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

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Date:	Tue, 4 Aug 2015 10:58:03 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.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?

Hi

On Tue, Aug 4, 2015 at 1:02 AM, Andy Lutomirski <luto@...nel.org> wrote:
>                                                I got Fedora
> Rawhide working under kdbus (thanks, everyone!), and I ran this little
> program:
>
> #include <systemd/sd-bus.h>
> #include <err.h>
>
> int main(int argc, char *argv[])
> {
>     while (1) {
>         sd_bus *bus;
>         if (sd_bus_open_system(&bus) < 0) {
>             /* warn("sd_bus_open_system"); */
>             continue;
>         }
>         sd_bus_close(bus);

You lack a call to sd_bus_unref() here. Without it, your loop contains:

        while (1)
                malloc(1024);

This simple malloc-loop already hogs your system. If I add the
required call to _unref(), your tool runs smoothly on my machine.

>     }
> }
>
> under both userspace dbus and under kdbus.  Userspace dbus burns some
> CPU -- no big deal.  I expected kdbus to fail to scale and burn a
> disproportionate amount of CPU (because I don't see how it /can/
> scale).  Instead it fell over completely.  I didn't bother debugging
> it, but offhand I'd guess that the system OOMed and didn't come back.

I cannot see the relation to kdbus.

> On very brief inspection, Rawhide seems to have a lot of kdbus
> connections with 16MiB of mapped tmpfs stuff each.  (53 of them
> mapped, and I don't know how many exist with tmpfs backing but aren't
> mapped.  Presumably the number only goes up as the degree of reliance
> on the userspace proxy goes down.

What does this have to do with the proxy? Why would resource
consumption go *up* as the proxy users decline? Please elaborate.

>                                         I don't know of any deployed
> systems that solve it by broadcasting the lifetime of everything to
> everyone and relying on those broadcasts going through, though.

Luckily, kdbus does not do this.

Thanks
David
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