linux-kernel - Re: [REPORT] cfs-v4 vs sd-0.44

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Date:	Mon, 23 Apr 2007 21:52:03 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Peter Williams <pwil3058@...pond.net.au>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Nick Piggin <npiggin@...e.de>,
	Juliusz Chroboczek <jch@....jussieu.fr>,
	Con Kolivas <kernel@...ivas.org>, ck list <ck@....kolivas.org>,
	Bill Davidsen <davidsen@....com>, Willy Tarreau <w@....eu>,
	William Lee Irwin III <wli@...omorphy.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Gene Heskett <gene.heskett@...il.com>
Subject: Re: [REPORT] cfs-v4 vs sd-0.44

> Within reason, it's not the number of clients that X has that causes its 
> CPU bandwidth use to sky rocket and cause problems.  It's more to to 
> with what type of clients they are.  Most GUIs (even ones that are 
> constantly updating visual data (e.g. gkrellm -- I can open quite a 
> large number of these without increasing X's CPU usage very much)) cause 
> very little load on the X server.  The exceptions to this are the 

there is actually 2 and not just 1 "X server", and they are VERY VERY
different in behavior.

Case 1: Accelerated driver

If X talks to a decent enough card it supports will with acceleration,
it will be very rare for X itself to spend any kind of significant
amount of CPU time, all the really heavy stuff is done in hardware, and
asynchronously at that. A bit of batching will greatly improve system
performance in this case.

Case 2: Unaccelerated VESA

Some drivers in X, especially the VESA and NV drivers (which are quite
common, vesa is used on all hardware without a special driver nowadays),
have no or not enough acceleration to matter for modern desktops. This
means the CPU is doing all the heavy lifting, in the X program. In this
case even a simple "move the window a bit" becomes quite a bit of a CPU
hog already.

The cases are fundamentally different in behavior, because in the first
case, X hardly consumes the time it would get in any scheme, while in
the second case X really is CPU bound and will happily consume any CPU
time it can get.

-- 
if you want to mail me at work (you don't), use arjan (at) linux.intel.com
Test the interaction between Linux and your BIOS via http://www.linuxfirmwarekit.org

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