Hi Olivier, > -----Original Message----- > From: Olivier MATZ [mailto:olivier.matz at 6wind.com] > Sent: Monday, June 15, 2015 3:31 PM > To: Richardson, Bruce > Cc: Ananyev, Konstantin; dev at dpdk.org; Damjan Marion (damarion) > Subject: Re: [dpdk-dev] rte_mbuf.next in 2nd cacheline > > > > On 06/15/2015 04:12 PM, Bruce Richardson wrote: > > On Mon, Jun 15, 2015 at 04:05:05PM +0200, Olivier MATZ wrote: > >> Hi, > >> > >> On 06/15/2015 03:54 PM, Ananyev, Konstantin wrote: > >>> > >>> > >>>> -----Original Message----- > >>>> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Bruce Richardson > >>>> Sent: Monday, June 15, 2015 2:44 PM > >>>> To: Olivier MATZ > >>>> Cc: dev at dpdk.org; Damjan Marion (damarion) > >>>> Subject: Re: [dpdk-dev] rte_mbuf.next in 2nd cacheline > >>>> > >>>> On Mon, Jun 15, 2015 at 03:20:22PM +0200, Olivier MATZ wrote: > >>>>> Hi Damjan, > >>>>> > >>>>> On 06/10/2015 11:47 PM, Damjan Marion (damarion) wrote: > >>>>>> > >>>>>> Hi, > >>>>>> > >>>>>> We noticed 7% performance improvement by simply moving rte_mbuf.next > >>>>>> field to the 1st cache line. > >>>>>> > >>>>>> Currently, it falls under /* second cache line - fields only used in > >>>>>> slow path or on TX */ > >>>>>> but it is actually used at several places in rx fast path. (e.g.: > >>>>>> i40e_rx_alloc_bufs() is setting that field to NULL). > >>>>>> > >>>>>> Is there anything we can do here (stop using next field, or move it to > >>>>>> 1st cache line)? > >>>>> > >>>>> Agree, this is also something I noticed, see: > >>>>> http://dpdk.org/ml/archives/dev/2015-February/014400.html > >>>>> > >>>>> I did not have the time to do performance testing, but it's something > >>>>> I'd like to do as soon as I can. I don't see any obvious reason not to > >>>>> do it. > >>>>> > >>>>> It seems we currently just have enough room to do it (8 bytes are > >>>>> remaining in the first cache line when compiled in 64 bits). > >>>> > >>>> This, to me, is the obvious reason not to do it! It prevents us from > >>>> taking in > >>>> any other offload fields in the RX fast-path into the mbuf. > >>>> > >>>> That being said, I can see why we might want to look to move it - but > >>>> from the > >>>> work done in the ixgbe driver, I'd be hopeful we can get as much > >>>> performance with > >>>> it on the second cache line for most cases, through judicious use of > >>>> prefetching, > >>>> or otherwise. > >>>> > >>>> It took a lot of work and investigation to get free space in the mbuf - > >>>> especially > >>>> in cache line 0, and I'd like to avoid just filling the cache line up > >>>> again as > >>>> long as we possibly can! > >>> > >>> Yep, agree with Bruce here. > >>> Plus, with packet_type going to be 4B and vlan_tci_outer, > >>> we just don't have 8 free bytes at the first cache line any more. > >> > >> I don't understand why m->next would not be a better candidate than > >> rx offload fields to be in the first cache line. For instance, m->next > >> is mandatory and must be initialized when allocating a mbuf (to be > >> compared with m->seqn for instance, which is also in the first cache > >> line). So if we want to do some room in the first cache line, I > >> think we can. > >> > >> To me, the only reason for not doing it now is because we don't > >> have a full performance test report (several use-cases, drivers, ...) > >> that shows it's better. > >> > > Because the "next" field is not mandatory to be set on initialization. It > > can > > instead be set only when needed, and cleared on free if it is used. > > > > The next pointers always start out as NULL when the mbuf pool is created. > > The > > only time it is set to non-NULL is when we have chained mbufs. If we never > > have > > any chained mbufs, we never need to touch the next field, or even read it - > > since > > we have the num-segments count in the first cache line. If we do have a > > multi-segment > > mbuf, it's likely to be a big packet, so we have more processing time > > available > > and we can then take the hit of setting the next pointer. Whenever we go to > > free that mbuf for that packet, the code to do the freeing obviously needs > > to > > read the next pointer so as to free all the buffers in the chain, and so it > > can > > also reset the next pointer to NULL when doing so. > > > > In this way, we can ensure that the next pointer on cache line 1 is not a > > problem > > in our fast path. > > This is a good idea, but looking at the drivers, it seems that today > they all set m->next to NULL in the rx function. What you are suggesting > is to remove all of them, and document somewhere that all mbufs in a > pool are supposed to have their m->next set to NULL, correct? > > I think what you are describing could also apply to reference counter > (set to 1 by default), right?
We probably can reset next to NULL at __rte_pktmbuf_prefree_seg(), at the same time we do reset refcnt to 0. Is that what you suggesting? Konstantin > > > Olivier > > > > > > /Bruce > >
