How to write a large buffer into a binary file in C fast

Question

I m trying to write huge amounts of data onto my SSD solid state drive   And by huge amounts I mean 80GB   I browsed the web for solutions  but the best I came up with was this    include  lt fstream gt  const unsigned long long size   64ULL 1024ULL 1024ULL  unsigned long long a size   int main         std  fstream myfile      myfile   std  fstream  file binary   std  ios  out   std  ios  binary         Here would be some error handling     for int i   0  i  lt  32    i             Some calculations to fill a           myfile write  char   amp a size sizeof unsigned long long              myfile close        Compiled with Visual Studio 2010 and full optimizations and run under Windows7 this program maxes out around 20MB s  What really bothers me is that Windows can copy files from an other SSD to this SSD at somewhere between 150MB s and 200MB s  So at least 7 times faster  That s why I think I should be able to go faster   Any ideas how I can speed up my writing

User · Answer

If you want to write fast to file streams then you could make stream the read buffer larger:

wfstream f;
const size_t nBufferSize = 16184;
wchar_t buffer[nBufferSize];
f.rdbuf()->pubsetbuf(buffer, nBufferSize);

Also, when writing lots of data to files it is sometimes faster to logically extend the file size instead of physically, this is because when logically extending a file the file system does not zero the new space out before writing to it. It is also smart to logically extend the file more than you actually need to prevent lots of file extentions. Logical file extention is supported on Windows by calling SetFileValidData or xfsctl with XFS_IOC_RESVSP64 on XFS systems.

User · Answer

Try the following  in order    Smaller buffer size  Writing  2 MiB at a time might be a good start  On my last laptop   512 KiB was the sweet spot  but I haven t tested on my SSD yet   Note  I ve noticed that very large buffers tend to decrease performance  I ve noticed speed losses with using 16-MiB buffers instead of 512-KiB buffers before  Use  open  or  topen if you want to be Windows-correct  to open the file  then use  write  This will probably avoid a lot of buffering  but it s not certain to  Using Windows-specific functions like CreateFile and WriteFile  That will avoid any buffering in the standard library

User · Answer

If you copy something from disk A to disk B in explorer  Windows employs DMA  That means for most of the copy process  the CPU will basically do nothing other than telling the disk controller where to put  and get data from  eliminating a whole step in the chain  and one that is not at all optimized for moving large amounts of data - and I mean hardware   What you do involves the CPU a lot  I want to point you to the  Some calculations to fill a    part  Which I think is essential  You generate a    then you copy from a   to an output buffer  thats what fstream  write does   then you generate again  etc   What to do  Multithreading   I hope you have a multi-core processor    fork  Use one thread to generate a   data Use the other to write data from a   to disk You will need two arrays a1   and a2   and switch between them You will need some sort of synchronization between your threads  semaphores  message queue  etc   Use lower level  unbuffered  functions  like the the WriteFile function mentioned by Mehrdad

User · Answer

I d suggest trying file mapping  I used mmapin the past  in a UNIX environment  and I was impressed by the high performance I could achieve

User · Answer

This did the job  in the year 2012     include  lt stdio h gt  const unsigned long long size   8ULL 1024ULL 1024ULL  unsigned long long a size    int main         FILE  pFile      pFile   fopen  file binary    wb        for  unsigned long long j   0  j  lt  1024    j             Some calculations to fill a           fwrite a  1  size sizeof unsigned long long   pFile             fclose pFile       return 0      I just timed 8GB in 36sec  which is about 220MB s and I think that maxes out my SSD  Also worth to note  the code in the question used one core 100   whereas this code only uses 2-5    Thanks a lot to everyone   Update  5 years have passed it s 2017 now  Compilers  hardware  libraries and my requirements have changed  That s why I made some changes to the code and did some new measurements   First up the code    include  lt fstream gt   include  lt chrono gt   include  lt vector gt   include  lt cstdint gt   include  lt numeric gt   include  lt random gt   include  lt algorithm gt   include  lt iostream gt   include  lt cassert gt   std  vector lt uint64 t gt  GenerateData std  size t bytes        assert bytes   sizeof uint64 t     0       std  vector lt uint64 t gt  data bytes   sizeof uint64 t        std  iota data begin    data end    0       std  shuffle data begin    data end    std  mt19937  std  random device             return data     long long option 1 std  size t bytes        std  vector lt uint64 t gt  data   GenerateData bytes        auto startTime   std  chrono  high resolution clock  now        auto myfile   std  fstream  file binary   std  ios  out   std  ios  binary       myfile write  char   amp data 0   bytes       myfile close        auto endTime   std  chrono  high resolution clock  now         return std  chrono  duration cast lt std  chrono  milliseconds gt  endTime - startTime  count       long long option 2 std  size t bytes        std  vector lt uint64 t gt  data   GenerateData bytes        auto startTime   std  chrono  high resolution clock  now        FILE  file   fopen  file binary    wb        fwrite  amp data 0   1  bytes  file       fclose file       auto endTime   std  chrono  high resolution clock  now         return std  chrono  duration cast lt std  chrono  milliseconds gt  endTime - startTime  count       long long option 3 std  size t bytes        std  vector lt uint64 t gt  data   GenerateData bytes        std  ios base  sync with stdio false       auto startTime   std  chrono  high resolution clock  now        auto myfile   std  fstream  file binary   std  ios  out   std  ios  binary       myfile write  char   amp data 0   bytes       myfile close        auto endTime   std  chrono  high resolution clock  now         return std  chrono  duration cast lt std  chrono  milliseconds gt  endTime - startTime  count       int main         const std  size t kB   1024      const std  size t MB   1024   kB      const std  size t GB   1024   MB       for  std  size t size   1   MB  size  lt   4   GB  size    2  std  cout  lt  lt   option1     lt  lt  size   MB  lt  lt   MB     lt  lt  option 1 size   lt  lt   ms   lt  lt  std  endl      for  std  size t size   1   MB  size  lt   4   GB  size    2  std  cout  lt  lt   option2     lt  lt  size   MB  lt  lt   MB     lt  lt  option 2 size   lt  lt   ms   lt  lt  std  endl      for  std  size t size   1   MB  size  lt   4   GB  size    2  std  cout  lt  lt   option3     lt  lt  size   MB  lt  lt   MB     lt  lt  option 3 size   lt  lt   ms   lt  lt  std  endl       return 0      This code compiles with Visual Studio 2017 and g   7 2 0  a new requirements   I ran the code with two setups    Laptop  Core i7  SSD  Ubuntu 16 04  g   Version 7 2 0 with -std c  11 -march native -O3 Desktop  Core i7  SSD  Windows 10  Visual Studio 2017 Version 15 3 1 with  Ox  Ob2  Oi  Ot  GT  GL  Gy   Which gave the following measurements  after ditching the values for 1MB  because they were obvious outliers     Both times option1 and option3 max out my SSD  I didn t expect this to see  because option2 used to be the fastest code on my old machine back then   TL DR  My measurements indicate to use std  fstream over FILE

User · Answer

Try to use memory-mapped files

User · Answer

The best solution is to implement an async writing with double buffering   Look at the time line   ------------------------------------------------ gt  FF WWWWWWWW FF WWWWWWWW FF WWWWWWWW FF WWWWWWWW    The  F  represents time for buffer filling  and  W  represents time for writing buffer to disk  So the problem in wasting time between writing buffers to file  However  by implementing writing on a separate thread  you can start filling the next buffer right away like this   ------------------------------------------------ gt   main thread  fills buffers  FF ff       FF       ff                 ------------------------------------------------ gt   writer thread     WWWWWWWW wwwwwwww WWWWWWWW wwwwwwww    F - filling 1st buffer f - filling 2nd buffer W - writing 1st buffer to file w - writing 2nd buffer to file   - wait while operation is completed  This approach with buffer swaps is very useful when filling a buffer requires more complex computation  hence  more time   I always implement a CSequentialStreamWriter class that hides asynchronous writing inside  so for the end-user the interface has just Write function s    And the buffer size must be a multiple of disk cluster size  Otherwise  you ll end up with poor performance by writing a single buffer to 2 adjacent disk clusters   Writing the last buffer  When you call Write function for the last time  you have to make sure that the current buffer is being filled should be written to disk as well  Thus CSequentialStreamWriter should have a separate method  let s say Finalize  final buffer flush   which should write to disk the last portion of data   Error handling  While the code start filling 2nd buffer  and the 1st one is being written on a separate thread  but write fails for some reason  the main thread should be aware of that failure   ------------------------------------------------ gt   main thread  fills buffers  FF fX  ------------------------------------------------ gt   writer thread     X    Let s assume the interface of a CSequentialStreamWriter has Write function returns bool or throws an exception  thus having an error on a separate thread  you have to remember that state  so next time you call Write or Finilize on the main thread  the method will return False or will throw an exception  And it does not really matter at which point you stopped filling a buffer  even if you wrote some data ahead after the failure - most likely the file would be corrupted and useless

User · Answer

I see no difference between std  stream FILE device  Between buffering and non buffering  Also note   SSD drives  quot tend quot  to slow down  lower transfer rates  as they fill up  SSD drives  quot tend quot  to slow down  lower transfer rates  as they get older  because of non working bits    I am seeing the code run in 63 secondds  Thus a transfer rate of  260M s  my SSD look slightly faster than yours   64   1024   1024   8   sizeof unsigned long long       32   Chunks      16G   16G 63   260M s  I get a no increase by moving to FILE  from std  fstream   include  lt stdio h gt   using namespace std   int main              FILE  stream   fopen  quot binary quot    quot w quot         for int loop 0 loop  lt  32   loop                 fwrite a  sizeof unsigned long long   size  stream             fclose stream       So the C   stream are working as fast as the underlying library will allow  But I think it is unfair comparing the OS to an application that is built on-top of the OS  The application can make no assumptions  it does not know the drives are SSD  and thus uses the file mechanisms of the OS for transfer  While the OS does not need to make any assumptions  It can tell the types of the drives involved and use the optimal technique for transferring the data  In this case a direct memory to memory transfer  Try writing a program that copies 80G from 1 location in memory to another and see how fast that is  Edit I changed my code to use the lower level calls  ie no buffering   include  lt fcntl h gt   include  lt unistd h gt    const unsigned long long size   64ULL 1024ULL 1024ULL  unsigned long long a size   int main         int data   open  quot test quot   O WRONLY   O CREAT  0777       for int loop   0  loop  lt  32    loop                   write data  a  size   sizeof unsigned long long                 close data      This made no diffference  NOTE  My drive is an SSD drive if you have a normal drive you may see a difference between the two techniques above  But as I expected non buffering and buffering  when writting large chunks greater than buffer size  make no difference  Edit 2  Have you tried the fastest method of copying files in C   int main         std  ifstream  input  quot input quot        std  ofstream  output  quot ouptut quot         output  lt  lt  input rdbuf

User · Answer

fstreams are not slower than C streams  per se  but they use more CPU  especially if buffering is not properly configured   When a CPU saturates  it limits the I O rate   At least the MSVC 2015 implementation copies 1 char at a time to the output buffer when a stream buffer is not set  see streambuf  xsputn   So make sure to set a stream buffer   0    I can get a write speed of 1500MB s  the full speed of my M 2 SSD  with fstream using this code    include  lt iostream gt   include  lt fstream gt   include  lt chrono gt   include  lt memory gt   include  lt stdio h gt   ifdef   linux    include  lt unistd h gt   endif using namespace std  using namespace std  chrono  const size t sz   512   1024   1024  const int numiter   20  const size t bufsize   1024   1024  int main int argc  char  argv      unique ptr lt char   gt  data new char sz      unique ptr lt char   gt  buf new char bufsize      for  size t p   0  p  lt  sz  p    16        memcpy  amp data p    BINARY DATA        16         unlink  file binary      int64 t total   0    if  argc  lt  2    strcmp argv 1    fopen      0        cout  lt  lt   fstream mode n       ofstream myfile  file binary   ios  out   ios  binary       if   myfile          cerr  lt  lt   open failed n   return 1            myfile rdbuf  - gt pubsetbuf buf get    bufsize      IMPORTANT     for  int i   0  i  lt  numiter    i          auto tm1   high resolution clock  now          myfile write data get    sz         if   myfile          cerr  lt  lt   write failed n         auto tm    duration cast lt milliseconds gt  high resolution clock  now   - tm1  count           cout  lt  lt  tm  lt  lt    ms n         total    tm            myfile close          else       cout  lt  lt   fopen mode n       FILE  pFile   fopen  file binary    wb        if   pFile          cerr  lt  lt   open failed n   return 1            setvbuf pFile  buf get     IOFBF  bufsize      NOT important     auto tm1   high resolution clock  now        for  int i   0  i  lt  numiter    i          auto tm1   high resolution clock  now          if  fwrite data get    sz  1  pFile     1          cerr  lt  lt   write failed n         auto tm    duration cast lt milliseconds gt  high resolution clock  now   - tm1  count           cout  lt  lt  tm  lt  lt    ms n         total    tm            fclose pFile       auto tm2   high resolution clock  now          cout  lt  lt   Total     lt  lt  total  lt  lt    ms     lt  lt   sz numiter   1000    1024 0   1024   total    lt  lt    MB s n       I tried this code on other platforms  Ubuntu  FreeBSD  and noticed no I O rate differences  but a CPU usage difference of about 8 1  fstream used 8 times more CPU   So one can imagine  had I a faster disk  the fstream write would slow down sooner than the stdio version

User · Answer

Try using open   write   close   API calls and experiment with the output buffer size  I mean do not pass the whole  many-many-bytes  buffer at once  do a couple of writes  i e   TotalNumBytes   OutBufferSize    OutBufferSize can be from 4096 bytes to megabyte   Another try - use WinAPI OpenFile CreateFile and use this MSDN article to turn off buffering  FILE FLAG NO BUFFERING   And this MSDN article on WriteFile   shows how to get the block size for the drive to know the optimal buffer size   Anyway  std  ofstream is a wrapper and there might be blocking on I O operations  Keep in mind that traversing the entire N-gigabyte array also takes some time  While you are writing a small buffer  it gets to the cache and works faster

User · Answer

im compiling my program in  gcc in GNU Linux and mingw in win 7 and win xp and worked good  you can use my program and to create a 80 GB file just change the line 33 to  makeFile  Text txt  1024 8192000     when exit the program the file will be destroyed then check the file when it is running   to have the program that you want just change the program  firt one is the windows program and the second is for GNU Linux  http   mustafajf persiangig com Projects File WinFile cpp  http   mustafajf persiangig com Projects File File cpp

User · Answer

Could you use FILE  instead  and the measure the performance you ve gained  A couple of options is to use fwrite write instead of fstream    include  lt stdio h gt   int main        FILE   pFile    char buffer        x     y     z       pFile   fopen    myfile bin     w b       fwrite  buffer   1   sizeof buffer    pFile      fclose  pFile     return 0      If you decide to use write  try something similar    include  lt unistd h gt   include  lt fcntl h gt   int main void        int filedesc   open  testfile txt   O WRONLY   O APPEND        if  filedesc  lt  0            return -1             if  write filedesc   This will be output to testfile txt n   36     36            write 2   There was an error writing to testfile txt n   43           return -1             return 0      I would also advice you to look into memory map  That may be your answer  Once I had to process a 20GB file in other to store it in the database  and the file as not even opening  So the solution as to utilize moemory map  I did that in Python though

[c++] How to write a large buffer into a binary file in C++, fast?

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