Kqueue : Generic Event notification


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A talk on Kqueue - at Gnunify

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Kqueue : Generic Event notification

  1. 1. Kqueue : Generic Event NotificationMahendra MMahendra_M@infosys.comhttp://www.infosys.comThis work is licensed under a Creative Commons Licensehttp://creativecommons.org/licenses/by-sa/2.5/
  2. 2. Agenda Traditional ways of multiplexing I/O Methods and issues in handling asynchronous events. Enter Kqueue The Kqueue architecture. Kqueue possibilities.
  3. 3. Traditional File/Socket handling Traditionally a single file can be handled as below /* No error checking here */ while ( i = read( fd, ... ) ) { do_something( with_this_data ); } The above case works fine for one file descriptor What about the case where we have two or more such descriptors ( for sockets ) and data can appear on any one of the socket at any given point of time ? – Basically, we need a mechanism for event driven applications. – This is a case for multiplexing I/O ( or events ) !!
  4. 4. Traditional I/O multiplexing Use select() and/or poll() select() or poll() pass a list of file descriptors to the kernel and wait for updates to happen. On receiving an update these calls have the list of file descriptors that got updated. File descriptors passed as a bitmap – with each bit being set or unset to represent a file descriptor. Select() and poll() can watch for read/write/exception events on the list of file descriptors. On return, the applications have to parse the entire bitmap to see which file descriptors have to be handled.
  5. 5. Traditional I/O multiplexing ( contd.. )fd_set fds;FD_ZERO( &fds );FD_SET( 5, &fds );n = select( 1, &fds, NULL, NULL, NULL );j = 0;for ( i = 0; (i < MAX) && (j < n); i++ ) { if ( FD_ISSET( i ) ) { read_something_from_socket( i ); j++; }}
  6. 6. Issues with select()/poll() Problems of scalability – Entire descriptor set has to be passed to each invocation of the system call ( specially with poll() - which uses an array ) – Massive copies from user space to kernel space and vice- versa – Not all descriptors may have activity all the time – On return, apps had to parse the entire list to check for updated descriptors. ( duplicated effort in kernel and app ) - O(N) activity – Results in inefficient memory usage within the kernel – In case of sleep, the list has to be parsed three times. sleep()/poll() can handle only file descriptors Coding was clunky for select() – Descriptor set is a bitmap of fixed size ( default 255 )
  7. 7. Other forms of interesting events Asynchronous signal notifications – Required in libraries that may want to be notified of signals Asynchronous timer expiry Asynchronous Read/Write ( aio_read(), aio_write() ) VFS changes Process state Changes Thread state changes Device driver notifications Anything else – that will require some asynchronous event notification – and the design allowing it.
  8. 8. Available solutions Linux 2.4 : SIGIO Sun Solaris : /dev/poll Linux 2.4 : /dev/epoll – Use ioctl() to manipulate the above. Even Microsoft Windows had something to offer. Kqueue – for BSD boxes. – We shall be talking about that now !!
  9. 9. Kqueue - Goals A generic event notification framework – File descriptors (read/write/exceptions), Signals, Asynchronous I/O ( not in OSFR ), Vnodes monitoring, process monitoring, Timer events. A single system call to handle all this. Capability to add new functionality. Efficient use of memory – Memory should be allocated as per need. – Should be able to register/receive interested number of events. – Events should be combined ( eg: data arriving over a socket ) Should be good replacements for standard calls. Should be possible to extend this functionality easily
  10. 10. Kqueue APIs int32_t kqueue( void ); – Creates a kernel queue. It is identical to a file descriptor. It can be deleted using the close() system call. int32_t kevent( kq, changes, nc, events, ne, timeout ); – To register events in the kernel queue – To receive events that occurred between consecutive calls. – Can simulate select(), poll() - Using different values of timeout – No need to store the event descriptors locally in the application. EV_SET( &event, ident, filter, flags, fflags, data, udata) – Used to prepare an event for registering in the kernel queue.
  11. 11. Kqueue sample codekq = kqueue();struct kevent kev[10];// Prepare an eventEV_SET( &kev[0], fd, EVFILT_READ, EV_ADD, 0, 0, 0);// Register an eventkevent( kq, &kev, 10, NULL, 0, timeout );// Receive eventsn = kevent( kq, NULL, 0, &kev, 10, timeout );for ( i = 0; i < n; i++ ) { // Do something}
  12. 12. Kqueue filter types READ : Returns when data is available for read from sockets, vnodes, fifos, pipes – ident = descriptor – Data = amount of data to be read – Flags = can be EOF etc. WRITE : Returns when it is possible to write to a descriptor ( ident ). – Data = amount of data that can be written VNODE : Returns when a file descriptor changes – fflags = delete, write, extend, attrib, link, rename, revoke
  13. 13. Kqueue filter types ( contd... ) PROC : Monitors a process – Ident = pid of the process to be monitored. – Fflags = Exit, fork, exec, track, trackerr SIGNAL : Returns when a signal is delivered to a process. – Ident = signal number – Data = no of times the signal was delivered. – Co-exists with signal() and sigaction() - and has a lower precedence. – Is delivered even if SIG_IGN is set for the signal TIMER : Establishes a timer – ident = timer id, Data = timeout in milliseconds, or no of times – Periodic by default unless ONESHOT is specified
  14. 14. Kqueue Flags ADD : To add an event to the queue ENABLE : To enable a disabled event DISABLE : To temporarily disable an event ( not deleted ) DELETE : Remove an event from the kernel queue ONESHOT : Cause the event to happen only once. CLEAR : Clear the state of the filter after it is received EOF : End – of – File ERROR : Specific errors.
  15. 15. Kqueue – Good things As you would have seen – It is extremely scalable in handling large file descriptors – Eliminates most of the deficiencies of select()/poll() – Currently, efforts are underway to migrate some popular daemons ( Apache ) to use Kqueue. It supports a wide range of events – not just file descriptors. Is easily extensible. New kqueue filters can be added very easily inside the BSD kernels. Opens up a lot of interesting possibilities.
  16. 16. Issues with Kqueue Kqueue calls are not part of POSIX specifications. – Most of the Unix systems do not implement it. – Breaks portability across Unices Third party code may still use select(), poll() etc. We may have to migrate this or allow these to co-exist Relatively new in the play field – Not time-tested.
  17. 17. References Kqueue: A generic and scalable event notification facility - Jonathan Lemon http://people.freebsd.org/~jlemon/papers/kqueue.pdf Man pages for kqueue, knote, kfilter_register Read the source, Luke !!
  18. 18. Finally ... Questions ?? Thanks to – Organizers for giving me a chance to speak at GNUnify 2006 – NetBSD and Linux developers who helped me during my work – To Infosys for sponsoring my visit to GNUnify 2006 Special thanks to YOU for listening... You can contact me at : Mahendra_M@infosys.com
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