Re: [dpdk-dev] [PATCH 1/1] mbuf: move pool pointer in first half

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> On Sat, Nov 07, 2020 at 07:57:05PM +0100, Morten Brørup wrote:
> > > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > > Sent: Saturday, November 7, 2020 4:53 PM
> > >
> > > The mempool pointer in the mbuf struct is moved
> > > from the second to the first half.
> > > It should increase performance on most systems having 64-byte cache line,
> > > i.e. mbuf is split in two cache lines.
> >
> > Perhaps with the #define DEV_TX_OFFLOAD_MBUF_FAST_FREE mentioned by Konstantin, it might be better moving m->next instead, at
> least for applications with the opportunity to set this flag (e.g. applications with only one mbuf pool).
> >
> > Unfortunately, the information about the DEV_TX_OFFLOAD_MBUF_FAST_FREE flag came to light after the techboard meeting, and I
> don't have any benchmarks to support this suggestion, so I guess it's hard to change the techboard's decision to move the pool pointer.
> >
> 
> The thing with FAST_FREE is that it doesn't just indicate that there is
> only one mbuf pool, but rather that none of the packets are chained mbufs
> either.

Hmm, wonder where such assumption comes from?
There is nothing about forbidding chained mbufs in rte_ethdev.h right now:
#define DEV_TX_OFFLOAD_MBUF_FAST_FREE   0x00010000
/**< Device supports optimization for fast release of mbufs.
 *   When set application must guarantee that per-queue all mbufs comes from
 *   the same mempool and has refcnt = 1.
 */ 

> Therefore, the flag applies equally to both next and pool pointers
> for optimization. [And the flag should perhaps be two flags!]
> 

I think what Morten means here:
For FAST_FREE PMD can store pool pointer somewhere in its queue metadata
and doesn't need to read it from the mbuf.
So if we move next to first line - mbuf_free code-path for FAST_FREE case can
avoid touching  second cache line in the mbuf.