linux-kernel - Re: RFC: THE OFFLINE SCHEDULER

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Date:	Sun, 23 Aug 2009 11:52:13 +0200
From:	Mike Galbraith <efault@....de>
To:	raz ben yehuda <raziebe@...il.com>
Cc:	riel@...hat.com, mingo@...e.hu, peterz@...radead.org,
	andrew motron <akpm@...l.org>, wiseman@...s.biu.ac.il,
	lkml <linux-kernel@...r.kernel.org>,
	linux-rt-users@...r.kernel.org
Subject: Re: RFC: THE OFFLINE SCHEDULER

On Sun, 2009-08-23 at 14:05 +0300, raz ben yehuda wrote:
> On Sun, 2009-08-23 at 09:30 +0200, Mike Galbraith wrote:
> > On Sun, 2009-08-23 at 12:09 +0300, raz ben yehuda wrote:
> > > On Sun, 2009-08-23 at 07:21 +0200, Mike Galbraith wrote:
> > 
> > > > Seems to me this boils down to a different way to make a SW box in a HW
> > > > box, which already exists.  What does this provide that  partitioning a
> > > > box with csets and virtualization doesn't?
> > > OFFSCHED does not compete with cpu sets nor virtualization.it is
> > > different.
> > > 
> > > 1. Neither virtuallization nor cpu sets provide hard real time. OFFSCHED
> > > does this with a little cost and no impact on the OS.OFFSCHED is not
> > > just accurate , it is also extremely fast,after all, it is NMI'ed
> > > processor.
> > 
> > Why not?  Why can't I run an RT kernel with an RTOS guest and let it do
> > it's deadline management thing?
> Have you ever tested how long a single context switch cost ? can you run
> this system with a 1us accuracy ? you cannot.try ftrac'ing your system.
> the interrupt alone costs several hundreds nano seconds. By the time you
> will be reaching your code, the deadline will be nearly gone.  

I've measured context switch cost many times.  The point though, wasn't
how tight a constraint may be, you maintained that realtime was out the
window, and I didn't see any reason for that to be the case.

> > > 2. OFFSCHED has a access to every piece of memory in the system. so it
> > > can act as a centry for the system, or use linux facilities. Also, the
> > > kernel can access OFFSCHED memory, it is the same address space.
> > 
> > Hm.  That appears to be a self negating argument.
> correct. but I can receive packets over napi and transmit packets over hard_start_xmit
> much faster than any guest OS. I can disable interrupts and move to poll
> mode, thus helping the operating system. can a guest OS help linux?

Depends entirely on the job at hand.  If the job is running a firewall
in kernel mode, no it won't cut the mustard.

(no offense intended, but this all sounds like a great big kernel module
to me, one which doesn't even taint the kernel)
  
> > > 3. OFFSCHED can improve the linux OS ( NAPI,OFFSCHED firewall,RTOP ),
> > > while a guest OS cannot.
> > > 
> > > 4. cpu sets cannot replace softirqs and hardirqs. OFFSCHED can. cpu sets
> > > deals with kernel threads and user space threads. in OFFSCHED we use
> > > offlets.
> > 
> > Which still looks like OS-fu to me.
> I do not understand this remark.

Whether it's offlet, tasklet, insert buzz-word of the day, it's thread
of execution management, which I called OS-fu, ie one of those things
that OSs do.

The rest, I'll leave off replying to, we're kinda splitting hairs.  I
don't see a big generic benefit to OFFSCHED or ilk, others do.

	-Mike


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