Re: [GIT PULL] kdbus for 4.1-rc1

Havoc Pennington
2015-04-15T16:45:48Z

On Wed, Apr 15, 2015 at 12:00 PM, Rik van Riel <riel@redhat.com> wrote:
> On 04/15/2015 07:06 AM, One Thousand Gnomes wrote:
>
>>> that anyone here does either. In the many years I've spent working on
>>> this, dbus has seemed to be odd, and strange, to the way that the kernel
>>> has normally worked, because it is. And that's not a bad thing, it's
>>> just different, and for us to support real needs and requirements of our
>>> users, is the requirement of the Linux kernel.
>>
>> There are I think a set of intertwined problems here
>>
>> - An efficient delivery system for multicast messages delivered locally
>> (be that MPI, dbus whatever - it's not "dbus or nothing")
>>
>> - A kernel side dynamic namespace to describe what goes where
>>
>> - A kernel side security model to describe who may receive what, and
>> which additional information/tags/cred info
>>
>> - Something that provides state to stuff that needs it (and probably
>> belongs in userspace - dbus name service etc)
>>
>> - Something that maps dbus and other models onto the kernel security
>> model (and we have tools like EBPF which are very powerful)
>>
>> - Something that maps the kernel layer onto models like MPI-3

When trying to split apart problems, for dbus it's important to keep
ordering guarantees.

That is, with dbus if I send a broadcast message, then send a unicast
request to another client, then drop the connection causing the bus to
broadcast that I've dropped; then the other client will see those
things in that order - the broadcast, then the request, and then that
I've dropped the connection.

If you have separate facilities for these things, it could get hard to
keep them in order. dbus uses the simple model that they stay in order
because the bus conceptually has a single dispatch queue.

By pushing everything through one queue, dbus is trying to reduce the
number of codepaths in applications. Apps have a lot of new problems
to solve if messages get their order scrambled.

(dbus does NOT guarantee order across multiple clients, of course -
there's no guarantee that all clients get the broadcast, before anyone
gets the next message - each client has its own buffer on both read
and write. The ordering is only with respect to each client's message
stream.)

Ordering is vital for tracking state, because if you're sending out
events to describe changes in state, the order of those changes is
important.

Of course there are more complex ways to handle this over in
distributed-systems-world.

Havoc