Re: [dpdk-dev] [RFC v2 1/1] lib/ring: add scatter gather APIs

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> > > > Hi Honnappa,
> > > >
> > > > From a quick walkthrough, I have some questions/comments, please
> > > > see below.
> > > Hi Olivier, appreciate your input.
> > >
> > > >
> > > > On Tue, Oct 06, 2020 at 08:29:05AM -0500, Honnappa Nagarahalli wrote:
> > > > > Add scatter gather APIs to avoid intermediate memcpy. Use cases
> > > > > that involve copying large amount of data to/from the ring can
> > > > > benefit from these APIs.
> > > > >
> > > > > Signed-off-by: Honnappa Nagarahalli
> > > > > <honnappa.nagarahalli@arm.com>
> > > > > ---
> > > > >  lib/librte_ring/meson.build        |   3 +-
> > > > >  lib/librte_ring/rte_ring_elem.h    |   1 +
> > > > >  lib/librte_ring/rte_ring_peek_sg.h | 552
> > > > > +++++++++++++++++++++++++++++
> > > > >  3 files changed, 555 insertions(+), 1 deletion(-)  create mode
> > > > > 100644 lib/librte_ring/rte_ring_peek_sg.h
> > > > >
> > > > > diff --git a/lib/librte_ring/meson.build
> > > > > b/lib/librte_ring/meson.build index 31c0b4649..377694713 100644
> > > > > --- a/lib/librte_ring/meson.build
> > > > > +++ b/lib/librte_ring/meson.build
> > > > > @@ -12,4 +12,5 @@ headers = files('rte_ring.h',
> > > > >  		'rte_ring_peek.h',
> > > > >  		'rte_ring_peek_c11_mem.h',
> > > > >  		'rte_ring_rts.h',
> > > > > -		'rte_ring_rts_c11_mem.h')
> > > > > +		'rte_ring_rts_c11_mem.h',
> > > > > +		'rte_ring_peek_sg.h')
> > > > > diff --git a/lib/librte_ring/rte_ring_elem.h
> > > > > b/lib/librte_ring/rte_ring_elem.h index 938b398fc..7d3933f15
> > > > > 100644
> > > > > --- a/lib/librte_ring/rte_ring_elem.h
> > > > > +++ b/lib/librte_ring/rte_ring_elem.h
> > > > > @@ -1079,6 +1079,7 @@ rte_ring_dequeue_burst_elem(struct
> > > > > rte_ring *r, void *obj_table,
> > > > >
> > > > >  #ifdef ALLOW_EXPERIMENTAL_API
> > > > >  #include <rte_ring_peek.h>
> > > > > +#include <rte_ring_peek_sg.h>
> > > > >  #endif
> > > > >
> > > > >  #include <rte_ring.h>
> > > > > diff --git a/lib/librte_ring/rte_ring_peek_sg.h
> > > > > b/lib/librte_ring/rte_ring_peek_sg.h
> > > > > new file mode 100644
> > > > > index 000000000..97d5764a6
> > > > > --- /dev/null
> > > > > +++ b/lib/librte_ring/rte_ring_peek_sg.h
> > > > > @@ -0,0 +1,552 @@
> > > > > +/* SPDX-License-Identifier: BSD-3-Clause
> > > > > + *
> > > > > + * Copyright (c) 2020 Arm
> > > > > + * Copyright (c) 2007-2009 Kip Macy kmacy@freebsd.org
> > > > > + * All rights reserved.
> > > > > + * Derived from FreeBSD's bufring.h
> > > > > + * Used as BSD-3 Licensed with permission from Kip Macy.
> > > > > + */
> > > > > +
> > > > > +#ifndef _RTE_RING_PEEK_SG_H_
> > > > > +#define _RTE_RING_PEEK_SG_H_
> > > > > +
> > > > > +/**
> > > > > + * @file
> > > > > + * @b EXPERIMENTAL: this API may change without prior notice
> > > > > + * It is not recommended to include this file directly.
> > > > > + * Please include <rte_ring_elem.h> instead.
> > > > > + *
> > > > > + * Ring Peek Scatter Gather APIs
> > > >
> > > > I am not fully convinced by the API name. To me, "scatter/gather"
> > > > is associated to iovecs, like for instance in [1]. The wikipedia
> > > > definition [2] may be closer though.
> > > >
> > > > [1]
> > > >
> https://www.gnu.org/software/libc/manual/html_node/Scatter_002dGat
> > > > he
> > > > r.html
> > > > [2]
> > > > https://en.wikipedia.org/wiki/Gather-scatter_(vector_addressing)
> > > The way I understand scatter-gather is, the data to be sent to
> > > something (like a device) is coming from multiple sources. It would
> > > require
> > copying to put the data together to be contiguous. If the device
> > supports scatter-gather, such copying is not required. The device can
> collect data from multiple locations and make it contiguous.
> > >
> > > In the case I was looking at, one part of the data was coming from
> > > the user of the API and another was generated by the API itself. If
> > these two pieces of information have to be enqueued as a single object
> > on the ring, I had to create an intermediate copy. But by exposing the ring
> memory to the user, the intermediate copy is avoided. Hence I called it
> scatter-gather.
> > >
> > > >
> > > > What about "zero-copy"?
> > > I think no-copy (nc for short) or user-copy (uc for short) would
> > > convey the meaning better. These would indicate that the rte_ring
> > > APIs are
> > not copying the objects and it is left to the user to do the actual copy.
> > >
> > > >
> > > > Also, the "peek" term looks also a bit confusing to me, but it
> > > > existed before your patch. I understand it for dequeue, but not for
> enqueue.
> > > I kept 'peek' there because the API still offers the 'peek' API
> > > capabilities. I am also not sure on what 'peek' means for enqueue
> > > API. The
> > enqueue 'peek' API was provided to be symmetric with dequeue peek API.
> > >
> > > >
> > > > Or, what about replacing the existing experimental peek API by this one?
> > > > They look quite similar to me.
> > > I agree, scatter gather APIs provide the peek capability and the no-copy
> benefits.
> > > Konstantin, any opinions here?
> 
> Sorry, didn't have time yet, to look at this RFC properly.
> Will try to do it next week, as I understand that's for 21.02 anyway?
This is committed for 20.11. We should be able to get into RC2.

> 
> > > >
> > > > > + * Introduction of rte_ring with scatter gather serialized
> > > > > + producer/consumer
> > > > > + * (HTS sync mode) makes it possible to split public
> > > > > + enqueue/dequeue API
> > > > > + * into 3 phases:
> > > > > + * - enqueue/dequeue start
> > > > > + * - copy data to/from the ring
> > > > > + * - enqueue/dequeue finish
> > > > > + * Along with the advantages of the peek APIs, these APIs
> > > > > + provide the ability
> > > > > + * to avoid copying of the data to temporary area.
> > > > > + *
> > > > > + * Note that right now this new API is available only for two sync
> modes:
> > > > > + * 1) Single Producer/Single Consumer (RTE_RING_SYNC_ST)
> > > > > + * 2) Serialized Producer/Serialized Consumer
> (RTE_RING_SYNC_MT_HTS).
> > > > > + * It is a user responsibility to create/init ring with
> > > > > + appropriate sync
> > > > > + * modes selected.
> > > > > + *
> > > > > + * Example usage:
> > > > > + * // read 1 elem from the ring:
> > > >
> > > > Comment should be "prepare enqueuing 32 objects"
> > > >
> > > > > + * n = rte_ring_enqueue_sg_bulk_start(ring, 32, &sgd, NULL);
> > > > > + * if (n != 0) {
> > > > > + *	//Copy objects in the ring
> > > > > + *	memcpy (sgd->ptr1, obj, sgd->n1 * sizeof(uintptr_t));
> > > > > + *	if (n != sgd->n1)
> > > > > + *		//Second memcpy because of wrapround
> > > > > + *		n2 = n - sgd->n1;
> > > > > + *		memcpy (sgd->ptr2, obj[n2], n2 * sizeof(uintptr_t));
> > > >
> > > > Missing { }
> > > >
> > > > > + *	rte_ring_dequeue_sg_finish(ring, n);
> > > >
> > > > Should be enqueue
> > > >
> > > Thanks, will correct them.
> > >
> > > > > + * }
> > > > > + *
> > > > > + * Note that between _start_ and _finish_ none other thread can
> > > > > + proceed
> > > > > + * with enqueue(/dequeue) operation till _finish_ completes.
> > > > > + */
> > > > > +
> > > > > +#ifdef __cplusplus
> > > > > +extern "C" {
> > > > > +#endif
> > > > > +
> > > > > +#include <rte_ring_peek_c11_mem.h>
> > > > > +
> > > > > +/* Rock that needs to be passed between reserve and commit APIs
> > > > > +*/ struct rte_ring_sg_data {
> > > > > +	/* Pointer to the first space in the ring */
> > > > > +	void **ptr1;
> > > > > +	/* Pointer to the second space in the ring if there is wrap-
> around */
> > > > > +	void **ptr2;
> > > > > +	/* Number of elements in the first pointer. If this is equal to
> > > > > +	 * the number of elements requested, then ptr2 is NULL.
> > > > > +	 * Otherwise, subtracting n1 from number of elements
> requested
> > > > > +	 * will give the number of elements available at ptr2.
> > > > > +	 */
> > > > > +	unsigned int n1;
> > > > > +};
> > > >
> > > > Would it be possible to simply return the offset instead of this structure?
> > > > The wrap could be managed by a __rte_ring_enqueue_elems()
> function.
> > > Trying to use __rte_ring_enqueue_elems() will force temporary copy.
> See below.
> > >
> > > >
> > > > I mean something like this:
> > > >
> > > > 	uint32_t start;
> > > > 	n = rte_ring_enqueue_sg_bulk_start(ring, 32, &start, NULL);
> > > > 	if (n != 0) {
> > > > 		/* Copy objects in the ring. */
> > > > 		__rte_ring_enqueue_elems(ring, start, obj, sizeof(uintptr_t),
> > > > n);
> > > For ex: 'obj' here is temporary copy.
> > >
> > > > 		rte_ring_enqueue_sg_finish(ring, n);
> > > > 	}
> > > >
> > > > It would require to slightly change __rte_ring_enqueue_elems() to
> > > > support to be called with prod_head >= size, and wrap in that case.
> > > >
> > > The alternate solution I can think of requires 3 things 1) the base
> > > address of the ring 2) Index to where to copy 3) the mask. With
> > > these 3
> > things one could write the code like below:
> > > for (i = 0; i < n; i++) {
> > > 	ring_addr[(index + i) & mask] = obj[i]; // ANDing with mask will take
> care of wrap-around.
> > > }
> > >
> > > However, I think this does not allow for passing the address in the
> > > ring to another function/API to copy the data (It is possible, but
> > > the user
> > has to calculate the actual address, worry about the wrap-around, second
> pointer etc).
> > >
> > > The current approach hides some details and provides flexibility to the
> application to use the pointer the way it wants.
> >
> > I agree that doing the access + masking manually looks too complex.
> >
> > However I'm not sure to get why using __rte_ring_enqueue_elems()
> would
> > result in an additional copy. I suppose the object that you want to
> > enqueue is already stored somewhere?
I think this is the key. The object is not stored any where (yet), it is getting generated. When it is generated, it should get stored directly into the ring. I have provided some examples below.

> >
> > For instance, let's say you have 10 objects to enqueue, located at
> > different places:
> >
> > 	void *obj_0_to_3 = <place where objects 0 to 3 are stored>;
> > 	void *obj_4_to_7 = ...;
> > 	void *obj_8_to_9 = ...;
> > 	uint32_t start;
> > 	n = rte_ring_enqueue_sg_bulk_start(ring, 10, &start, NULL);
> > 	if (n != 0) {
> > 		__rte_ring_enqueue_elems(ring, start, obj_0_to_3,
> > 			sizeof(uintptr_t), 4);
> > 		__rte_ring_enqueue_elems(ring, start + 4, obj_4_to_7,
> > 			sizeof(uintptr_t), 4);
> > 		__rte_ring_enqueue_elems(ring, start + 8, obj_8_to_9,
> > 			sizeof(uintptr_t), 2);
> > 		rte_ring_enqueue_sg_finish(ring, 10);
> > 	}
> >
> 
> 
> As I understand, It is not about different objects stored in different places, it
> is about:
> a) object is relatively big (16B+ ?)
> b) You compose objects from values stored in few different places.
> 
> Let say you have:
> struct elem_obj {uint64_t a; uint32_t b, c;};
> 
> And then you'd like to copy 'a' value from one location, 'b' from second, and
> 'c' from third one.
> 
> Konstantin
> 
I think there are multiple use cases. Some I have in mind are:

1)
Code without this patch:

struct rte_mbuf *pkts_burst[32];

/* Create ring with sync type RTE_RING_SYNC_ST or RTE_RING_SYNC_MT_HTS */

/* Pkt I/O core polls packets from the NIC, pkts_burst is the temporary store */
nb_rx = rte_eth_rx_burst(portid, queueid, pkts_burst, 32);
/* Provide packets to the packet processing cores */
rte_ring_enqueue_burst(ring, pkts_burst, 32, &free_space);

Code with the patch:

/* Create ring with sync type RTE_RING_SYNC_ST or RTE_RING_SYNC_MT_HTS */

/* Reserve space on the ring */
n = rte_ring_enqueue_sg_burst_start(ring, 32, &sgd, NULL);
/* Pkt I/O core polls packets from the NIC */
if (n == 32)
	nb_rx = rte_eth_rx_burst(portid, queueid, sgd->ptr1, 32);
else
	nb_rx = rte_eth_rx_burst(portid, queueid, sgd->ptr1, sgd->n1);
/* Provide packets to the packet processing cores */
/* Temporary storage 'pkts_burst' is not required */
rte_ring_enqueue_sg_finish(ring, nb_rx);


2) This is same/similar to what Konstantin mentioned

Code without this patch:

struct elem_obj {uint64_t a; uint32_t b, c;};
struct elem_obj obj;

/* Create ring with sync type RTE_RING_SYNC_ST or RTE_RING_SYNC_MT_HTS */

obj.a = rte_get_a();
obj.b = rte_get_b();
obj.c = rte_get_c();
/* obj is the temporary storage and results in memcpy in the following call */
rte_ring_enqueue_elem(ring, sizeof(struct elem_obj), 1, &obj, NULL);

Code with the patch:

struct elem_obj *obj;
/* Reserve space on the ring */
n = rte_ring_enqueue_sg_bulk_elem_start(ring, sizeof(elem_obj), 1, &sgd, NULL);

obj = (struct elem_obj *)sgd->ptr1;
obj.a = rte_get_a();
obj.b = rte_get_b();
obj.c = rte_get_c();
/* obj is not a temporary storage */
rte_ring_enqueue_sg_elem_finish(ring, n);