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linux-specific aio implemented using clone

Linux::AIO - linux-specific aio implemented using clone


Linux::AIO - linux-specific aio implemented using clone


 use Linux::AIO;

 # This module has been mostly superseded by IO::AIO.


This module has been mostly superseded by IO::AIO, which is API compatible.

This module implements asynchronous I/O using the means available to Linux - clone. It does not hook into the POSIX aio_* functions because Linux does not yet support these in the kernel (even as of 2.6.12, only O_DIRECT files are supported) and even if, it would only allow aio_read and write, not open, stat and so on.

Instead, in this module a number of (non-posix) threads are started that execute your read/writes and signal their completion. You don't need thread support in your libc or perl, and the threads created by this module will not be visible to the pthreads library.

NOTICE: the threads created by this module will automatically be killed when the thread calling min_parallel exits. Make sure you only ever call min_parallel from the same thread that loaded this module.

Although the module will work with in the presence of other threads, it is not reentrant, so use appropriate locking yourself.


All the aio_* calls are more or less thin wrappers around the syscall with the same name (sans aio_). The arguments are similar or identical, and they all accept an additional $callback argument which must be a code reference. This code reference will get called with the syscall return code (e.g. most syscalls return -1 on error, unlike perl, which usually delivers ``false'') as it's sole argument when the given syscall has been executed asynchronously.

All functions that expect a filehandle will also accept a file descriptor.

The filenames you pass to these routines must be absolute. The reason is that at the time the request is being executed, the current working directory could have changed. Alternatively, you can make sure that you never change the current working directory.

Linux::AIO::min_parallel $nthreads
Set the minimum number of AIO threads to $nthreads. The default is 1, which means a single asynchronous operation can be done at one time (the number of outstanding operations, however, is unlimited).

It is recommended to keep the number of threads low, as some linux kernel versions will scale negatively with the number of threads (higher parallelity => MUCH higher latency).

Under normal circumstances you don't need to call this function, as this module automatically starts a single async thread.

Linux::AIO::max_parallel $nthreads
Sets the maximum number of AIO threads to $nthreads. If more than the specified number of threads are currently running, kill them. This function blocks until the limit is reached.

This module automatically runs max_parallel 0 at program end, to ensure that all threads are killed and that there are no outstanding requests.

Under normal circumstances you don't need to call this function.

$fileno = Linux::AIO::poll_fileno
Return the request result pipe filehandle. This filehandle must be polled for reading by some mechanism outside this module (e.g. Event or select, see below). If the pipe becomes readable you have to call poll_cb to check the results.

See poll_cb for an example.

Process all outstanding events on the result pipe. You have to call this regularly. Returns the number of events processed. Returns immediately when no events are outstanding.

You can use Event to multiplex, e.g.:

   Event->io (fd => Linux::AIO::poll_fileno,

              poll => 'r', async => 1,

              cb => \&Linux::AIO::poll_cb);

Wait till the result filehandle becomes ready for reading (simply does a select on the filehandle. This is useful if you want to synchronously wait for some requests to finish).

See nreqs for an example.

Returns the number of requests currently outstanding.

Example: wait till there are no outstanding requests anymore:

   Linux::AIO::poll_wait, Linux::AIO::poll_cb

      while Linux::AIO::nreqs;

aio_open $pathname, $flags, $mode, $callback
Asynchronously open or create a file and call the callback with the filedescriptor (NOT a perl filehandle, sorry for that, but watch out, this might change in the future).

The pathname passed to aio_open must be absolute. See API NOTES, above, for an explanation.

The $mode argument is a bitmask. See the Fcntl module for a list. They are the same as used in sysopen.


   aio_open "/etc/passwd", O_RDONLY, 0, sub {

      if ($_[0] >= 0) {

         open my $fh, "<&=$_[0]";

         print "open successful, fh is $fh\n";


      } else {

         die "open failed: $!\n";



aio_close $fh, $callback
Asynchronously close a file and call the callback with the result code.

aio_read $fh,$offset,$length, $data,$dataoffset,$callback
aio_write $fh,$offset,$length, $data,$dataoffset,$callback
Reads or writes length bytes from the specified fh and offset into the scalar given by data and offset dataoffset and calls the callback without the actual number of bytes read (or -1 on error, just like the syscall).

Example: Read 15 bytes at offset 7 into scalar $buffer, strating at offset 0 within the scalar:

   aio_read $fh, 7, 15, $buffer, 0, sub {

      $_[0] >= 0 or die "read error: $!";

      print "read <$buffer>\n";


aio_readahead $fh,$offset,$length, $callback
Asynchronously reads the specified byte range into the page cache, using the readahead syscall.

readahead() populates the page cache with data from a file so that subsequent reads from that file will not block on disk I/O. The $offset argument specifies the starting point from which data is to be read and $length specifies the number of bytes to be read. I/O is performed in whole pages, so that offset is effectively rounded down to a page boundary and bytes are read up to the next page boundary greater than or equal to (off-set+length). aio_readahead() does not read beyond the end of the file. The current file offset of the file is left unchanged.

aio_stat $fh_or_path, $callback
aio_lstat $fh, $callback
Works like perl's stat or lstat in void context. The callback will be called after the stat and the results will be available using stat _ or -s _ etc...

The pathname passed to aio_stat must be absolute. See API NOTES, above, for an explanation.

Currently, the stats are always 64-bit-stats, i.e. instead of returning an error when stat'ing a large file, the results will be silently truncated unless perl itself is compiled with large file support.

Example: Print the length of /etc/passwd:

   aio_stat "/etc/passwd", sub {

      $_[0] and die "stat failed: $!";

      print "size is ", -s _, "\n";


aio_unlink $pathname, $callback
Asynchronously unlink (delete) a file and call the callback with the result code.

aio_fsync $fh, $callback
Asynchronously call fsync on the given filehandle and call the callback with the fsync result code.

aio_fdatasync $fh, $callback
Asynchronously call fdatasync on the given filehandle and call the callback with the fdatasync result code.


This module has been extensively tested in a large and very busy webserver for many years now.

   - aio_open gives a fd, but all other functions expect a perl filehandle.


Coro, the IO::AIO manpage.


 Marc Lehmann <>