Re: [dpdk-dev] [RFC v2] doc compression API for DPDK

["Verma, Shally" <Shally.Verma@cavium.com>]

HI Ahmed

> -----Original Message-----
> From: Ahmed Mansour [mailto:ahmed.mansour@nxp.com]
> Sent: 10 January 2018 00:38
> To: Verma, Shally <Shally.Verma@cavium.com>; Trahe, Fiona
> <fiona.trahe@intel.com>; dev@dpdk.org
> Cc: Athreya, Narayana Prasad <NarayanaPrasad.Athreya@cavium.com>;
> Gupta, Ashish <Ashish.Gupta@cavium.com>; Sahu, Sunila
> <Sunila.Sahu@cavium.com>; De Lara Guarch, Pablo
> <pablo.de.lara.guarch@intel.com>; Challa, Mahipal
> <Mahipal.Challa@cavium.com>; Jain, Deepak K <deepak.k.jain@intel.com>;
> Hemant Agrawal <hemant.agrawal@nxp.com>; Roy Pledge
> <roy.pledge@nxp.com>; Youri Querry <youri.querry_1@nxp.com>
> Subject: Re: [RFC v2] doc compression API for DPDK
> 
> Hi Shally,
> 
> Thanks for the summary. It is very helpful. Please see comments below
> 
> 
> On 1/4/2018 6:45 AM, Verma, Shally wrote:
> > This is an RFC v2 document to brief understanding and requirements on
> compression API proposal in DPDK. It is based on "[RFC v3] Compression API
> in DPDK
> https://emea01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fdpd
> k.org%2Fdev%2Fpatchwork%2Fpatch%2F32331%2F&data=02%7C01%7Cahm
> ed.mansour%40nxp.com%7C80bd3270430c473fa71d08d55368a0e1%7C686ea
> 1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C636506631207323264&sdata=JF
> tOnJxajgXX7s3DMZ79K7VVM7TXO8lBd6rNeVlsHDg%3D&reserved=0 ".
> > Intention of this document is to align on concepts built into compression
> API, its usage and identify further requirements.
> >
> > Going further it could be a base to Compression Module Programmer
> Guide.
> >
> > Current scope is limited to
> > - definition of the terminology which makes up foundation of compression
> API
> > - typical API flow expected to use by applications
> > - Stateless and Stateful operation definition and usage after RFC v1 doc
> review
> https://emea01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fdev.
> dpdk.narkive.com%2FCHS5l01B%2Fdpdk-dev-rfc-v1-doc-compression-api-
> for-
> dpdk&data=02%7C01%7Cahmed.mansour%40nxp.com%7C80bd3270430c473
> fa71d08d55368a0e1%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C6
> 36506631207323264&sdata=Fy7xKIyxZX97i7vEM6NqgrvnqKrNrWOYLwIA5dEH
> QNQ%3D&reserved=0
> >
> > 1. Overview
> > ~~~~~~~~~~~
> >
> > A. Compression Methodologies in compression API
> > ===========================================
> > DPDK compression supports two types of compression methodologies:
> > - Stateless - each data object is compressed individually without any
> reference to previous data,
> > - Stateful -  each data object is compressed with reference to previous data
> object i.e. history of data is needed for compression / decompression
> > For more explanation, please refer RFC
> https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fw
> ww.ietf.org%2Frfc%2Frfc1951.txt&data=02%7C01%7Cahmed.mansour%40nx
> p.com%7C80bd3270430c473fa71d08d55368a0e1%7C686ea1d3bc2b4c6fa92cd9
> 9c5c301635%7C0%7C0%7C636506631207323264&sdata=pfp2VX1w3UxH5YLcL
> 2R%2BvKXNeS7jP46CsASq0B1SETw%3D&reserved=0
> >
> > To support both methodologies, DPDK compression introduces two key
> concepts: Session and Stream.
> >
> > B. Notion of a session in compression API
> > ==================================
> > A Session in DPDK compression is a logical entity which is setup one-time
> with immutable parameters i.e. parameters that don't change across
> operations and devices.
> > A session can be shared across multiple devices and multiple operations
> simultaneously.
> > A typical Session parameters includes info such as:
> > - compress / decompress
> > - compression algorithm and associated configuration parameters
> >
> > Application can create different sessions on a device initialized with
> same/different xforms. Once a session is initialized with one xform it cannot
> be re-initialized.
> >
> > C. Notion of stream in compression API
> >  =======================================
> > Unlike session which carry common set of information across operations, a
> stream in DPDK compression is a logical entity which identify related set of
> operations and carry operation specific information as needed by device
> during its processing.
> > It is device specific data structure which is opaque to application, setup and
> maintained by device.
> >
> > A stream can be used with *only* one op at a time i.e. no two operations
> can share same stream simultaneously.
> > A stream is *must* for stateful ops processing and optional for stateless
> (Please see respective sections for more details).
> >
> > This enables sharing of a session by multiple threads handling different
> data set as each op carry its own context (internal states, history buffers et
> el) in its attached stream.
> > Application should call rte_comp_stream_create() and attach to op before
> beginning of  operation processing and free via rte_comp_stream_free()
> after its complete.
> >
> > C. Notion of burst operations in compression API
> >  =======================================
> > A burst in DPDK compression is an array of operations where each op carry
> independent set of data. i.e. a burst can look like:
> >
> >                                       ---------------------------------------------------------------------
> ------------------------------------
> >               enque_burst (|op1.no_flush | op2.no_flush | op3.flush_final |
> op4.no_flush | op5.no_flush |)
> >                                        --------------------------------------------------------------------
> -------------------------------------
> >
> > Where, op1 .. op5 are all independent of each other and carry entirely
> different set of data.
> > Each op can be attached to same/different session but *must* be attached
> to different stream.
> >
> > Each op (struct rte_comp_op) carry compression/decompression
> operational parameter and is both an input/output parameter.
> > PMD gets source, destination and checksum information at input and
> update it with bytes consumed and produced and checksum at output.
> >
> > Since each operation in a burst is independent and thus can complete out-
> of-order,  applications which need ordering, should setup per-op user data
> area with reordering information so that it can determine enqueue order at
> deque.
> >
> > Also if multiple threads calls enqueue_burst() on same queue pair then it's
> application onus to use proper locking mechanism to ensure exclusive
> enqueuing of operations.
> >
> > D. Stateless Vs Stateful
> > ===================
> > Compression API provide RTE_COMP_FF_STATEFUL feature flag for PMD
> to reflect its support for Stateful operation. Each op carry an op type
> indicating if it's to be processed stateful or stateless.
> >
> > D.1 Compression API Stateless operation
> > ------------------------------------------------------
> > An op is processed stateless if it has
> > -              flush value is set to RTE_FLUSH_FULL or RTE_FLUSH_FINAL
> (required only on compression side),
> > -	 op_type set to RTE_COMP_OP_STATELESS
> > -              All-of the required input and sufficient large output buffer to store
> output i.e. OUT_OF_SPACE can never occur.
> >
> > When all of the above conditions are met, PMD initiates stateless
> processing and releases acquired resources after processing of current
> operation is complete i.e. full input consumed and full output written.
> > Application can optionally attach a stream to such ops. In such case,
> application must attach different stream to each op.
> >
> > Application can enqueue stateless burst via making consecutive
> enque_burst() calls i.e. Following is relevant usage:
> >
> > enqueued = rte_comp_enque_burst (dev_id, qp_id, ops1, nb_ops);
> > enqueued = rte_comp_enque_burst(dev_id, qp_id, ops2, nb_ops);
> >
> > *Note - Every call has different ops array i.e.  same rte_comp_op array
> *cannot be re-enqueued* to process next batch of data until previous ones
> are completely processed.
> >
> > D.1.1 Stateless and OUT_OF_SPACE
> > ------------------------------------------------
> > OUT_OF_SPACE is a condition when output buffer runs out of space and
> where PMD still has more data to produce. If PMD run into such condition,
> then it's an error condition in stateless processing.
> > In such case, PMD resets itself and return with status
> RTE_COMP_OP_STATUS_OUT_OF_SPACE with produced=consumed=0 i.e.
> no input read, no output written.
> > Application can resubmit an full input with larger output buffer size.
> 
> [Ahmed] Can we add an option to allow the user to read the data that was
> produced while still reporting OUT_OF_SPACE? this is mainly useful for
> decompression applications doing search.

[Shally] It is there but applicable for stateful operation type (please refer to handling out_of_space under "Stateful Section").
By definition, "stateless" here means that application (such as IPCOMP) knows maximum output size guaranteedly and ensure that uncompressed data size cannot grow more than provided output buffer.
Such apps can submit an op with type = STATELESS and provide full input, then PMD assume it has sufficient input and output and thus doesn't need to maintain any contexts after op is processed. 
If application doesn't know about max output size, then it should process it as stateful op i.e. setup op with type = STATEFUL and attach a stream so that PMD can maintain relevant context to handle such condition.

> 
> > D.2 Compression API Stateful operation
> > ----------------------------------------------------------
> >  A Stateful operation in DPDK compression means application invokes
> enqueue burst() multiple times to process related chunk of data either
> because
> > - Application broke data into several ops, and/or
> > - PMD ran into out_of_space situation during input processing
> >
> > In case of either one or all of the above conditions, PMD is required to
> maintain state of op across enque_burst() calls and
> > ops are setup with op_type RTE_COMP_OP_STATEFUL, and begin with
> flush value = RTE_COMP_NO/SYNC_FLUSH and end at flush value
> RTE_COMP_FULL/FINAL_FLUSH.
> >
> > D.2.1 Stateful operation state maintenance
> > ---------------------------------------------------------------
> > It is always an ideal expectation from application that it should parse
> through all related chunk of source data making its mbuf-chain and enqueue
> it for stateless processing.
> > However, if it need to break it into several enqueue_burst() calls, then an
> expected call flow would be something like:
> >
> > enqueue_burst( |op.no_flush |)
> 
> [Ahmed] The work is now in flight to the PMD.The user will call dequeue
> burst in a loop until all ops are received. Is this correct?
> 
> > deque_burst(op) // should dequeue before we enqueue next

[Shally] Yes. Ideally every submitted op need to be dequeued. However this illustration is specifically in context of stateful op processing to reflect if a stream is broken into chunks, then each chunk should be submitted as one op at-a-time with type = STATEFUL and need to be dequeued first before next chunk is enqueued.

> > enqueue_burst( |op.no_flush |)
> > deque_burst(op) // should dequeue before we enqueue next
> > enqueue_burst( |op.full_flush |)
> 
> [Ahmed] Why now allow multiple work items in flight? I understand that
> occasionaly there will be OUT_OF_SPACE exception. Can we just distinguish
> the response in exception cases?

[Shally] Multiples ops are allowed in flight, however condition is each op in such case is independent of each other i.e. belong to different streams altogether.
Earlier (as part of RFC v1 doc) we did consider the proposal to process all related chunks of data in single burst by passing them as ops array but later found that as not-so-useful for PMD handling for various reasons. You may please refer to RFC v1 doc review comments for same.
 
> >
> > Here an op *must* be attached to a stream and every subsequent
> enqueue_burst() call should carry *same* stream. Since PMD maintain ops
> state in stream, thus it is mandatory for application to attach stream to such
> ops.
> >
> > D.2.2 Stateful and Out_of_Space
> > --------------------------------------------
> > If PMD support stateful and run into OUT_OF_SPACE situation, then it is
> not an error condition for PMD. In such case, PMD return with status
> RTE_COMP_OP_STATUS_OUT_OF_SPACE with consumed = number of input
> bytes read and produced = length of complete output buffer.
> > Application should enqueue op with source starting at consumed+1 and
> output buffer with available space.
> 
> [Ahmed] Related to OUT_OF_SPACE. What status does the user recieve in a
> decompression case when the end block is encountered before the end of
> the input? Does the PMD continue decomp? Does it stop there and return
> the stop index?
> 

[Shally] Before I could answer this, please help me understand your use case . When you say  "when the end block is encountered before the end of the input?" Do you mean -
"Decompressor process a final block (i.e. has BFINAL=1 in its header) and there's some footer data after that?" Or 
you mean "decompressor process one block and has more to process till its final block?"
What is "end block" and "end of input" reference here?

> >
> > D.2.3 Sliding Window Size
> > ------------------------------------
> > Every PMD will reflect in its algorithm capability structure maximum length
> of Sliding Window in bytes which would indicate maximum history buffer
> length used by algo.
> >
> > 2. Example API illustration
> > ~~~~~~~~~~~~~~~~~~~~~~~
> >
> > Following is an illustration on API usage  (This is just one flow, other variants
> are also possible):
> > 1. rte_comp_session *sess = rte_compressdev_session_create
> (rte_mempool *pool);
> > 2. rte_compressdev_session_init (int dev_id, rte_comp_session *sess,
> rte_comp_xform *xform, rte_mempool *sess_pool);
> > 3. rte_comp_op_pool_create(rte_mempool ..)
> > 4. rte_comp_op_bulk_alloc (struct rte_mempool *mempool, struct
> rte_comp_op **ops, uint16_t nb_ops);
> > 5. for every rte_comp_op in ops[],
> >     5.1 rte_comp_op_attach_session (rte_comp_op *op, rte_comp_session
> *sess);
> >     5.2 op.op_type = RTE_COMP_OP_STATELESS
> >     5.3 op.flush = RTE_FLUSH_FINAL
> > 6. [Optional] for every rte_comp_op in ops[],
> >     6.1 rte_comp_stream_create(int dev_id, rte_comp_session *sess, void
> **stream);
> >     6.2 rte_comp_op_attach_stream(rte_comp_op *op, rte_comp_session
> *stream);
> 
> [Ahmed] What is the semantic effect of attaching a stream to every op? will
> this application benefit for this given that it is setup with op_type STATELESS

[Shally] By role, stream is data structure that hold all information that PMD need to maintain for an op processing and thus it's marked device specific. It is required for stateful processing but optional for statelss as PMD doesn't need to maintain context once op is processed unlike stateful.
It may be of advantage to use stream for stateless to some of the PMD. They can be designed to do one-time per op setup (such as mapping session params) during stream_create() in control path than data path.

> 
> > 7.for every rte_comp_op in ops[],
> >      7.1 set up with src/dst buffer
> > 8. enq = rte_compressdev_enqueue_burst (dev_id, qp_id, &ops, nb_ops);
> > 9. do while (dqu < enq) // Wait till all of enqueued are dequeued
> >     9.1 dqu = rte_compressdev_dequeue_burst (dev_id, qp_id, &ops, enq);
> 
> [Ahmed] I am assuming that waiting for all enqueued to be dequeued is not
> strictly necessary, but is just the chosen example in this case
> 

[Shally] Yes. By design, for burst_size>1 each op is independent of each other. So app may proceed as soon as it dequeue any.

> > 10. Repeat 7 for next batch of data
> > 11. for every ops in ops[]
> >       11.1 rte_comp_stream_free(op->stream);
> > 11. rte_comp_session_clear (sess) ;
> > 12. rte_comp_session_terminate(ret_comp_sess *session)
> >
> > Thanks
> > Shally
> >
> >