Re: RCU scaling on large systems

[Jesse Barnes <jbarnes@engr.sgi.com>]

On Sunday, May 2, 2004 11:28 am, Paul E. McKenney wrote:
> From your numbers below, I would guess that if you have at least
> 8 CPUs per NUMA node, a two-level tree would suffice.  If you have
> only 4 CPUs per NUMA node, you might well need a per-node level,
> a per-4-nodes level, and a global level to get the global lock
> contention reduced sufficiently.

Actually, only 2, but it sounds like your approach would work.

> Cute!  However, it is not clear to me that this approach is
> compatible with real-time use of RCU, since it results in CPUs
> processing their callbacks less frequently, and thus getting
> more of them to process at a time.

I think it was just a proof-of-concept--the current RCU design obviously 
wasn't designed with this machine in mind :).

> But it is not clear to me that anyone is looking for realtime
> response from a 512-CPU machine (yow!!!), so perhaps this
> is not a problem...

There are folks that would like realtime (or close to realtime) response on 
such systems, so it would be best not to do anything that would explicitly 
prevent it.

> This patch certainly seems simple enough, and I would guess that
> "jiffies" is referenced often enough that it is warm in the cache
> despite being frequently updated.
>
> Other thoughts?

On a big system like this though, won't reading jiffies frequently be another 
source of contention?

Jesse