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perl-IO-AIO - Asynchronous Input/Output…  more info»


    IO::AIO - Asynchronous Input/Output

     use IO::AIO;

     aio_open "/etc/passwd", O_RDONLY, 0, sub {
        my $fh = shift
           or die "/etc/passwd: $!";

     aio_unlink "/tmp/file", sub { };

     aio_read $fh, 30000, 1024, $buffer, 0, sub {
        $_[0] > 0 or die "read error: $!";

     # version 2+ has request and group objects
     use IO::AIO 2;

     aioreq_pri 4; # give next request a very high priority
     my $req = aio_unlink "/tmp/file", sub { };
     $req->cancel; # cancel request if still in queue

     my $grp = aio_group sub { print "all stats done\n" };
     add $grp aio_stat "..." for ...;

     # AnyEvent integration
     open my $fh, "<&=" . IO::AIO::poll_fileno or die "$!";
     my $w = AnyEvent->io (fh => $fh, poll => 'r', cb => sub { IO::AIO::poll_cb });

     # Event integration
     Event->io (fd => IO::AIO::poll_fileno,
                poll => 'r',
                cb => \&IO::AIO::poll_cb);

     # Glib/Gtk2 integration
     add_watch Glib::IO IO::AIO::poll_fileno,
               in => sub { IO::AIO::poll_cb; 1 };

     # Tk integration
     Tk::Event::IO->fileevent (IO::AIO::poll_fileno, "",
                               readable => \&IO::AIO::poll_cb);

     # Danga::Socket integration
     Danga::Socket->AddOtherFds (IO::AIO::poll_fileno =>

    This module implements asynchronous I/O using whatever means your
    operating system supports.

    Asynchronous means that operations that can normally block your program
    (e.g. reading from disk) will be done asynchronously: the operation will
    still block, but you can do something else in the meantime. This is
    extremely useful for programs that need to stay interactive even when
    doing heavy I/O (GUI programs, high performance network servers etc.),
    but can also be used to easily do operations in parallel that are
    normally done sequentially, e.g. stat'ing many files, which is much
    faster on a RAID volume or over NFS when you do a number of stat
    operations concurrently.

    While most of this works on all types of file descriptors (for example
    sockets), using these functions on file descriptors that support
    nonblocking operation (again, sockets, pipes etc.) is very inefficient
    or might not work (aio_read fails on sockets/pipes/fifos). Use an event
    loop for that (such as the Event module): IO::AIO will naturally fit
    into such an event loop itself.

    In this version, a number of threads are started that execute your
    requests and signal their completion. You don't need thread support in
    perl, and the threads created by this module will not be visible to
    perl. In the future, this module might make use of the native aio
    functions available on many operating systems. However, they are often
    not well-supported or restricted (GNU/Linux doesn't allow them on normal
    files currently, for example), and they would only support aio_read and
    aio_write, so the remaining functionality would have to be implemented
    using threads anyway.

    Although the module will work with in the presence of other (Perl-)
    threads, it is currently not reentrant in any way, so use appropriate
    locking yourself, always call "poll_cb" from within the same thread, or
    never call "poll_cb" (or other "aio_" functions) recursively.

    This is a simple example that uses the Event module and loads
    /etc/passwd asynchronously:

       use Fcntl;
       use Event;
       use IO::AIO;

       # register the IO::AIO callback with Event
       Event->io (fd => IO::AIO::poll_fileno,
                  poll => 'r',
                  cb => \&IO::AIO::poll_cb);

       # queue the request to open /etc/passwd
       aio_open "/etc/passwd", O_RDONLY, 0, sub {
          my $fh = shift
             or die "error while opening: $!";

          # stat'ing filehandles is generally non-blocking
          my $size = -s $fh;

          # queue a request to read the file
          my $contents;
          aio_read $fh, 0, $size, $contents, 0, sub {
             $_[0] == $size
                or die "short read: $!";

             close $fh;

             # file contents now in $contents
             print $contents;

             # exit event loop and program

       # possibly queue up other requests, or open GUI windows,
       # check for sockets etc. etc.

       # process events as long as there are some:

    Every "aio_*" function creates a request. which is a C data structure
    not directly visible to Perl.

    If called in non-void context, every request function returns a Perl
    object representing the request. In void context, nothing is returned,
    which saves a bit of memory.

    The perl object is a fairly standard ref-to-hash object. The hash
    contents are not used by IO::AIO so you are free to store anything you
    like in it.

    During their existance, aio requests travel through the following
    states, in order:

        Immediately after a request is created it is put into the ready
        state, waiting for a thread to execute it.

        A thread has accepted the request for processing and is currently
        executing it (e.g. blocking in read).

        The request has been executed and is waiting for result processing.

        While request submission and execution is fully asynchronous, result
        processing is not and relies on the perl interpreter calling
        "poll_cb" (or another function with the same effect).

        The request results are processed synchronously by "poll_cb".

        The "poll_cb" function will process all outstanding aio requests by
        calling their callbacks, freeing memory associated with them and
        managing any groups they are contained in.

        Request has reached the end of its lifetime and holds no resources
        anymore (except possibly for the Perl object, but its connection to
        the actual aio request is severed and calling its methods will
        either do nothing or result in a runtime error).

    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 (and optional) $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 expecting a filehandle keep a copy of the filehandle
    internally until the request has finished.

    All functions return request objects of type IO::AIO::REQ that allow
    further manipulation of those requests while they are in-flight.

    The pathnames you pass to these routines *must* be absolute and encoded
    as octets. The reason for the former 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 anywhere in the program and then use relative paths.

    To encode pathnames as octets, either make sure you either: a) always
    pass in filenames you got from outside (command line, readdir etc.)
    without tinkering, b) are ASCII or ISO 8859-1, c) use the Encode module
    and encode your pathnames to the locale (or other) encoding in effect in
    the user environment, d) use Glib::filename_from_unicode on unicode
    filenames or e) use something else to ensure your scalar has the correct

    This works, btw. independent of the internal UTF-8 bit, which IO::AIO
    handles correctly wether it is set or not.

    $prev_pri = aioreq_pri [$pri]
        Returns the priority value that would be used for the next request
        and, if $pri is given, sets the priority for the next aio request.

        The default priority is 0, the minimum and maximum priorities are -4
        and 4, respectively. Requests with higher priority will be serviced

        The priority will be reset to 0 after each call to one of the
        "aio_*" functions.

        Example: open a file with low priority, then read something from it
        with higher priority so the read request is serviced before other
        low priority open requests (potentially spamming the cache):

           aioreq_pri -3;
           aio_open ..., sub {
              return unless $_[0];

              aioreq_pri -2;
              aio_read $_[0], ..., sub {

    aioreq_nice $pri_adjust
        Similar to "aioreq_pri", but subtracts the given value from the
        current priority, so the effect is cumulative.

    aio_open $pathname, $flags, $mode, $callback->($fh)
        Asynchronously open or create a file and call the callback with a
        newly created filehandle for the file.

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

        The $flags argument is a bitmask. See the "Fcntl" module for a list.
        They are the same as used by "sysopen".

        Likewise, $mode specifies the mode of the newly created file, if it
        didn't exist and "O_CREAT" has been given, just like perl's
        "sysopen", except that it is mandatory (i.e. use 0 if you don't
        create new files, and 0666 or 0777 if you do). Note that the $mode
        will be modified by the umask in effect then the request is being
        executed, so better never change the umask.


           aio_open "/etc/passwd", O_RDONLY, 0, sub {
              if ($_[0]) {
                 print "open successful, fh is $_[0]\n";
              } else {
                 die "open failed: $!\n";

    aio_close $fh, $callback->($status)
        Asynchronously close a file and call the callback with the result
        code. *WARNING:* although accepted, you should not pass in a perl
        filehandle here, as perl will likely close the file descriptor
        another time when the filehandle is destroyed. Normally, you can
        safely call perls "close" or just let filehandles go out of scope.

        This is supposed to be a bug in the API, so that might change. It's
        therefore best to avoid this function.

    aio_read $fh,$offset,$length, $data,$dataoffset, $callback->($retval)
    aio_write $fh,$offset,$length, $data,$dataoffset, $callback->($retval)
        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).

        The $data scalar *MUST NOT* be modified in any way while the request
        is outstanding. Modifying it can result in segfaults or WW3 (if the
        necessary/optional hardware is installed).

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

           aio_read $fh, 7, 15, $buffer, 0, sub {
              $_[0] > 0 or die "read error: $!";
              print "read $_[0] bytes: <$buffer>\n";

    aio_sendfile $out_fh, $in_fh, $in_offset, $length, $callback->($retval)
        Tries to copy $length bytes from $in_fh to $out_fh. It starts
        reading at byte offset $in_offset, and starts writing at the current
        file offset of $out_fh. Because of that, it is not safe to issue
        more than one "aio_sendfile" per $out_fh, as they will interfere
        with each other.

        This call tries to make use of a native "sendfile" syscall to
        provide zero-copy operation. For this to work, $out_fh should refer
        to a socket, and $in_fh should refer to mmap'able file.

        If the native sendfile call fails or is not implemented, it will be
        emulated, so you can call "aio_sendfile" on any type of filehandle
        regardless of the limitations of the operating system.

        Please note, however, that "aio_sendfile" can read more bytes from
        $in_fh than are written, and there is no way to find out how many
        bytes have been read from "aio_sendfile" alone, as "aio_sendfile"
        only provides the number of bytes written to $out_fh. Only if the
        result value equals $length one can assume that $length bytes have
        been read.

    aio_readahead $fh,$offset,$length, $callback->($retval)
        "aio_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.

        If that syscall doesn't exist (likely if your OS isn't Linux) it
        will be emulated by simply reading the data, which would have a
        similar effect.

    aio_stat $fh_or_path, $callback->($status)
    aio_lstat $fh, $callback->($status)
        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

        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, $callback->($status)
        Asynchronously unlink (delete) a file and call the callback with the
        result code.

    aio_mknod $path, $mode, $dev, $callback->($status)

        Asynchronously create a device node (or fifo). See mknod(2).

        The only (POSIX-) portable way of calling this function is:

           aio_mknod $path, IO::AIO::S_IFIFO | $mode, 0, sub { ...

    aio_link $srcpath, $dstpath, $callback->($status)
        Asynchronously create a new link to the existing object at $srcpath
        at the path $dstpath and call the callback with the result code.

    aio_symlink $srcpath, $dstpath, $callback->($status)
        Asynchronously create a new symbolic link to the existing object at
        $srcpath at the path $dstpath and call the callback with the result

    aio_readlink $path, $callback->($link)
        Asynchronously read the symlink specified by $path and pass it to
        the callback. If an error occurs, nothing or undef gets passed to
        the callback.

    aio_rename $srcpath, $dstpath, $callback->($status)
        Asynchronously rename the object at $srcpath to $dstpath, just as
        rename(2) and call the callback with the result code.

    aio_mkdir $pathname, $mode, $callback->($status)
        Asynchronously mkdir (create) a directory and call the callback with
        the result code. $mode will be modified by the umask at the time the
        request is executed, so do not change your umask.

    aio_rmdir $pathname, $callback->($status)
        Asynchronously rmdir (delete) a directory and call the callback with
        the result code.

    aio_readdir $pathname, $callback->($entries)
        Unlike the POSIX call of the same name, "aio_readdir" reads an
        entire directory (i.e. opendir + readdir + closedir). The entries
        will not be sorted, and will NOT include the "." and ".." entries.

        The callback a single argument which is either "undef" or an
        array-ref with the filenames.

    aio_load $path, $data, $callback->($status)
        This is a composite request that tries to fully load the given file
        into memory. Status is the same as with aio_read.

    aio_copy $srcpath, $dstpath, $callback->($status)
        Try to copy the *file* (directories not supported as either source
        or destination) from $srcpath to $dstpath and call the callback with
        the 0 (error) or -1 ok.

        This is a composite request that it creates the destination file
        with mode 0200 and copies the contents of the source file into it
        using "aio_sendfile", followed by restoring atime, mtime, access
        mode and uid/gid, in that order.

        If an error occurs, the partial destination file will be unlinked,
        if possible, except when setting atime, mtime, access mode and
        uid/gid, where errors are being ignored.

    aio_move $srcpath, $dstpath, $callback->($status)
        Try to move the *file* (directories not supported as either source
        or destination) from $srcpath to $dstpath and call the callback with
        the 0 (error) or -1 ok.

        This is a composite request that tries to rename(2) the file first.
        If rename files with "EXDEV", it copies the file with "aio_copy"
        and, if that is successful, unlinking the $srcpath.

    aio_scandir $path, $maxreq, $callback->($dirs, $nondirs)
        Scans a directory (similar to "aio_readdir") but additionally tries
        to efficiently separate the entries of directory $path into two sets
        of names, directories you can recurse into (directories), and ones
        you cannot recurse into (everything else, including symlinks to

        "aio_scandir" is a composite request that creates of many sub
        requests_ $maxreq specifies the maximum number of outstanding aio
        requests that this function generates. If it is "<= 0", then a
        suitable default will be chosen (currently 4).

        On error, the callback is called without arguments, otherwise it
        receives two array-refs with path-relative entry names.


           aio_scandir $dir, 0, sub {
              my ($dirs, $nondirs) = @_;
              print "real directories: @$dirs\n";
              print "everything else: @$nondirs\n";

        Implementation notes.

        The "aio_readdir" cannot be avoided, but "stat()"'ing every entry

        After reading the directory, the modification time, size etc. of the
        directory before and after the readdir is checked, and if they match
        (and isn't the current time), the link count will be used to decide
        how many entries are directories (if >= 2). Otherwise, no knowledge
        of the number of subdirectories will be assumed.

        Then entries will be sorted into likely directories (everything
        without a non-initial dot currently) and likely non-directories
        (everything else). Then every entry plus an appended "/." will be
        "stat"'ed, likely directories first. If that succeeds, it assumes
        that the entry is a directory or a symlink to directory (which will
        be checked seperately). This is often faster than stat'ing the entry
        itself because filesystems might detect the type of the entry
        without reading the inode data (e.g. ext2fs filetype feature).

        If the known number of directories (link count - 2) has been
        reached, the rest of the entries is assumed to be non-directories.

        This only works with certainty on POSIX (= UNIX) filesystems, which
        fortunately are the vast majority of filesystems around.

        It will also likely work on non-POSIX filesystems with reduced
        efficiency as those tend to return 0 or 1 as link counts, which
        disables the directory counting heuristic.

    aio_rmtree $path, $callback->($status)
        Delete a directory tree starting (and including) $path, return the
        status of the final "rmdir" only. This is a composite request that
        uses "aio_scandir" to recurse into and rmdir directories, and unlink
        everything else.

    aio_fsync $fh, $callback->($status)
        Asynchronously call fsync on the given filehandle and call the
        callback with the fsync result code.

    aio_fdatasync $fh, $callback->($status)
        Asynchronously call fdatasync on the given filehandle and call the
        callback with the fdatasync result code.

        If this call isn't available because your OS lacks it or it couldn't
        be detected, it will be emulated by calling "fsync" instead.

    aio_group $callback->(...)
        This is a very special aio request: Instead of doing something, it
        is a container for other aio requests, which is useful if you want
        to bundle many requests into a single, composite, request with a
        definite callback and the ability to cancel the whole request with
        its subrequests.

        Returns an object of class IO::AIO::GRP. See its documentation below
        for more info.


           my $grp = aio_group sub {
              print "all stats done\n";

           add $grp
              (aio_stat ...),
              (aio_stat ...),

    aio_nop $callback->()
        This is a special request - it does nothing in itself and is only
        used for side effects, such as when you want to add a dummy request
        to a group so that finishing the requests in the group depends on
        executing the given code.

        While this request does nothing, it still goes through the execution
        phase and still requires a worker thread. Thus, the callback will
        not be executed immediately but only after other requests in the
        queue have entered their execution phase. This can be used to
        measure request latency.

    IO::AIO::aio_busy $fractional_seconds, $callback->() *NOT EXPORTED*
        Mainly used for debugging and benchmarking, this aio request puts
        one of the request workers to sleep for the given time.

        While it is theoretically handy to have simple I/O scheduling
        requests like sleep and file handle readable/writable, the overhead
        this creates is immense (it blocks a thread for a long time) so do
        not use this function except to put your application under
        artificial I/O pressure.

    All non-aggregate "aio_*" functions return an object of this class when
    called in non-void context.

    cancel $req
        Cancels the request, if possible. Has the effect of skipping
        execution when entering the execute state and skipping calling the
        callback when entering the the result state, but will leave the
        request otherwise untouched. That means that requests that currently
        execute will not be stopped and resources held by the request will
        not be freed prematurely.

    cb $req $callback->(...)
        Replace (or simply set) the callback registered to the request.

    This class is a subclass of IO::AIO::REQ, so all its methods apply to
    objects of this class, too.

    A IO::AIO::GRP object is a special request that can contain multiple
    other aio requests.

    You create one by calling the "aio_group" constructing function with a
    callback that will be called when all contained requests have entered
    the "done" state:

       my $grp = aio_group sub {
          print "all requests are done\n";

    You add requests by calling the "add" method with one or more
    "IO::AIO::REQ" objects:

       $grp->add (aio_unlink "...");

       add $grp aio_stat "...", sub {
          $_[0] or return $grp->result ("error");

          # add another request dynamically, if first succeeded
          add $grp aio_open "...", sub {
             $grp->result ("ok");

    This makes it very easy to create composite requests (see the source of
    "aio_move" for an application) that work and feel like simple requests.

    * The IO::AIO::GRP objects will be cleaned up during calls to
    "IO::AIO::poll_cb", just like any other request.
    * They can be canceled like any other request. Canceling will cancel not
    only the request itself, but also all requests it contains.
    * They can also can also be added to other IO::AIO::GRP objects.
    * You must not add requests to a group from within the group callback
    (or any later time).

    Their lifetime, simplified, looks like this: when they are empty, they
    will finish very quickly. If they contain only requests that are in the
    "done" state, they will also finish. Otherwise they will continue to

    That means after creating a group you have some time to add requests.
    And in the callbacks of those requests, you can add further requests to
    the group. And only when all those requests have finished will the the
    group itself finish.

    add $grp ...
    $grp->add (...)
        Add one or more requests to the group. Any type of IO::AIO::REQ can
        be added, including other groups, as long as you do not create
        circular dependencies.

        Returns all its arguments.

        Cancel all subrequests and clears any feeder, but not the group
        request itself. Useful when you queued a lot of events but got a
        result early.

    $grp->result (...)
        Set the result value(s) that will be passed to the group callback
        when all subrequests have finished and set thre groups errno to the
        current value of errno (just like calling "errno" without an error
        number). By default, no argument will be passed and errno is zero.

    $grp->errno ([$errno])
        Sets the group errno value to $errno, or the current value of errno
        when the argument is missing.

        Every aio request has an associated errno value that is restored
        when the callback is invoked. This method lets you change this value
        from its default (0).

        Calling "result" will also set errno, so make sure you either set $!
        before the call to "result", or call c<errno> after it.

    feed $grp $callback->($grp)
        Sets a feeder/generator on this group: every group can have an
        attached generator that generates requests if idle. The idea behind
        this is that, although you could just queue as many requests as you
        want in a group, this might starve other requests for a potentially
        long time. For example, "aio_scandir" might generate hundreds of
        thousands "aio_stat" requests, delaying any later requests for a
        long time.

        To avoid this, and allow incremental generation of requests, you can
        instead a group and set a feeder on it that generates those
        requests. The feed callback will be called whenever there are few
        enough (see "limit", below) requests active in the group itself and
        is expected to queue more requests.

        The feed callback can queue as many requests as it likes (i.e. "add"
        does not impose any limits).

        If the feed does not queue more requests when called, it will be
        automatically removed from the group.

        If the feed limit is 0, it will be set to 2 automatically.


           # stat all files in @files, but only ever use four aio requests concurrently:

           my $grp = aio_group sub { print "finished\n" };
           limit $grp 4;
           feed $grp sub {
              my $file = pop @files
                 or return;

              add $grp aio_stat $file, sub { ... };

    limit $grp $num
        Sets the feeder limit for the group: The feeder will be called
        whenever the group contains less than this many requests.

        Setting the limit to 0 will pause the feeding process.

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

        See "poll_cb" for an example.

        Process some 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. The amount of events
        processed depends on the settings of "IO::AIO::max_poll_req" and

        If not all requests were processed for whatever reason, the
        filehandle will still be ready when "poll_cb" returns.

        Example: Install an Event watcher that automatically calls
        IO::AIO::poll_cb with high priority:

           Event->io (fd => IO::AIO::poll_fileno,
                      poll => 'r', async => 1,
                      cb => \&IO::AIO::poll_cb);

    IO::AIO::max_poll_reqs $nreqs
    IO::AIO::max_poll_time $seconds
        These set the maximum number of requests (default 0, meaning
        infinity) that are being processed by "IO::AIO::poll_cb" in one
        call, respectively the maximum amount of time (default 0, meaning
        infinity) spent in "IO::AIO::poll_cb" to process requests (more
        correctly the mininum amount of time "poll_cb" is allowed to use).

        Setting "max_poll_time" to a non-zero value creates an overhead of
        one syscall per request processed, which is not normally a problem
        unless your callbacks are really really fast or your OS is really
        really slow (I am not mentioning Solaris here). Using
        "max_poll_reqs" incurs no overhead.

        Setting these is useful if you want to ensure some level of
        interactiveness when perl is not fast enough to process all requests
        in time.

        For interactive programs, values such as 0.01 to 0.1 should be fine.

        Example: Install an Event watcher that automatically calls
        IO::AIO::poll_cb with low priority, to ensure that other parts of
        the program get the CPU sometimes even under high AIO load.

           # try not to spend much more than 0.1s in poll_cb
           IO::AIO::max_poll_time 0.1;

           # use a low priority so other tasks have priority
           Event->io (fd => IO::AIO::poll_fileno,
                      poll => 'r', nice => 1,
                      cb => &IO::AIO::poll_cb);

        If there are any outstanding requests and none of them in the result
        phase, 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.

        Waits until some requests have been handled.

        Returns the number of requests processed, but is otherwise strictly
        equivalent to:

           IO::AIO::poll_wait, IO::AIO::poll_cb

        Wait till all outstanding AIO requests have been handled.

        Strictly equivalent to:

           IO::AIO::poll_wait, IO::AIO::poll_cb
              while IO::AIO::nreqs;

    IO::AIO::min_parallel $nthreads
        Set the minimum number of AIO threads to $nthreads. The current
        default is 8, which means eight asynchronous operations can execute
        concurrently at any one time (the number of outstanding requests,
        however, is unlimited).

        IO::AIO starts threads only on demand, when an AIO request is queued
        and no free thread exists. Please note that queueing up a hundred
        requests can create demand for a hundred threads, even if it turns
        out that everything is in the cache and could have been processed
        faster by a single thread.

        It is recommended to keep the number of threads relatively low, as
        some Linux kernel versions will scale negatively with the number of
        threads (higher parallelity => MUCH higher latency). With current
        Linux 2.6 versions, 4-32 threads should be fine.

        Under most circumstances you don't need to call this function, as
        the module selects a default that is suitable for low to moderate

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

        While $nthreads are zero, aio requests get queued but not executed
        until the number of threads has been increased again.

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

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

    IO::AIO::max_idle $nthreads
        Limit the number of threads (default: 4) that are allowed to idle
        (i.e., threads that did not get a request to process within 10
        seconds). That means if a thread becomes idle while $nthreads other
        threads are also idle, it will free its resources and exit.

        This is useful when you allow a large number of threads (e.g. 100 or
        1000) to allow for extremely high load situations, but want to free
        resources under normal circumstances (1000 threads can easily
        consume 30MB of RAM).

        The default is probably ok in most situations, especially if thread
        creation is fast. If thread creation is very slow on your system you
        might want to use larger values.

    $oldmaxreqs = IO::AIO::max_outstanding $maxreqs
        This is a very bad function to use in interactive programs because
        it blocks, and a bad way to reduce concurrency because it is
        inexact: Better use an "aio_group" together with a feed callback.

        Sets the maximum number of outstanding requests to $nreqs. If you to
        queue up more than this number of requests, the next call to the
        "poll_cb" (and "poll_some" and other functions calling "poll_cb")
        function will block until the limit is no longer exceeded.

        The default value is very large, so there is no practical limit on
        the number of outstanding requests.

        You can still queue as many requests as you want. Therefore,
        "max_oustsanding" is mainly useful in simple scripts (with low
        values) or as a stop gap to shield against fatal memory overflow
        (with large values).

        Returns the number of requests currently in the ready, execute or
        pending states (i.e. for which their callback has not been invoked

        Example: wait till there are no outstanding requests anymore:

           IO::AIO::poll_wait, IO::AIO::poll_cb
              while IO::AIO::nreqs;

        Returns the number of requests currently in the ready state (not yet

        Returns the number of requests currently in the pending state
        (executed, but not yet processed by poll_cb).

    This module should do "the right thing" when the process using it forks:

    Before the fork, IO::AIO enters a quiescent state where no requests can
    be added in other threads and no results will be processed. After the
    fork the parent simply leaves the quiescent state and continues
    request/result processing, while the child frees the request/result
    queue (so that the requests started before the fork will only be handled
    in the parent). Threads will be started on demand until the limit set in
    the parent process has been reached again.

    In short: the parent will, after a short pause, continue as if fork had
    not been called, while the child will act as if IO::AIO has not been
    used yet.

    Per-request usage:

    Each aio request uses - depending on your architecture - around 100-200
    bytes of memory. In addition, stat requests need a stat buffer (possibly
    a few hundred bytes), readdir requires a result buffer and so on. Perl
    scalars and other data passed into aio requests will also be locked and
    will consume memory till the request has entered the done state.

    This is now awfully much, so queuing lots of requests is not usually a

    Per-thread usage:

    In the execution phase, some aio requests require more memory for
    temporary buffers, and each thread requires a stack and other data
    structures (usually around 16k-128k, depending on the OS).

    Known bugs will be fixed in the next release.


     Marc Lehmann <>

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