Re: [Libguestfs] [nbdkit PATCH 1/2] plugins: Add .thread_model callback

On 5/20/19 8:36 AM, Richard W.M. Jones wrote: So far, we've come up with two potential models to add: VDDK's model where only the main thread is used but where parallel connections can still be made (that is slightly different than SERIALIZE_ALL_REQUESTS, not sure where it would lie conceptually between existing models), and a model that allows parallel requests from a single connection but delays responses to be in order (perhaps named SERIALIZE_RESPONSES, conceptually lies between SERIALIZE_REQUESTS and PARALLEL). We could add a utility helper function that, when given two constants, returns -1/0/1 depending on whether the first parameter is stricter/equal/looser than the second, to at least ease any out-of-order defined integer values back into a hierarchical scheme. I also wonder if we need a utility function for a plugin to learn which threading model ACTUALLY got selected, usable during .open or later (if the plugin supports PARALLEL but is run with a filter that forces SERIALIZE_ALL_REQUESTS, the plugin might be able to optimize out the use of a mutex). I thought about that, but my initial idea was to instead just declare that attempts at a more parallel model are just silently ignored (with at most a debug message visible during -fv). -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org