04-23-2009
Read/Write a fairly large amount of data to a file as fast as possible
Hi,
I'm trying to figure out the best solution to the following problem, and I'm not
yet that much experienced like you. :-)
Basically I have to read a fairly large file, composed of "messages" , in order
to display all of them through an user interface (made with QT).
The messages that I write into the file, comes all at once from a socket, so
in oder to write it quickly without loosing any of them I plan to do the following:
- Create a list of preallocated pages (3-4 by default but the list grows if needed)
- Write the data that comes from the socket to the preallocated buffer
- Once a page is full, schedule a write with aio_write (AIO - Asyncronous IO).
- On callback schedule another one if any is full.
And so on..
This is the best I could come up with in the writing part, but if any of you have
a better idea, please let me know.
Now the problem comes when I have to read the file, at a later time, and display all
the messages in order to analyze them as fast as possible.
I first thought of mmap'ing the file in order to copy the data only once, from the file
to the kernel cache (if I understood correctly how mmap works internally) and then
accessing it from the application. But I'm not sure this can be done, or convenient, as
the file might be pretty big (2,3 Giga bytes, although I'm not sure about the magnitude).
Beside the kernel could unload the pages and many page faults could occurred.
So I discarded this idea.
I also thought about the opposite of what I do for writing but I'm not sure is a good idea.
The main problem is that I have to decode the messages before displaying them, as they are of different type and variable length. So reading the whole file at once and then decoding them to copy to another memory location seem time consuming to me. As it
requires 4 copies (disk -> kernel -> user space -> user space after decoding).
Anyway, now it's your turn. :-)
Any help would be appreciated.
Thanks.
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LEARN ABOUT SUSE
sendfile
SENDFILE(2) Linux Programmer's Manual SENDFILE(2)
NAME
sendfile - transfer data between file descriptors
SYNOPSIS
#include <sys/sendfile.h>
ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);
DESCRIPTION
sendfile() copies data between one file descriptor and another. Because this copying is done within the kernel, sendfile() is more effi-
cient than the combination of read(2) and write(2), which would require transferring data to and from user space.
in_fd should be a file descriptor opened for reading and out_fd should be a descriptor opened for writing.
If offset is not NULL, then it points to a variable holding the file offset from which sendfile() will start reading data from in_fd. When
sendfile() returns, this variable will be set to the offset of the byte following the last byte that was read. If offset is not NULL, then
sendfile() does not modify the current file offset of in_fd; otherwise the current file offset is adjusted to reflect the number of bytes
read from in_fd.
If offset is NULL, then data will be read from in_fd starting at the current file offset, and the file offset will be updated by the call.
count is the number of bytes to copy between the file descriptors.
Presently (Linux 2.6.9): in_fd, must correspond to a file which supports mmap(2)-like operations (i.e., it cannot be a socket); and out_fd
must refer to a socket.
Applications may wish to fall back to read(2)/write(2) in the case where sendfile() fails with EINVAL or ENOSYS.
RETURN VALUE
If the transfer was successful, the number of bytes written to out_fd is returned. On error, -1 is returned, and errno is set appropri-
ately.
ERRORS
EAGAIN Nonblocking I/O has been selected using O_NONBLOCK and the write would block.
EBADF The input file was not opened for reading or the output file was not opened for writing.
EFAULT Bad address.
EINVAL Descriptor is not valid or locked, or an mmap(2)-like operation is not available for in_fd.
EIO Unspecified error while reading from in_fd.
ENOMEM Insufficient memory to read from in_fd.
VERSIONS
sendfile() is a new feature in Linux 2.2. The include file <sys/sendfile.h> is present since glibc 2.1.
CONFORMING TO
Not specified in POSIX.1-2001, or other standards.
Other Unix systems implement sendfile() with different semantics and prototypes. It should not be used in portable programs.
NOTES
If you plan to use sendfile() for sending files to a TCP socket, but need to send some header data in front of the file contents, you will
find it useful to employ the TCP_CORK option, described in tcp(7), to minimize the number of packets and to tune performance.
In Linux 2.4 and earlier, out_fd could refer to a regular file, and sendfile() changed the current offset of that file.
SEE ALSO
mmap(2), open(2), socket(2), splice(2)
COLOPHON
This page is part of release 3.25 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2010-02-15 SENDFILE(2)