more complex scenarios it’s very likely that Access’ performance would degrade more than SQL Server, but it’s nice to see that Access isn’t a sloth." So is worth bother with some db server when data is less than 2 Gb and users are about 20 (knowing that MS Access theorically supports up to 255 concurrent users though practically it's around a dozen concurrent users only). Are there any real world studies that really compare MS Access with other db in these specific use case ? Because professionaly speaking I keep hearing people systematically recommend DB server from people who have never used Access just because they think DB Server can only perform better in every case which I used to think myself I confess.

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  • Single page app with high number of images working extremely slow on iOS8 safari/Webview

    - by NikhilWanpal
    We are working on a WebView (not WKWebView, yet) app, are are observing that the app runs extremely slow on iOS 8. The same app runs smooth on lower versions of OS like iOS7 and iOS6. So we tried it in safari on iOS8 and the performance is similar to iOS6 and 7. The app is filled with images and many are high resolution. While trying to trace the issue (trial and error!) we reduced the sizes and resolutions of the images and the performance improved, but it is still not at par with versions 6 and 7. We are unable to find any such issues reported elsewhere and are stuck. It would be great if we could get some pointers on this one.

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  • Best way to merge two identical ASPNET web sites?

    - by ase69s
    We have two websites which only diference is in the design (Diferent images, styles, layouts..etc) but the web structure of files and cs code is the same so we want to simplify its manteinance... The actual structure would be: DefaultA.aspx DefaultA.aspx.cs DefaultB.aspx DefaultB.aspx.cs LoginA.aspx LoginA.aspx.cs LoginB.aspx LoginB.aspx.cs One idea would be changing the design differencies at runtime depending of the origin website, but we dont like much this because performance, abstraction in designing them and url confusion... Another one is sharing the cs (both aspx inheriting and using the same cs) file but we never have done or seen it done in any website before so we wonder if its a good aproach... What do you think? Any other way better in terms of performance vs development-ease?

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  • How have your coding values changed since graduating?

    - by Matt
    We all walked out of school with the stars in our eyes and little experience in "real-world" programming. How have your opinions on programming as a craft changed since you've gained more experience away from academia? I've become more and more about design a la McConnell : wide use of encapsulation, quality code that gives you warm fuzzy feelings when you read it, maintainability over execution performance, etc..., whereas many of my co-workers have followed a different path of fewer middlemen layers getting in the way, code that is right out in the open and easier to locate, even if harder to read, and performance-centric designs. What have you learned about the craft of software design which has changed the way you approach coding since leaving the academic world?

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  • Indexed key vs indexed separate columns, which one is faster ?

    - by Jerry
    In MYSQL, from a pure performance perspective, if I have a table with large amount of data with 10/1 read/write ratio. is it faster in read/write performance to have 4 search criteria in separate columns and all indexed or have them combined in to one single string acting as a key and store in one indexed column ? e.g. say this table with 5 columns, first name, last name, sex, country and file where the first four columns will ALWAYS be given as a part of search parameters in a search or have a table with two columns, key and file. where the value of key can be john-smith-male-australia ?? I don't quite get the pros and cons. the point I try to stress is the fact that all parameters will be given.in a search.

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  • how to get apache log like debugging in Android logcat

    - by Nav
    I am currently using a webview to load a webpage which contains lots of javascript and having lots of trouble debugging what exactly gets loaded and when in the webview . Then I saw this post where the op seems to be using apache log to monitor webpage load events in his webview. Enhance webView performance (should be the same performance as native Web Browser) Can I get a similar utility plugin or anything so that I can use it with logcat in ddms view. If possible please provide some resource as to how to configure it for android.

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  • Do MySQL Locked Tables affect related Views?

    - by CogitoErgoSum
    So after reading http://stackoverflow.com/questions/1415602/performance-in-pdo-php-mysql-transaction-versus-direct-execution in regards to performance issues I was thinking about I did some research on locking tables in MySQL. On http://dev.mysql.com/doc/refman/5.0/en/table-locking.html Table locking enables many sessions to read from a table at the same time, but if a session wants to write to a table, it must first get exclusive access. During the update, all other sessions that want to access this particular table must wait until the update is done. This part struck me particularly becuase most of our queries will be updates rather than inserts. I was wondering if one created a table called foo on which all updates/inserts were carried out and then a view called foo_view (A copy of foo, or perhaps foo and a linkage of several other tables plus foo) on which all selects occured, would this locking issue still occur? That is, would SELECT quries on foo_view still have to wait for an update to finish on foo?

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