Re: [PATCH RFC ticketlock] Auto-queued ticketlock

From: Paul E. McKenney
Date: Tue Jun 11 2013 - 13:20:19 EST

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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