Re: [GIT PULL] kdbus for 4.1-rc1

From: Linus Torvalds
Date: Mon Apr 27 2015 - 18:14:58 EST

On Mon, Apr 27, 2015 at 3:00 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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.

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