Re: [dpdk-dev] [PATCH] mbuf: optimize refcnt handling during free

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, Mar 26, 2015 at 09:00:33PM +0000, Wiles, Keith wrote:
> 
> 
> On 3/26/15, 1:10 PM, "Zoltan Kiss" <zoltan.kiss@linaro.org> wrote:
> 
> >The current way is not the most efficient: if m->refcnt is 1, the second
> >condition never evaluates, and we set it to 0. If refcnt > 1, the 2nd
> >condition fails again, although the code suggest otherwise to branch
> >prediction. Instead we should keep the second condition only, and remove
> >the
> >duplicate set to zero.
> >
> >Signed-off-by: Zoltan Kiss <zoltan.kiss@linaro.org>
> >---
> > lib/librte_mbuf/rte_mbuf.h | 5 +----
> > 1 file changed, 1 insertion(+), 4 deletions(-)
> >
> >diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> >index 17ba791..3ec4024 100644
> >--- a/lib/librte_mbuf/rte_mbuf.h
> >+++ b/lib/librte_mbuf/rte_mbuf.h
> >@@ -764,10 +764,7 @@ __rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> > {
> > 	__rte_mbuf_sanity_check(m, 0);
> > 
> >-	if (likely (rte_mbuf_refcnt_read(m) == 1) ||
> >-			likely (rte_mbuf_refcnt_update(m, -1) == 0)) {
> >-
> >-		rte_mbuf_refcnt_set(m, 0);
> >+	if (likely (rte_mbuf_refcnt_update(m, -1) == 0)) {
> > 
> > 		/* if this is an indirect mbuf, then
> > 		 *  - detach mbuf
> 
> I fell for this one too, but read Bruce¹s email
> http://dpdk.org/ml/archives/dev/2015-March/014481.html

Looks like a code comment that really, really needs to be added to the code itself!

/Bruce

> >-- 
> >1.9.1
> >
>