How to increase mbuf size in dpdk version 17.11

[Stephen Hemminger]

On Tue, 1 Mar 2022 13:37:07 -0800
Cliff Burdick <shaklee3 at gmail.com> wrote:

> Can you verify how many buffers you're allocating? I don't see how many
> you're allocating in this thread.
> 
> On Tue, Mar 1, 2022 at 1:30 PM Lombardo, Ed <Ed.Lombardo at netscout.com>
> wrote:
> 
> > Hi Stephen,
> > The VM is configured to have 32 GB of memory.
> > Will dpdk consume the 2GB of hugepage memory for the mbufs?
> > I don't mind having less mbufs with mbuf size of 16K vs original mbuf size
> > of 2K.
> >
> > Thanks,
> > Ed
> >
> > -----Original Message-----
> > From: Stephen Hemminger <stephen at networkplumber.org>
> > Sent: Tuesday, March 1, 2022 2:57 PM
> > To: Lombardo, Ed <Ed.Lombardo at netscout.com>
> > Cc: users at dpdk.org
> > Subject: Re: How to increase mbuf size in dpdk version 17.11
> >
> > External Email: This message originated outside of NETSCOUT. Do not click
> > links or open attachments unless you recognize the sender and know the
> > content is safe.
> >
> > On Tue, 1 Mar 2022 18:34:22 +0000
> > "Lombardo, Ed" <Ed.Lombardo at netscout.com> wrote:
> >  
> > > Hi,
> > > I have an application built with dpdk 17.11.
> > > During initialization I want to change the mbuf size from 2K to 16K.
> > > I want to receive packet sizes of 8K or more in one mbuf.
> > >
> > > The VM running the application is configured to have 2G hugepages.
> > >
> > > I tried many things and I get an error when a packet arrives.
> > >
> > > I read online that there is #define DEFAULT_MBUF_DATA_SIZE that I  
> > changed from 2176 to ((2048*8)+128), where 128 is for headroom.  
> > > The call to rte_pktmbuf_pool_create() returns success with my changes.
> > > From the rte_mempool_dump() - "rx_nombuf" - Total number of Rx mbuf  
> > allocation failures.  This value increments each time a packet arrives.  
> > >
> > > Is there any reference document explaining what causes this error?
> > > Is there a user guide I should follow to make the mbuf size change,  
> > starting with the hugepage value?  
> > >
> > > Thanks,
> > > Ed  
> >
> > Did you check that you have enough memory in the system for the larger
> > footprint?
> > Using 16K per mbuf is going to cause lots of memory to be consumed.

A little maths you can fill in your own values.

Assuming you want 16K of data.

You need at a minimum [1]
    num_rxq := total number of receive queues
    num_rxd := number of receive descriptors per receive queue
    num_txq := total number of transmit queues (assume all can be full)
    num_txd := number of transmit descriptors
    num_mbufs = num_rxq * num_rxd + num_txq * num_txd + num_cores * burst_size

Assuming you are using code copy/pasted from some example like l3fwd.
With 4 Rxq

    num_mbufs = 4 * 1024 + 4 * 1024 + 4 * 32 = 8320

Each mbuf element requires [2]
    elt_size = sizeof(struct rte_mbuf) + HEADROOM + mbuf_size
             = 128 + 128 + 16K = 16640

    obj_size = rte_mempool_calc_obj_size(elt_size, 0, NULL)
             = 16832

So total pool is
    num_mbufs * obj_size = 8320 * 16832 = 140,042,240 ~ 139M


[1] Some devices line bnxt need multiple buffers per packet.
[2] Often applications want additional space per mbuf for meta-data.