Re: [GIT PULL] kdbus for 4.1-rc1

David Lang
2015-04-28T20:35:16Z

On Tue, 28 Apr 2015, Havoc Pennington wrote:

> On Tue, Apr 28, 2015 at 1:19 PM, David Lang <david@lang.hm> wrote:
>> If the examples that are being used to show the performance advantage of
>> kdbus vs normal dbus are doing the wrong thing, then we need to get some
>> other examples available to people who don't live and breath dbus that 'so
>> things right' so that the kernel developers can see what you think is the
>> real problem and how kdbus addresses it.
>>
>> So far, this 'wrong' example is the only thing that's been posted to show
>> the performance advantage of kdbus.
>
> I'm hopeful someone will do that.
>
> fwiw, I would be suspicious of a broken benchmark if it didn't show:
>
> * the bus daemon means an extra read/parse and marshal/write per
> message, so 4 vs. 2
> * the existence of the bus daemon therefore makes a message
> send/receive take roughly twice as long
>
> https://lwn.net/Articles/580194/ has a bit more elaboration about
> number of copies, validations, and context switches in each case.
>
> From what I can tell, the core performance claim for kdbus is that for
> a userspace daemon to be a routing intermediary, it has to receive and
> re-send messages. If the baseline performance of IPC is the cost to
> send once and receive once, adding the daemon means there's twice as
> much to do (1 more receive, 1 more send). However fast you make
> send/receive, the daemon always means there are twice as many
> send/receives as there would be with no daemon.

there are twice as many context switches, nobody disputes that, the question is
if it matters.

It doesn't matter if the message router is in kernel space or user space, it
still needs to read/parse, marshal/write the data, so you aren't saving that
time due to it being in the kernel.

> If that isn't what a benchmark shows, then there's a mystery to
> explain... (one disruption to the ratio of course could be if the
> clients use a much faster or slower dbus lib than the daemon)
>
> As noted many times, of course this 2x penalty for the daemon was a
> conscious tradeoff - kdbus is trying to escape the tradeoff in order
> to extend usage of dbus to more use cases. Given the tradeoff,
> _existing_ uses of dbus seem to prefer the performance hit to the loss
> of useful semantics, but potential new users would like to or need to
> have both.

If there is a 2x performance improvement for being in the kernel, but a 100x
performance improvement from fixing the userspace code, the effort should be
spent on the userspace code, not on moving things to kernel space.

Remember the Tux in-kernel webserver? it showed performance improvements from
putting the http daemon in the kernel, and a lot of the arguments about it sound
very similar (reduced context switches, etc)

David Lang