[dpdk-dev] [PATCH v8 1/3] mempool: support external mempool operations

[Shreyansh Jain]

Hi Jerin,

> -----Original Message-----
> From: Jerin Jacob [mailto:jerin.jacob at caviumnetworks.com]
> Sent: Thursday, June 09, 2016 4:02 PM
> To: Hunt, David <david.hunt at intel.com>
> Cc: Shreyansh Jain <shreyansh.jain at nxp.com>; dev at dpdk.org;
> olivier.matz at 6wind.com; viktorin at rehivetech.com
> Subject: Re: [dpdk-dev] [PATCH v8 1/3] mempool: support external mempool
> operations
> 
> On Thu, Jun 09, 2016 at 10:39:46AM +0100, Hunt, David wrote:
> > Hi Shreyansh,
> >
> > On 8/6/2016 2:48 PM, Shreyansh Jain wrote:
> > > Hi David,
> > >
> > > Thanks for explanation. I have some comments inline...
> > >
> > > > -----Original Message-----
> > > > From: Hunt, David [mailto:david.hunt at intel.com]
> > > > Sent: Tuesday, June 07, 2016 2:56 PM
> > > > To: Shreyansh Jain <shreyansh.jain at nxp.com>; dev at dpdk.org
> > > > Cc: olivier.matz at 6wind.com; viktorin at rehivetech.com;
> > > > jerin.jacob at caviumnetworks.com
> > > > Subject: Re: [dpdk-dev] [PATCH v8 1/3] mempool: support external
> mempool
> > > > operations
> > > >
> > > > Hi Shreyansh,
> > > >
> > > > On 6/6/2016 3:38 PM, Shreyansh Jain wrote:
> > > > > Hi,
> > > > >
> > > > > (Apologies for overly-eager email sent on this thread earlier. Will
> be more
> > > > careful in future).
> > > > > This is more of a question/clarification than a comment. (And I have
> taken
> > > > only some snippets from original mail to keep it cleaner)
> > > > > <snip>
> > > > > > +MEMPOOL_REGISTER_OPS(ops_mp_mc);
> > > > > > +MEMPOOL_REGISTER_OPS(ops_sp_sc);
> > > > > > +MEMPOOL_REGISTER_OPS(ops_mp_sc);
> > > > > > +MEMPOOL_REGISTER_OPS(ops_sp_mc);
> > > > > <snip>
> > > > >
> > > > >   From the above what I understand is that multiple packet pool
> handlers can
> > > > be created.
> > > > > I have a use-case where application has multiple pools but only the
> packet
> > > > pool is hardware backed. Using the hardware for general buffer
> requirements
> > > > would prove costly.
> > > > >   From what I understand from the patch, selection of the pool is
> based on
> > > > the flags below.
> > > >
> > > > The flags are only used to select one of the default handlers for
> > > > backward compatibility through
> > > > the rte_mempool_create call. If you wish to use a mempool handler that
> > > > is not one of the
> > > > defaults, (i.e. a new hardware handler), you would use the
> > > > rte_create_mempool_empty
> > > > followed by the rte_mempool_set_ops_byname call.
> > > > So, for the external handlers, you create and empty mempool, then set
> > > > the operations (ops)
> > > > for that particular mempool.
> > > I am concerned about the existing applications (for example, l3fwd).
> > > Explicit calls to 'rte_create_mempool_empty->rte_mempool_set_ops_byname'
> model would require modifications to these applications.
> > > Ideally, without any modifications, these applications should be able to
> use packet pools (backed by hardware) and buffer pools (backed by
> ring/others) - transparently.
> > >
> > > If I go by your suggestions, what I understand is, doing the above
> without modification to applications would be equivalent to:
> > >
> > >    struct rte_mempool_ops custom_hw_allocator = {...}
> > >
> > > thereafter, in config/common_base:
> > >
> > >    CONFIG_RTE_DEFAULT_MEMPOOL_OPS="custom_hw_allocator"
> > >
> > > calls to rte_pktmbuf_pool_create would use the new allocator.
> >
> > Yes, correct. But only for calls to rte_pktmbuf_pool_create(). Calls to
> > rte_mempool_create will continue to use the default handlers (ring based).
> > > But, another problem arises here.
> > >
> > > There are two distinct paths for allocations of a memory pool:
> > > 1. A 'pkt' pool:
> > >     rte_pktmbuf_pool_create
> > >       \- rte_mempool_create_empty
> > >       |   \- rte_mempool_set_ops_byname(..ring_mp_mc..)
> > >       |
> > >       `- rte_mempool_set_ops_byname
> > >             (...RTE_MBUF_DEFAULT_MEMPOOL_OPS..)
> > >             /* Override default 'ring_mp_mc' of
> > >              * rte_mempool_create */
> > >
> > > 2. Through generic mempool create API
> > >     rte_mempool_create
> > >       \- rte_mempool_create_empty
> > >             (passing pktmbuf and pool constructors)
> > > I found various instances in example applications where
> rte_mempool_create() is being called directly for packet pools - bypassing
> the more semantically correct call to rte_pktmbuf_* for packet pools.
> > >
> > > In (2) control path, RTE_MBUF_DEFAULT_MEMPOOLS_OPS wouldn't be able to
> replace custom handler operations for packet buffer allocations.
> > >
> > >  From a performance point-of-view, Applications should be able to select
> between packet pools and non-packet pools.
> >
> > This is intended for backward compatibility, and API consistency. Any
> > applications that use
> > rte_mempool_create directly will continue to use the default mempool
> > handlers. If the need
> > to use a custeom hander, they will need to be modified to call the newer
> > API,
> > rte_mempool_create_empty and rte_mempool_set_ops_byname.
> >
> >
> > > > > <snip>
> > > > > > +	/*
> > > > > > +	 * Since we have 4 combinations of the SP/SC/MP/MC examine the
> flags to
> > > > > > +	 * set the correct index into the table of ops structs.
> > > > > > +	 */
> > > > > > +	if (flags & (MEMPOOL_F_SP_PUT | MEMPOOL_F_SC_GET))
> > > > > > +		rte_mempool_set_ops_byname(mp, "ring_sp_sc");
> > > > > > +	else if (flags & MEMPOOL_F_SP_PUT)
> > > > > > +		rte_mempool_set_ops_byname(mp, "ring_sp_mc");
> > > > > > +	else if (flags & MEMPOOL_F_SC_GET)
> > > > > > +		rte_mempool_set_ops_byname(mp, "ring_mp_sc");
> > > > > > +	else
> > > > > > +		rte_mempool_set_ops_byname(mp, "ring_mp_mc");
> > > > > > +
> > > My suggestion is to have an additional flag, 'MEMPOOL_F_PKT_ALLOC',
> which, if specified, would:
> > >
> > > ...
> > > #define MEMPOOL_F_SC_GET    0x0008
> > > #define MEMPOOL_F_PKT_ALLOC 0x0010
> > > ...
> > >
> > > in rte_mempool_create_empty:
> > >     ... after checking the other MEMPOOL_F_* flags...
> > >
> > >      if (flags & MEMPOOL_F_PKT_ALLOC)
> > >          rte_mempool_set_ops_byname(mp, RTE_MBUF_DEFAULT_MEMPOOL_OPS)
> > >
> > > And removing the redundant call to rte_mempool_set_ops_byname() in
> rte_pktmbuf_create_pool().
> > >
> > > Thereafter, rte_pktmbuf_pool_create can be changed to:
> > >
> > >        ...
> > >      mp = rte_mempool_create_empty(name, n, elt_size, cache_size,
> > > -        sizeof(struct rte_pktmbuf_pool_private), socket_id, 0);
> > > +        sizeof(struct rte_pktmbuf_pool_private), socket_id,
> > > +        MEMPOOL_F_PKT_ALLOC);
> > >      if (mp == NULL)
> > >          return NULL;
> >
> > Yes, this would simplify somewhat the creation of a pktmbuf pool, in that
> it
> > replaces
> > the rte_mempool_set_ops_byname with a flag bit. However, I'm not sure we
> > want
> > to introduce a third method of creating a mempool to the developers. If we
> > introduced this, we would then have:
> > 1. rte_pktmbuf_pool_create()
> > 2. rte_mempool_create_empty() with MEMPOOL_F_PKT_ALLOC set (which would
> >    use the configured custom handler)
> > 3. rte_mempool_create_empty() with MEMPOOL_F_PKT_ALLOC __not__ set followed
> >    by a call to rte_mempool_set_ops_byname() (would allow several different
> > custom
> >    handlers to be used in one application
> >
> > Does anyone else have an opinion on this? Oliver, Jerin, Jan?
> 
> As I mentioned earlier, My take is not to create the separate API's for
> external mempool handlers.In my view, It's same,  just that sepreate
> mempool handler through function pointers.
> 
> To keep the backward compatibility, I think we can extend the flags
> in rte_mempool_create and have a single API external/internal pool
> creation(makes easy for existing applications too, add a just mempool
> flag command line argument to existing applications to choose the
> mempool handler)

May be I am interpreting it wrong, but, are you suggesting a single mempool handler for all buffer/packet needs of an application (passed as command line argument)?
That would be inefficient especially for cases where pool is backed by a hardware. The application wouldn't want its generic buffers to consume hardware resource which would be better used for packets.

I was hoping that external mempool handler would help segregate such use-cases and allow applications to tap into accelerated packet processors.

Do correct me if I my understanding is wrong.

> 
> Jerin
> 
> >
> > Regards,
> > Dave.
> >
> >

-
Shreyansh