Re: [GIT PULL] kdbus for 4.1-rc1

David Herrmann
2015-04-16T18:20:11Z

Hi

On Wed, Apr 15, 2015 at 8:18 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Wed, Apr 15, 2015 at 11:11 AM, Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
>> On Wed, Apr 15, 2015 at 01:33:27PM -0400, Steven Rostedt wrote:
>>>
>>> I'll argue that you can't fix the later one. One thing that I've observed over
>>> the years of having faster computers is, as soon as you make it faster, people
>>> will write slower software.
>>>
>>> Currently the issue is that we have thousands of dbus queries, you make dbus
>>> 10x faster, I guarantee that people will write software with 10 thousand dbus
>>> queries and we are no better off than we are today.
>>
>> Then they get to buy a faster machine :)
>
> Is there actually a performance issue?
>
> I've seen this claimed, but I have never seen any actual numbers. What
> speeds up? By how much? is it actually measurable?

For us, boot speed-up has not been the primary concern. The boot-time
speedup that kdbus provides is unlikely to be significant on a generic
linux distro today, given that nowadays the slow parts during bootup
are firmware and hw initialization. The number of dbus messages sent
during bootup of a general purpose Linux distro is relatively small.
OTOH, during start-up of desktop environments, the dbus traffic is
substantial, but until the porting of Qt and glib to kdbus has been
completed and merged the real-world effect of this is minimal.

Our interest in improved raw performance is mainly motivated by making
dbus a viable protocol in situations where its semantics are
appropriate. But for performance reasons one had to use custom
protocols instead so far. Greg gave some examples in the cover letter,
multi-media being the most obvious area where the ten-fold decrease in
latency and the ability to efficiently copy large chunks of memory
from one process to another is relevant.

> Maybe they've marched past me in this thread-from-hell. But I can't
> recall having seen any (not now, not before).
>
> That said, I think the more serious issue is that if Luto complains
> about the capability-capturing code being completely broken, then
> people need to take that *seriously*.

Absolutely. In v3 of the patch set we addressed Andy's concerns, and
now he seems to be convinced that the kdbus code is not vulnerable
[1].

Thanks
David

[1] http://lists.freedesktop.org/archives/systemd-devel/2015-April/030776.html