linux-kernel - Re: [GIT PULL] kdbus for 4.1-rc1

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Message-ID: <20150622232401.7d3f105a@holly>
Date:	Mon, 22 Jun 2015 23:24:01 +0200
From:	Jindřich Makovička <makovick@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Lukasz Skalski <l.skalski@...sung.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Tom Gundersen <teg@...m.no>, Jiri Kosina <jkosina@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Daniel Mack <daniel@...que.org>,
	David Herrmann <dh.herrmann@...il.com>,
	Djalal Harouni <tixxdz@...ndz.org>
Subject: Re: [GIT PULL] kdbus for 4.1-rc1

On Mon, 27 Apr 2015 15:14:49 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Mon, Apr 27, 2015 at 3:00 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > IOW, all the people who say that it's about avoiding context
> > switches are probably just full of shit. It's not about context
> > switches, it's about bad user-level code.
> 
> Just to make sure, I did a system-wide profile (so that you can
> actually see the overhead of context switching better), and that
> didn't change the picture.
> 
> The scheduler overhead *might* be 1% or so.
> 
> So really. The people who talk about how kdbus improves performance
> are just full of sh*t. Yes, it improves things, but the improvement
> seems to be 100% "incidental", in that it avoids a few trips down the
> user-space problems.
> 
> The real problems seem to be in dbus memory management (suggestion:
> keep a small per-thread cache of those message allocations) and to a
> smaller degree in the crazy utf8 validation (why the f*ck does it do
> that anyway?), with some locking problems thrown in for good measure.

In case someone actually still reads this, I guess the global rw_lock in
gobject/gtype.c, used by the GDbus binding, is one of the culprits.
Every GType instance allocation/ deallocation is serialized using this
lock, which pretty much disqualifies GObject from being used for
anything scalable to multiple threads.

GStreamer used to have serious performance issues due to that, which
AFAIK have been solved by removing GType from GStreamer core in the 1.0
release.

Regards,

-- 
Jindrich Makovicka
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