I have a C application which generates a lot of output and for which speed is critical. The program is basically a loop over a large (8-12GB) binary input file which must be read sequentially. In each iteration the read bytes are processed and output is generated and written to multiple files, but never to multiple files at the same time. So if you are at the point where output is generated and there are 4 output files you write to either file 0 or 1 or 2, or 3. At the end of the iteration I now write the output using fwrite()
, thereby waiting for the write operation to finish. The total number of output operations is large, up to 4 million per file, and output size of files ranges from 100mb to 3.5GB. The program runs on a basic multicore processor.
I want to write output in a separate thread and I know this can be done with
I have 2 type of questions, namely conceptual and code specific.
What would be the best approach. Note that the application should be portable to Linux, however, I don't see how that would be very important for my choice for 1-3, since I would write a wrapper around anything kernel/API specific. For me the most important criteria is speed. I have read that option 1 is not that likely to increase the performance of the program and that the kernel in any case creates new threads for the i/o operation, so then why not use option (2) immediately with the advantage that it seems easier to program (also since I did not succeed with option (1), see code issues below).
Note that I read https://stackoverflow.com/questions/3689759/how-can-i-run-a-specific-function-of-thread-asynchronously-in-c-c, but I dont see a motivation on what to use based on the nature of the application. So I hope somebody could provide me with some advice what would be best in my situation. Also from the book "Windows System Programming" by Johnson M. Hart, I know that the recommendation is using threads, mainly because of the simplicity. However, will it also be fastest?
This question involves the attempts I made so far to make asynchronous I/O work. I understand that its a big piece of code so that its not that easy to look into. In any case I would really appreciate any attempt.
To decrease execution time I try to write the output by means of a new thread using WINAPI via CreateFile()
with FILE_FLAGGED_OVERLAP
with an overlapped structure. I have created a sample program in which I try to get this to work. However, I encountered 2 problems:
The file is only opened in overlapped mode when I delete an already existing file (I have tried using CreateFile
in different modes (CREATE_ALWAYS
, CREATE_NEW
, OPEN_EXISTING
), but this does not help).
Only the first WriteFile
is executed asynchronously. The remainder of WriteFile
commands is synchronous. For this problem I already consulted http://support.microsoft.com/kb/156932. It seems that the problem I have is related to the fact that "any write operation to a file that extends its length will be synchronous". I've already tried to solve this by increasing file size/valid data size (commented region in code). However, I still do not get it to work. I'm aware of the fact that it could be the case that to get most out of asynchronous io i should CreateFile
with FILE_FLAG_NO_BUFFERING
, however I cannot get this to work as well.
Please note that the program creates a file of about 120mb in the path of execution. Also note that print statements "not ok" are not desireable, I would like to see "can do work in background" appear on my screen... What goes wrong here?
#include
#include
#include
#include
#include
#define ASYNC // remove this definition to run synchronously (i.e. using fwrite)
#ifdef ASYNC
struct _OVERLAPPED *pOverlapped;
HANDLE *pEventH;
HANDLE *pFile;
#else
FILE *pFile;
#endif
#define DIM_X 100
#define DIM_Y 150000
#define _PRINTERROR(msgs)\
{printf("file: %s, line: %d, %s",__FILE__,__LINE__,msgs);\
fflush(stdout);\
return 0;} \
#define _PRINTF(msgs)\
{printf(msgs);\
fflush(stdout);} \
#define _START_TIMER \
time_t time1,time2; \
clock_t clock1; \
time(&time1); \
printf("start time: %s",ctime(&time1)); \
fflush(stdout);
#define _END_TIMER\
time(&time2);\
clock1 = clock();\
printf("end time: %s",ctime(&time2));\
printf("elapsed processor time: %.2f\n",(((float)clock1)/CLOCKS_PER_SEC));\
fflush(stdout);
double aio_dat[DIM_Y] = {0};
double do_compute(double A,double B, int arr_len);
int main()
{
_START_TIMER;
const char *pName = "test1.bin";
DWORD dwBytesToWrite;
BOOL bErrorFlag = FALSE;
int j=0;
int i=0;
int fOverlapped=0;
#ifdef ASYNC
// create / open the file
pFile=CreateFile(pName,
GENERIC_WRITE, // open for writing
0, // share write access
NULL, // default security
CREATE_ALWAYS, // create new/overwrite existing
FILE_FLAG_OVERLAPPED, // | FILE_FLAG_NO_BUFFERING, // overlapped file
NULL); // no attr. template
// check whether file opening was ok
if(pFile==INVALID_HANDLE_VALUE){
printf("%x\n",GetLastError());
_PRINTERROR("file not opened properly\n");
}
// make the overlapped structure
pOverlapped = calloc(1,sizeof(struct _OVERLAPPED));
pOverlapped->Offset = 0;
pOverlapped->OffsetHigh = 0;
// put event handle in overlapped structure
if(!(pOverlapped->hEvent = CreateEvent(NULL,TRUE,FALSE,NULL))){
printf("%x\n",GetLastError());
_PRINTERROR("error in createevent\n");
}
#else
pFile = fopen(pName,"wb");
#endif
// create some output
for(j=0;j0){
//SetFilePointer(pFile,dwBytesToWrite,NULL,FILE_CURRENT);
//if(!(SetEndOfFile(pFile))){
// printf("%i\n",pFile);
// _PRINTERROR("error in set end of file\n");
//}
//SetFilePointer(pFile,-dwBytesToWrite,NULL,FILE_CURRENT);
//}
// write the bytes
if(!(bErrorFlag = WriteFile(pFile,aio_dat,dwBytesToWrite,NULL,pOverlapped))){
// check whether io pending or some other error
if(GetLastError()!=ERROR_IO_PENDING){
printf("lasterror: %x\n",GetLastError());
_PRINTERROR("error while writing file\n");
}
else{
fOverlapped=1;
}
}
else{
// if you get here output got immediately written; bad!
fOverlapped=0;
}
if(fOverlapped){
// do background, this msgs is what I want to see
for(j=0;jhEvent,INFINITE))!=WAIT_OBJECT_0){
_PRINTERROR("waiting did not succeed\n");
}
// reset event structure
if(!(ResetEvent(pOverlapped->hEvent))){
printf("%x\n",GetLastError());
_PRINTERROR("error in resetevent\n");
}
pOverlapped->Offset+=dwBytesToWrite;
#else
fwrite(aio_dat,sizeof(double),DIM_Y,pFile);
for(j=0;j
http://www.ibm.com/developerworks/linux/library/l-async/?ca=dgr-lnxw02aUsingPOISIXAIOAPI
http://www.flounder.com/asynchexplorer.htm#Asynchronous%20I/O
I know this is a big question and I would like to thank everybody in advance who takes the trouble reading it and perhaps even respond!