Subject: Re: [PATCH 3/3] Convert the UDP hash lock to RCU

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 7 Oct 2008 07:33:46 -0700

On Tue, Oct 07, 2008 at 09:16:13AM -0500, Christoph Lameter wrote:
> Evgeniy Polyakov wrote:
> > On Mon, Oct 06, 2008 at 06:08:09PM -0500, Corey Minyard ([email-masked]) wrote:
> >> Would using SLAB_DESTROY_BY_RCU be ok, or would that be too expensive?
> > 
> > I tested skb destruction via RCU path, and got 2.5 times worse numbers
> > with small-packets-bulk-transfer workload.
> 
> Was this with regular RCU freeing? This will cool down the cacheline before
> frees. You need SLAB_DESTROY_BY_RCU to keep the objects cache hot.

Indeed!

But care is required -- SLAB_DESTROY_BY_RCU permits objects to be freed
and reallocated while a reader holds a reference.  The only guarantee is
that the -type- of the data structure will not change while a reader holds
a reference.  With something like UDP, this might well be sufficient.

Just be careful!  ;-)

							Thanx, Paul