On 03/07/2011 05:59 PM, Anthony Liguori wrote:
How do async commands work? You mentioned them when talking about
QAPI but it's not obvious to me that there is any "native" support for
async commands?
Async commands are interesting..
Would there be anything in them other than starting each command in its
own thread? If it then drops the right locks it can execute in parallel
with other commands, if it doesn't, then it's synchronous (and
presumably doesn't depend on external or guest events).
Generating async commands with QAPI is easy. Errors are propagated
now so the callback would just need to take the return value and error
type. It would need to be a special callback type for each function
but that's easy with the code generator.
Async commands are problematic from the client perspective though. I
think there are two relevant use-cases for client software. There's
the probably most dominant, I don't really care about small pauses
users who are going to be executing the RPCs synchronously either
using libqmp or Python. They may use threading to have some
parallelism but generally, the expectation is going to be that QEMU
doesn't introduce too much delay in processing a given RPC.
It's more for intrinsically long-running commands like migration. These
can be split into a start-migration and cancel-migration command (and it
also helps for querying status, which doesn't follow naturally for an
asychronous command).
OTOH, there are users that will be purely event driven that will treat
every RPC asynchronously. In both cases, it's more or less
all-or-nothing. Having some commands delay for really long periods of
time means that you either force users to treat some commands
specially, or you force all users into an event driven model.
And of course, if you have async commands, you need to cancel
commands, and then the context is within a single QMP session which
means that you need to figure out what to do if you drop the session
while an async command is executing. For instance, if a management
tool executes the migrate command, and it's implemented as an async
command, if the management tool loses it's connection, should
migration be automatically cancelled?
Sounds reasonable. The rules could be:
- a command (all commands are async) takes a ref on the monitor context
- if the session is dropped, call the current command's cancel callback
as a result, a dropped session causes the command to either cancel or
complete.
I'm really on the fence about async commands. At the moment, I'm
leaning towards just not every implementing them.
Do we have a list of candidates for async commands?
--
error compiling committee.c: too many arguments to function
