Re: [Libguestfs] [nbdkit PATCH v3 0/5] Play with libnbd for nbdkit-nbd

On Wed, Jun 12, 2019 at 04:00:08PM -0500, Eric Blake wrote: This looks reasonable. There's a slow-down, but we may be able to fix that with a concurrent writer thread in future. The pipe-to-self trick is interesting and comes back to the issue of what is the thread model of libnbd. If we think that libnbd shouldn't create threads or do tricky Linux-specific stuff internally, then asking callers to do this is reasonable. If I understand how this works: * There is one "reader" thread (per connection) which is doing polls. Essentially this is a background thread as far as nbdkit is concerned, not a thread that nbdkit has created or knows about. * There is a pipe-to-self (per connection). When a command is submitted from an nbdkit-managed thread a byte is sent down this pipe. * The reader thread can either wake up because the socket is ready for read or write, or because of an indication on the pipe-to-self. * If the socket is ready for read or write then the normal nbd_aio_notify_read|write function is called which will move the state machine along, and also check for command completion. * If it's pipe-to-self indication then we will (after checking for command completion) check the direction flag again and reenter the poll. The reason for this is because when the other thread started an AIO command it might have changed the handle state. * The pipe ensures there is no race (I think). Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/