Date: Thu, 27 Jan 2005 09:35:06 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: "Jack O'Quin" <joq@io.com>, Paul Davis <paul@linuxaudiosystems.com>,
       Con Kolivas <kernel@kolivas.org>, linux <linux-kernel@vger.kernel.org>,
       rlrevell@joe-job.com, CK Kernel <ck@vds.kolivas.org>,
       utz <utz@s2y4n2c.de>, Andrew Morton <akpm@osdl.org>, alexn@dsv.su.se,
       Rui Nuno Capela <rncbc@rncbc.org>, Chris Wright <chrisw@osdl.org>,
       Arjan van de Ven <arjanv@redhat.com>
Subject: Re: [patch, 2.6.11-rc2] sched: RLIMIT_RT_CPU_RATIO feature
Message-ID: <20050127083506.GD22482@elte.hu>
References: <20050124125814.GA31471@elte.hu> <20050125135613.GA18650@elte.hu> <87sm4opxto.fsf@sulphur.joq.us> <20050126070404.GA27280@elte.hu> <87fz0neshg.fsf@sulphur.joq.us> <1106782165.5158.15.camel@npiggin-nld.site> <874qh3bo1u.fsf@sulphur.joq.us> <1106796360.5158.39.camel@npiggin-nld.site> <87pszr1mi1.fsf@sulphur.joq.us> <1106805249.5158.77.camel@npiggin-nld.site>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1106805249.5158.77.camel@npiggin-nld.site>
User-Agent: Mutt/1.4.1i
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1544
Lines: 32


* Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> Well in the context of a multi user system, this really is a
> privileged operation. Witness: a normal user isn't even allowed to
> raise the nice priority of a normal task. Note that I think everyone
> agrees here, but I'm just repeating the point.

i've seen this argument repeated a number of times, but i'd like to
point out that with the rlimit set to a sane value, a user can do 'less
damage' to the system via SCHED_FIFO than it could do via nice--20!

negative nice levels are a guaranteed way to monopolize the CPU. 
SCHED_FIFO with throttling could at most be used to 'steal' CPU time up
to the threshold. Also, if a task 'runs away' in SCHED_FIFO mode it will
be efficiently throttled. While if it 'runs away' in nice--20 mode, it
will take away 95+% of the CPU time quite agressively. Furthermore, more
nice--20 tasks will do much more damage (try thunk.c at nice--20!),
while more throttled SCHED_FIFO tasks only do damage to their own class
- the guaranteed share of SCHED_OTHER tasks (and privileged RT tasks) is
not affected.

so while it is true that in terms of priorities, throttled SCHED_FIFO
trumps all SCHED_OTHER tasks, but in the "potential damage" sense,
"throttled real-time" is less of a privilege than "nice--20".

	Ingo
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/