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Date: Mon, 27 Apr 2015 15:14:49 -0700
Message-ID: <CA+55aFxdguuO7CLKYYq_qM6S6FT2CicZ1E5D5xGE54OdJNQwew@mail.gmail.com>
Subject: Re: [GIT PULL] kdbus for 4.1-rc1
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Lukasz Skalski <l.skalski@samsung.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
        Tom Gundersen <teg@jklm.no>, Jiri Kosina <jkosina@suse.cz>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Daniel Mack <daniel@zonque.org>,
        David Herrmann <dh.herrmann@gmail.com>,
        Djalal Harouni <tixxdz@opendz.org>
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On Mon, Apr 27, 2015 at 3:00 PM, Linus Torvalds
<torvalds@linux-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.

                              Linus
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