linux-kernel - Re: kdbus: to merge or not to merge?

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Date:	Tue, 4 Aug 2015 17:18:00 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Djalal Harouni <tixxdz@...ndz.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Havoc Pennington <havoc.pennington@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Tom Gundersen <teg@...m.no>, Daniel Mack <daniel@...que.org>,
	"Kalle A. Sandstrom" <ksandstr@....fi>,
	Borislav Petkov <bp@...en8.de>, cee1 <fykcee1@...il.com>
Subject: Re: kdbus: to merge or not to merge?

On Tue, Aug 4, 2015 at 7:47 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Tue, Aug 4, 2015 at 7:09 AM, David Herrmann <dh.herrmann@...il.com> wrote:
>> Hi
>>
>> On Tue, Aug 4, 2015 at 3:46 PM, Linus Torvalds
>> <torvalds@...ux-foundation.org> wrote:
>>> On Tue, Aug 4, 2015 at 1:58 AM, David Herrmann <dh.herrmann@...il.com> wrote:
>>>>
>>>> You lack a call to sd_bus_unref() here.
>>>
>>> I assume it was intentional. Why would Andy talk about "scaling" otherwise?
>
> It was actually an error.  I assumed that, since the user version
> worked fine (at least for as long as I ran it) and the kernel version
> didn't (killed X and left a blinking cursor, no visible log messages
> even when run from a text console, and no obvious OOM recovery after a
> long wait) that it was a kdbus issue or issue with other kdbus
> clients.
>
> I'll play with it more today.
>

I added the missing sd_bus_unref call.

With userspace dbus, my program takes 95% CPU and dbus-daemon takes
88% CPU or so.

With kdbus, I see abuse-bus (my test), systemd-journald,
systemd-bus-proxy, auditd, gnome-shell, mission-control, sedispatch,
firewalld, polkitd, NetworkManager, systemd, avahi-daemon, audisp,
abrt-dump-jour* (whatever it's called -- it truncated), upowerd, and
systemd-logind all taking tons of CPU.  I've listed them in decreasing
order of amount of CPU burned -- the top several are taking about as
much as is possible.  Load average is over 13.  That's if I run it
from a text console while I'm logged in to gnome in a different VT.

If I run the program from a graphical terminal, everything freezes so
hard that the cursor doesn't even make it to the next line when I hit
enter.

So I still claim that kdbus doesn't scale.  I'm not even just saying
that it doesn't scale to large systems -- somewhat to my surprise, it
doesn't even seem to scale well enough for a mostly empty Rawhide
workstation system running just a graphical terminal.  And I didn't
even try to find stress tests more interesting than connecting and
disconnecting in a loop.

FWIW, the old test (without the unref) appeared to be allocating 16M
of mapped kdbus pool every iteration, which seems unlikely to have
helped matters.

--Andy
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