linux-kernel - Re: [PATCH RFC ticketlock] Auto-queued ticketlock

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Message-ID: <20130611171646.GM5146@linux.vnet.ibm.com>
Date:	Tue, 11 Jun 2013 10:16:46 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Waiman Long <waiman.long@...com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, torvalds@...ux-foundation.org
Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock

On Tue, Jun 11, 2013 at 01:01:55PM -0400, Steven Rostedt wrote:
> On Tue, 2013-06-11 at 09:36 -0700, Paul E. McKenney wrote:
> 
> > > I am a bit concern about the size of the head queue table itself.
> > > RHEL6, for example, had defined CONFIG_NR_CPUS to be 4096 which mean
> > > a table size of 256. Maybe it is better to dynamically allocate the
> > > table at init time depending on the actual number of CPUs in the
> > > system.
> > 
> > But if your kernel is built for 4096 CPUs, the 32*256=8192 bytes of memory
> > is way down in the noise.  Systems that care about that small an amount
> > of memory probably have a small enough number of CPUs that they can just
> > turn off queueing at build time using CONFIG_TICKET_LOCK_QUEUED=n, right?
> 
> If this turns out to work for large machines, that means that distros
> will enable it, and distros tend to up the NR_CPUS, which is defined at
> compile time and is set regardless of if you are running with 2 CPUs or
> a 1000 CPUs.
> 
> For now it's fine to use NR_CPUS, but I always try to avoid it. Working
> in the ARM and POWER environment you are use to lots of kernels compiled
> specifically for the target. But in the x86 world, it is basically one
> kernel for all environments, where NR_CPUS does make a big difference.

Fair point.  Something to worry about should this ever be in danger of
actually going upstream.  ;-)

							Thanx, Paul

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