Re: [dpdk-dev] [PATCH] mbuf: make rearm_data address naturally aligned

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

Hi everyone,
 
> On Thu, May 19, 2016 at 12:20:16AM +0530, Jerin Jacob wrote:
> > On Wed, May 18, 2016 at 05:43:00PM +0100, Bruce Richardson wrote:
> > > On Wed, May 18, 2016 at 07:27:43PM +0530, Jerin Jacob wrote:
> > > > To avoid multiple stores on fast path, Ethernet drivers
> > > > aggregate the writes to data_off, refcnt, nb_segs and port
> > > > to an uint64_t data and write the data in one shot
> > > > with uint64_t* at &mbuf->rearm_data address.
> > > >
> > > > Some of the non-IA platforms have store operation overhead
> > > > if the store address is not naturally aligned.This patch
> > > > fixes the performance issue on those targets.
> > > >
> > > > Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> > > > ---
> > > >
> > > > Tested this patch on IA and non-IA(ThunderX) platforms.
> > > > This patch shows 400Kpps/core improvement on ThunderX + ixgbe + vector environment.
> > > > and this patch does not have any overhead on IA platform.
> > > >
> > > > Have tried an another similar approach by replacing "buf_len" with "pad"
> > > > (in this patch context),
> > > > Since it has additional overhead on read and then mask to keep "buf_len" intact,
> > > > not much improvement is not shown.
> > > > ref: http://dpdk.org/ml/archives/dev/2016-May/038914.html
> > > >
> > > > ---
> > > While this will work and from your tests doesn't seem to have a performance
> > > impact, I'm not sure I particularly like it. It's extending out the end of
> > > cacheline0 of the mbuf by 16 bytes, though I suppose it's not technically using
> > > up any more space of it.
> >
> > Extending by 2 bytes. Right ?. Yes, I guess, Now we using only 56 out of 64 bytes
> > in the first 64-byte cache line.
> >
> > >
> > > What I'm wondering about though, is do we have any usecases where we need a
> > > variable buf_len for packets for RX. These mbufs come directly from a mempool,
> > > which is generally understood to be a set of fixed-sized buffers. I realise that
> > > this change was made in the past after some discussion, but one of the key points
> > > there [at least to my reading] was that - even though nobody actually made a
> > > concrete case where they had variable-sized buffers - having support for them
> > > made no performance difference.

I was going to point to vhost zcp support, but as Thomas pointed out
that functionality was removed  from dpdk.org recently.
So I am not aware does such case exist right now in the 'real world' or not.
Though I still think RX function should leave buf_len field intact. 

> > >
> > > The latter part of that has now changed, and supporting variable-sized mbufs
> > > from an mbuf pool has a perf impact. Do we definitely need that functionality,
> > > because the easiest fix here is just to move the rxrearm marker back above
> > > mbuf_len as it was originally in releases like 1.8?
> >
> > And initialize the buf_len with mp->elt_size - sizeof(struct rte_mbuf).
> > Right?
> >
> > I don't have a strong opinion on this, I can do this if there is no
> > objection on this. Let me know.
> >
> > However, I do see in future, "buf_len" may belong at the end of the first 64 byte
> > cache line as currently "port" is defined as uint8_t, IMO, that is less.
> > We may need to increase that uint16_t. The reason why I think that
> > because, Currently in ThunderX HW, we do have 128VFs per socket for
> > built-in NIC, So, the two node configuration and one external PCIe NW card
> > configuration can easily go beyond 256 ports.

I wonder does anyone really use mbuf port field?
My though was - could we to drop it completely?
Actually, after discussing it with Bruce offline, an interesting idea came out:
if we'll drop port and make mbuf_prefree() to reset nb_segs=1, then
we can reduce RX rearm_data to 4B. So with that layout:

struct rte_mbuf {

         MARKER cacheline0;

        void *buf_addr;           
        phys_addr_t buf_physaddr; 
        uint16_t buf_len;
        uint8_t nb_segs;
        uint8_t reserved_1byte;   /* former port */
        
        MARKER32 rearm_data;
        uint16_t data_off;
       uint16_t refcnt;
       
        uint64_t ol_flags;
        ...

We can keep buf_len at its place and avoid 2B gap, while making rearm_data
4B long and 4B aligned.

Another similar alternative, is to make mbuf_prefree() to set refcnt=1
(as it update it anyway). Then we can remove refcnt from the RX rearm_data,
and again make rearm_data 4B long and 4B aligned:

struct rte_mbuf {

         MARKER cacheline0;

        void *buf_addr;           
        phys_addr_t buf_physaddr; 
        uint16_t buf_len;
        uint16_t refcnt;

        MARKER32 rearm_data;
        uint16_t data_off;
        uint8_t nb_segs;
        uint8_t port;
        
        uint64_t ol_flags;
         ..

As additional plus, __rte_mbuf_raw_alloc() wouldn't need to modify mbuf contents at all -
which probably is a good thing.
As a drawback - we'll have a free mbufs in pool with refcnt==1, which probably reduce
debug ability of the mbuf code.  

Konstantin

> >
> Ok, good point. If you think it's needed, and if we are changing the mbuf
> structure, it might be a good time to extend that field while you are at it, save
> a second ABI break later on.
> 
> /Bruce
> 
> > >
> > > Regards,
> > > /Bruce
> > >
> > > Ref: http://dpdk.org/ml/archives/dev/2014-December/009432.html
> > >