Re: [dpdk-dev] [PATCH 2/5] ixgbe: add prefetch to improve slow-path tx perf

[Neil Horman <nhorman@tuxdriver.com>]

On Thu, Sep 18, 2014 at 02:36:13PM +0100, Bruce Richardson wrote:
> On Wed, Sep 17, 2014 at 01:59:36PM -0400, Neil Horman wrote:
> > On Wed, Sep 17, 2014 at 03:35:19PM +0000, Richardson, Bruce wrote:
> > > 
> > > > -----Original Message-----
> > > > From: Neil Horman [mailto:nhorman@tuxdriver.com]
> > > > Sent: Wednesday, September 17, 2014 4:21 PM
> > > > To: Richardson, Bruce
> > > > Cc: dev@dpdk.org
> > > > Subject: Re: [dpdk-dev] [PATCH 2/5] ixgbe: add prefetch to improve slow-path tx
> > > > perf
> > > > 
> > > > On Wed, Sep 17, 2014 at 11:01:39AM +0100, Bruce Richardson wrote:
> > > > > Make a small improvement to slow path TX performance by adding in a
> > > > > prefetch for the second mbuf cache line.
> > > > > Also move assignment of l2/l3 length values only when needed.
> > > > >
> > > > > Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> > > > > ---
> > > > >  lib/librte_pmd_ixgbe/ixgbe_rxtx.c | 12 +++++++-----
> > > > >  1 file changed, 7 insertions(+), 5 deletions(-)
> > > > >
> > > > > diff --git a/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> > > > b/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> > > > > index 6f702b3..c0bb49f 100644
> > > > > --- a/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> > > > > +++ b/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> > > > > @@ -565,25 +565,26 @@ ixgbe_xmit_pkts(void *tx_queue, struct rte_mbuf
> > > > **tx_pkts,
> > > > >  		ixgbe_xmit_cleanup(txq);
> > > > >  	}
> > > > >
> > > > > +	rte_prefetch0(&txe->mbuf->pool);
> > > > > +
> > > > 
> > > > Can you explain what all of these prefetches are doing?  It looks to me like
> > > > they're just fetching the first caheline of the mempool structure, which it
> > > > appears amounts to the pools name.  I don't see that having any use here.
> > > > 
> > > This does make a decent enough performance difference in my tests (the amount varies depending on the RX path being used by testpmd). 
> > > 
> > > What I've done with the prefetches is two-fold:
> > > 1) changed it from prefetching the mbuf (first cache line) to prefetching the mbuf pool pointer (second cache line) so that when we go to access the pool pointer to free transmitted mbufs we don't get a cache miss. When clearing the ring and freeing mbufs, the pool pointer is the only mbuf field used, so we don't need that first cache line.
> > ok, this makes some sense, but you're not guaranteed to either have that
> > prefetch be needed, nor are you certain it will still be in cache by the time
> > you get to the free call.  Seems like it might be preferable to prefecth the
> > data pointed to by tx_pkt, as you're sure to use that every loop iteration.
> 
> The vast majority of the times the prefetch is necessary, and it does help 
> performance doing things this way. If the prefetch is not necessary, it's 
> just one extra instruction, while, if it is needed, having the prefetch 
> occur 20 cycles before access (picking an arbitrary value) means that we 
> have cut down the time it takes to pull the data from cache when it is 
> needed by 20 cycles.
I understand how prefetch works. What I'm concerned about is its overuse, and
its tendency to frequently need re-calibration (though I admit I missed the &
operator in the patch, and thought you were prefetching the contents of the
struct, not the pointer value itself).  As you say, if the pool pointer is
almost certain to be used, then it may well make sense to prefetch the data, but
in doing so, you potentially evict something that you were about to use, so
you're not doing yourself any favors.  I understand that you've validated this
experimentally, and so it works, right now.  I just like to be very careful
about how prefetch happens, as it can easily (and sliently) start hurting far
more than it helps.

> As for the value pointed to by tx_pkt, since this is a 
> packet the app has just been working on, it's almost certainly already in 
> l1/l2 cache.  
> 
Not sure I follow you here.  tx_pkts is an array of mbufs passed to the pmd from
rte_eth_tx_burts, which in turn is called by the application.  I don't see any
reasonable guarantee that any of those packets have been touch in sufficiently
recent history that they are likely to be in cache.  It seems like, if you do
want to do prefetching, interrotagting nb_tx and doing a prefetch of an
approriate stride to fill multiple cachelines with successive mbuf headers might
provide superior performance.
Neil