Re: [GIT PULL] kdbus for 4.1-rc1

Richard Weinberger
2015-04-20T20:43:26Z

David,

On Thu, Apr 16, 2015 at 8:20 PM, David Herrmann <dh.herrmann@gmail.com> wrote:
> 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.

In which situation on a common Linux system is the current dbus too slow today?
I've never seen a issue like "Oh my system is slow because dbus is
eating too much CPU cycles".

dbus my have issues which are worth to fix. But moving dbus more or
less in the kernel
seems overkill.
So, what exactly are these issues and why can't we add new IPC
primitives to Linux which
allow a decent userland dbus?
To me kdbus seems much like an ad-hoc solution which is very dbus centric.
IIRC Alan asked the same question.

--
Thanks,
//richard