I have a text file, that could potentially have up to 1 Million lines in it, and I have a code for reading the file one line at a time, but this is taking a lot of time...lots and lots of time. Is there a method in c# to potentially optimize this process, and improve the reading. This is the code I'm using.
using(var file = new StreamReader(filePath))
{
while((line = file.ReadLine()) != null)
{
//do something.
}
}
Any suggestions on reading these lines in bulk or improving the process?
Thanks.
Thanks for all your comments. The issue had to do with the \do something where I was using the SmartXls library to write to Excel, which was causing the bottle neck. I have contacted the developers to address the issue. All the suggested solutions will work in other scenarios.
Well, this code would be simpler, if you're using .NET 4 or later you can use File.ReadLines:
foreach (var line in File.ReadLines())
{
// Do something
}
Note that this is not the same as ReadAllLines, as ReadLines returns an IEnumerable<string> which reads lines lazily, instead of reading the whole file in one go.
The effect at execution time will be broadly the same as your original code (it won't improve performance) - this is just simpler to read.
Fundamentally, if you're reading a large file, that can take a long time - but reading just a million lines shouldn't take "lots and lots of time". My guess is that whatever you're doing with the lines takes a long time. You might want to parallelize that, potentially using a producer/consumer queue (e.g. via BlockingCollection) or TPL Dataflow, or just use Parallel LINQ, Parallel.ForEach etc.
You should use a profiler to work out where the time is being spent. If you're reading from a very slow file system, then it's possible that it really is the reading which is taking the time. We don't have enough information to guide you on that, but you should be able to narrow it down yourself.
Try to use streamreader, see if it's faster
string filePath = "";
string fileData = "";
using (FileStream fs = new FileStream(filePath, FileMode.Open))
{
byte[] data = new byte[fs.Length];
fs.Seek(0, SeekOrigin.Begin);
fs.Read(data, 0, int.Parse(fs.Length.ToString()));
fileData = System.Text.Encoding.Unicode.GetString(data);
}
You can read more data at once using StreamReader's int ReadBlock(char[] buffer, int index, int count) rather than line by line. This avoids reading reading the entire file at once (File.ReadAllLines) but allows you to process larger chunks in RAM at a time.
To improve performance, consider performing whatever work you are currently doing in your loop by spawning another thread to handle the load.
Parallel.ForEach(file.ReadLines(), (line) =>
{
// do your business
});
If space is not an issue..Create a buffer of around 1mb..
using(BufferedStream bs=new BufferedStream(File.OpenRead(path),1024*1024))
{
int read=-1;
byte[] buffer=new byte[1024*1024];
while((read=bs.Read(buffer,0,buffer.Length))!=0)
{
//play with buffer
}
}
You can also use ReadAllLines(filepath) and load the file into an array of lines, like this:
string[] lines = System.IO.File.ReadAllLines(#"path");
Related
I have .txt file (contains more than million rows) which is around 1GB and I have one list of string, I am trying to remove all the rows from the file that exist in the list of strings and creating new file but it is taking long long time.
using (StreamReader reader = new StreamReader(_inputFileName))
{
using (StreamWriter writer = new StreamWriter(_outputFileName))
{
string line;
while ((line = reader.ReadLine()) != null)
{
if (!_lstLineToRemove.Contains(line))
writer.WriteLine(line);
}
}
}
How can I enhance the performance of my code?
You may get some speedup by using PLINQ to do the work in parallel, also switching from a list to a hash set will also greatly speed up the Contains( check. HashSet is thread safe for read-only operations.
private HashSet<string> _hshLineToRemove;
void ProcessFiles()
{
var inputLines = File.ReadLines(_inputFileName);
var filteredInputLines = inputLines.AsParallel().AsOrdered().Where(line => !_hshLineToRemove.Contains(line));
File.WriteAllLines(_outputFileName, filteredInputLines);
}
If it does not matter that the output file be in the same order as the input file you can remove the .AsOrdered() and get some additional speed.
Beyond this you are really just I/O bound, the only way to make it any faster is to get faster drives to run it on.
The code is particularly slow because the reader and writer never execute in parallel. Each has to wait for the other.
You can almost double the speed of file operations like this by having a reader thread and a writer thread. Put a BlockingCollection between them so you can communicate between the threads and limit how many rows you buffer in memory.
If the computation is really expensive (it isn't in your case), a third thread with another BlockingCollection doing the processing can help too.
Do not use buffered text routines. Use binary, unbuffered library routines and make your buffer size as big as possible. That's how to make it the fastest.
Have you considered using AWK
AWK is a very powerfull tool to process text files, you can find more information about how to filter lines that match a certain criteria Filter text with ASK
I am developing an application that reads lines from enormous text files (~2.5 GB), manipulates each line to a specific format, and then writes each line to a text file. Once the output text file has been closed, the program "Bulk Inserts" (SQL Server) the data into my database. It works, it's just slow.
I am using StreamReader and StreamWriter.
I'm pretty much stuck with reading one line at a time due to how I have to manipulate the text; however, I think that if I made a collection of lines and wrote out the collection every 1000 lines or so, it would speed things up at least a bit. The problem is (and this could be purely from my ignorance) that I cannot write a string[] using StreamWriter. After exploring StackOverflow and the rest of the internet, I came across File.WriteAllLines, which allows me to write string[]s to file, but I dont think my computer's memory can handle 2.5 GB of data being stored at one time. Also, the file is created, populated, and closed, so I would have to make a ton of smaller files to break down the 2 GB text files only to insert them into the database. So I would prefer to stay away from that option.
One hack job that I can think of is making a StringBuilder and using the AppendLine method to add each line to make a gigantic string. Then I could convert that StringBuilder to a string and write it to file.
But enough of my conjecturing. The method I have already implemented works, but I am wondering if anyone can suggest a better way to write chunks of data to a file?
Two things will increase the speed of output using StreamWriter.
First, make sure that the output file is on a different physical disk than the input file. If the input and output are on the same drive, then very often reads have to wait for writes and writes have to wait for reads. The disk can do only one thing at a time. Obviously not every read or write waits, because the StreamReader reads into a buffer and parses lines out of it, and the StreamWriter writes to a buffer and then pushes that to disk when the buffer is full. With the input and output files on separate drives, your reads and writes overlap.
What do I mean they overlap? The operating system will typically read ahead for you, so it can be buffering your file while you're processing. And when you do a write, the OS typically buffers that and writes it to the disk lazily. So there is some limited amount of asynchronous processing going on.
Second thing is to increase your buffer size. The default buffer size for StreamReader and StreamWriter is 4 kilobytes. So every 4K read or written incurs an operating system call. And, quite likely, a disk operation.
If you increase the buffer size to 64K, then you make 16 times fewer OS calls and 16 times fewer disk operations (not strictly true, but close). Going to a 64K buffer can cut more than 25% off your I/O time, and it's dead simple to do:
const int BufferSize = 64 * 1024;
var reader = new StreamReader(filename, Encoding.UTF8, true, BufferSize);
var writer = new StreamWriter(filename, Encoding.UTF8, BufferSize);
Those two things will speed your I/O more than anything else you can do. Trying to build buffers in memory using StringBuilder is just unnecessary work that does a bad job of duplicating what you can achieve by increasing the buffer size, and done incorrectly can easily make your program slower.
I would caution against buffer sizes larger than 64 KB. On some systems, you get marginally better results with buffers up to 256 KB, but on others you get dramatically worse performance--to the tune of 50% slower! I've never seen a system perform better with buffers larger than 256 KB than they do with buffers of 64 KB. In my experience, 64 KB is the sweet spot.
One other thing you can do is use three threads: a reader, a processor, and a writer. They communicate with queues. This can reduce your total time from (input-time + process-time + output-time) to something very close to max(input-time, process-time, output-time). And with .NET, it's really easy to set up. See my blog posts: Simple multithreading, Part 1 and Simple multithreading, Part 2.
According to the docs, StreamWriter does not automatically flush after every write by default, so it is buffered.
You could also use some of the lazy methods on the File class like so:
File.WriteAllLines("output.txt",
File.ReadLines("filename.txt").Select(ProcessLine));
where ProcessLine is declared like so:
private string ProcessLine(string input) {
string result = // do some calculation on input
return result;
}
Since ReadLines is lazy and WriteAllLines has a lazy overload, it will stream the file rather than attempting to read the whole thing.
What about building strings to write?
Something like
int cnt = 0;
StringBuilder s = new StringBuilder();
while(line = reader.readLine())
{
cnt++;
String x = (manipulate line);
s.append(x+"\n");
if(cnt%10000 == 0)
{
StreamWriter.write(s);
s=new StringBuilder();
}
}
Edited because comment below is right, should have used stringbuilder.
I'm parsing a 40MB CSV file.
It works nicely right now, and it's rather easy to parse, the only problem I have is performance, which of course is rather slow.
I'd like to know if there is a way I can improve this, as I only need to find by key I find and then stop looping, so if the entry is at the beginning of the file it finishes quickly, but if it's at the end it takes a while.
I could balance this by giving it a random start line, but the algorithm would still be O(n)... So I'm not sure if it's really worth it.
Is there a way I can improve my sequential parsing algorithm?
First: "Reading Huge CSV File" and "So I'm parsing a 40MB CSV file.". Ihave space delimited files here of 10+ GIGAbyte - what would you call those?
Also: the size of the file is irrelevant, you process them normally anyway line by line.
the only problem I have is performance, which of course is rather slow
Define. What do you think is slow? Parsing them is quite fast when done properly.
I'd like to know if there is a way I can improve this, as I only need to find by key I find and
then stop looping, so if the entry is at the beggining of the file it
finishes quickly, but if it's at the end it takes a while.
Do NOT use a CSV file? More than 60 years ago people invented databases for this.
Is there a way I can improve my secuential parsing algorithm?
YOu mean except pulling the parsing into a separate thread, and using an efficient code (which you may not have - noone knows).
Theoretically you could:
Read on one thread, with a decent buffer (less IO = faster)
Move field split into thread 2 (optional)
Use tasks to parse the fields (one per field per line) so you use all processors).
I am currently processing some (around 10.000) files (with sizes in double digit gigabte sadly) and... I go this way (Have to process them in a specific order) to use my computer fully.
That should give you a lot - and seriously, a 40mb file should load in 0.x seconds (0.5 - 0.6).
STILL that is very inefficient. Any reason you do not load the file into a database like all people do? CSV is good as some transport format, it sucks as a database.
Why don't you convert your csv to a normal database. Even sqlexpress will be fine.
Of course.
Say you order it alphabetically.
Then, start in the middle.
Each iteration, move to the middle of the top or bottom; whichever has the appropriate key.
This algorithm has O(log n).
This is called a "binary search," and is what "Mike Christianson" suggests in his comment.
Will suggest you to break one 40Mb File into smaller size few files.
And using Parallel.ForEach you could improve file processing performace
You can load the CSV into DataTable and use available operations that could be faster than looping through
Loading it to database and perform your operation on that is another option
This, I believe, is the fastest way to read a CSV file sequentially. There may be other ways to extract data from CSV, but if you limited to this approach, then this solution might work for you.
const int BUFFER_SIZE = 0x8000; //represents 32768 bytes
public unsafe void parseCSV(string filePath)
{
byte[] buffer = new byte[BUFFER_SIZE];
int workingSize = 0; //store how many bytes left in buffer
int bufferSize = 0; //how many bytes were read by the file stream
StringBuilder builder = new StringBuilder();
char cByte; //character representation of byte
using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read))
{
do
{
bufferSize = fs.Read(buffer, 0, BUFFER_SIZE);
workingSize = bufferSize;
fixed (byte* bufferPtr = buffer)
{
byte* workingBufferPtr = bufferptr;
while (workingSize-- > 0)
{
switch (cByte = (char)*workingBufferPtr++)
{
case '\n':
break;
case '\r':
case ',':
builder.ToString();
builder.Clear();
break;
default:
builder.Append(cByte);
break;
}
}
}
} while (bufferSize != 0);
}
}
Explanation:
Reading the file into a byte buffer. This will be done using the basic Filestream class, which gives access to the always fast Read()
Unsafe code. While I generally recommend not using unsafe code, when traversing any kind of buffer, using pointers can bring a speedup.
StringBuilder since we will be concatenating bytes into workable strings to test againt the key. StringBuilder is by far the fastest way to append bytes together and get a workable string out them.
Note that this method fairly complaint with RFC 4180, but if you deal with quotes, you can easily modify the code I posted to handle trimming.
I have a list of large text files to process. I wonder which is the fastest method, because reading line by line is slow.
I have something like that:
int cnt = this.listView1.Items.Count;
for (int i = 0; i < this.listView1.Items.Count; i++)
{
FileStream fs = new FileStream(this.listView1.Items[i].Text.ToString(), FileMode.Open, FileAccess.Read);
using (StreamReader reader = new StreamReader(fs))
while (reader.Peek() != -1)
{
//code part
}
}
I read about using blocks(like 100k lines each) via backgroundworkers with multiple threads would help, but I don't know how to implement it. Or if you have better ideas to improve the performance ... your expert advice would be appreciated.
First you need to decide what is your bottleneck - I/O (reading the files) or CPU (processing them). If it's I/O, reading multiple files concurrently is not going to help you much, the most you can achieve is have one thread read files, and another process them. The processing thread will be done before the next file is available.
I agree with #asawyer, if it's only 100MB, you should read the file entirely into memory in one swoop. You might as well read 5 of them entirely into memory, it's really not a big deal.
EDIT: After realizing all the files are on a single hard-drive, and that processing takes longer than reading the file.
You should have on thread reading the files sequentially. Once a file is read, fire up another thread that handles the processing, and start reading the second file in the first thread. Once the second file is read, fire up another thread, and so on.
You should make sure you don't fire more processing threads than the numbers of cores you have, but for starters just use the thread-pool for this, and optimize later.
You're missing a little bit of performance, because the time you spend reading the first file is not used for any processing. This should be neglible, reading 100MBs of data to memory shouldn't take more than a few seconds.
I assume that you are processing files line by line. You also said that loading of files is faster than processing them. There are few ways you can do what you need. One for example:
Create a thread that reads files one by one, line by line. Sequentially, because when doing this in parallel you'll only hammer your HDD and possibly get worse results. You can use Queue<string> for that. Use Queue.Enqueue() to add lines you've red.
Run another thread that is processing the queue. Use Queue.Dequeue() to get (and remove) lines from beginning of your queue. Process the line and write it to the output file. Eventually you can put processed lines in another queue or list and write them at once when you finish processing.
If order of lines in output file is not important you can create as many threads as you have CPU cores (or use ThreadPool class) to do the processing (that would speed up things significantly).
[Edit]
If order of lines in the output file is important you should limit line processing to one thread. Or process them in parallel using separate threads and implement mechanism that would control output order. For example you may do that by numbering lines you read from input file (the easy way) or processing lines by each thread in chunks of n-lines and writing output chunk by chunk in the same order you started processing threads.
here is a simple threading code you can use: (.Net 4)
//firstly get file paths from listview so you won't block the UI thread
List<string> filesPaths = new List<string>();
for (int i = 0; i < this.listView1.Items.Count; i++)
{
filesPaths.Add(listView1.Items[i].Text.ToString());
}
//this foreach loop will fire 50 threads at same time to read 50 files
Parallel.ForEach(filesPaths, new ParallelOptions() { MaxDegreeOfParallelism = 50 }, (filepath, i, j) =>
{
//read file contents
string data = File.ReadAllText(filepath);
//do whatever you want with the contents
});
not tested though...
How to raise the speed of the streamwriter to write a 83MB csv file.
I have inceresed the buffersize to 65536 but its also consuming more time. How to improve the speed.
StreamWriter writer =new streamWriter(
new FileStream(filePath, FileMode.CreateNew), Encoding.UTF8, 65536))
string str=string.Empty;
while((str = reader.ReadLine())!=null)
writer.WriteLine(str)}
writer.Close()
Depending on the number of lines your CSV file contains, you could possibly end up with a loop that executes millions of times. Not a good idea to access the disk that many times.
The easy way out (if you have memory) is to read the entire CSV file in a string[] in memory (using File.ReadAll() i think), do your processing and write it all once (File.WriteAll() i think).
This will greatly increase your performance.
The other way out is to use asynchronous read/writes, increase buffer size AND create a mechanism to read bigger chunks of data. Having a big buffer if you are only reading 1 line will not help you.
I would try raising the buffer even more. At that buffer size it's going to take over 8200 writes to create the whole file. Try a buffer around 256K or 512K.
Based on your comment showing what your code is actualy doing:
File.WriteAllText( filePath, reader.ReadToEnd() );