17. Compression Device Library

The compression framework provides a generic set of APIs to perform compression services as well as to query and configure compression devices both physical(hardware) and virtual(software) to perform those services. The framework currently only supports lossless compression schemes: Deflate and LZS.

17.1. Device Management

17.1.1. Device Creation

Physical compression devices are discovered during the bus probe of the EAL function which is executed at DPDK initialization, based on their unique device identifier. For e.g. PCI devices can be identified using PCI BDF (bus/bridge, device, function). Specific physical compression devices, like other physical devices in DPDK can be white-listed or black-listed using the EAL command line options.

Virtual devices can be created by two mechanisms, either using the EAL command line options or from within the application using an EAL API directly.

From the command line using the –vdev EAL option

--vdev  '<PMD name>,socket_id=0'

Note

  • If a DPDK application requires multiple software compression PMD devices then the required number of --vdev args with appropriate libraries are to be added.
  • An application with multiple compression device instances exposed by the same PMD must specify a unique name for each device.

Example: --vdev  'pmd0' --vdev  'pmd1'

Or, by using the rte_vdev_init API within the application code.

rte_vdev_init("<pmd_name>","socket_id=0")

All virtual compression devices support the following initialization parameters:

  • socket_id - socket on which to allocate the device resources on.

17.1.2. Device Identification

Each device, whether virtual or physical, is uniquely designated by two identifiers:

  • A unique device index used to designate the compression device in all functions exported by the compressdev API.
  • A device name used to designate the compression device in console messages, for administration or debugging purposes.

17.1.3. Device Configuration

The configuration of each compression device includes the following operations:

  • Allocation of resources, including hardware resources if a physical device.
  • Resetting the device into a well-known default state.
  • Initialization of statistics counters.

The rte_compressdev_configure API is used to configure a compression device.

The rte_compressdev_config structure is used to pass the configuration parameters.

See the DPDK API Reference for details.

17.1.4. Configuration of Queue Pairs

Each compression device queue pair is individually configured through the rte_compressdev_queue_pair_setup API.

The max_inflight_ops is used to pass maximum number of rte_comp_op that could be present in a queue at a time. The PMD can then allocate resources accordingly on a specified socket.

See the DPDK API Reference for details.

17.1.5. Logical Cores, Memory and Queue Pair Relationships

The Compressdev library supports NUMA similarly as described in Cryptodev library section.

A queue pair cannot be shared, and should be exclusively used by a single processing context for enqueuing operations or dequeuing operations on the same compression device, since sharing would require global locks and hinder performance. It is however possible to use a different logical core to dequeue an operation on a queue pair from the logical core on which it was enqueued. This means that for a compression burst, enqueue/dequeue APIs are a logical place to transition from one logical core to another in a data processing pipeline.

17.2. Device Features and Capabilities

Compression devices define their functionality through two mechanisms, global device features and algorithm features. Global device features identify device wide level features which are applicable to the whole device, such as supported hardware acceleration and CPU features. List of compression device features can be seen in the RTE_COMPDEV_FF_XXX macros.

The algorithm features are features which the device supports per-algorithm, such as a stateful compression/decompression, checksums operation etc. The list of algorithm features can be seen in the RTE_COMP_FF_XXX macros.

17.2.1. Capabilities

Each PMD has a list of capabilities, including algorithms listed in the enum rte_comp_algorithm, its associated feature flag, and sliding window range in log base 2 value. The sliding window range defines the minimum and maximum size of a lookup window that an algorithm uses to find duplicates.

See the DPDK API Reference for details.

Each Compression poll mode driver defines its array of capabilities for each algorithm it supports. See the PMD implementation for capability initialization.

17.2.2. Capabilities Discovery

PMD capability and features are discovered via the rte_compressdev_info_get function.

The rte_compressdev_info structure contains all the relevant information for the device.

See the DPDK API Reference for details.

17.3. Compression Operation

DPDK compression supports two types of compression methodologies:

  • Stateless - data associated with a compression operation is compressed without any reference to another compression operation.
  • Stateful - data in each compression operation is compressed with reference to previous compression operations in the same data stream i.e. history of data is maintained between the operations.

For more explanation, please refer to the RFC https://www.ietf.org/rfc/rfc1951.txt

17.3.1. Operation Representation

A compression operation is described via struct rte_comp_op, which contains both input and output data. The operation structure includes the operation type (stateless or stateful), the operation status, the priv_xform/stream handle, source, destination and checksum buffer pointers. It also contains the source mempool from which the operation is allocated. The PMD updates the consumed field with the amount of data read from the source buffer, and the produced field with the amount of data written into the destination buffer, along with status of operation. See the section Produced, Consumed And Operation Status: for more details.

The compression operations mempool also has the ability to allocate private memory with the operation for the application’s use. The application software is responsible for specifying all the operation specific fields in the rte_comp_op structure, which are then used by the compression PMD to process the requested operation.

17.3.2. Operation Management and Allocation

The compressdev library provides an API set for managing compression operations which utilize the Mempool Library to allocate operation buffers. Therefore, it ensures that the compression operation is interleaved optimally across the channels and ranks for optimal processing.

A rte_comp_op contains a field indicating the pool it originated from.

rte_comp_op_alloc() and rte_comp_op_bulk_alloc() are used to allocate compression operations from a given compression operation mempool. The operation gets reset before being returned to a user so that the operation is always in a good known state before use by the application.

rte_comp_op_free() is called by the application to return an operation to its allocating pool.

See the DPDK API Reference for details.

17.3.3. Passing source data as mbuf-chain

If input data is scattered across several different buffers, then the application can either parse through all such buffers and make one mbuf-chain and enqueue it for processing or, alternatively, it can make multiple sequential enqueue_burst() calls for each of them, processing them statefully. See Compression API Stateful operation: for stateful processing of ops.

17.3.4. Operation Status

Each operation carries status information updated by the PMD after it is processed. The following are currently supported:

  • RTE_COMP_OP_STATUS_SUCCESS,

    Operation is successfully completed

  • RTE_COMP_OP_STATUS_NOT_PROCESSED,

    Operation has not yet been processed by the device

  • RTE_COMP_OP_STATUS_INVALID_ARGS,

    Operation failed due to invalid arguments in request

  • RTE_COMP_OP_STATUS_ERROR,

    Operation failed because of internal error

  • RTE_COMP_OP_STATUS_INVALID_STATE,

    Operation is invoked in invalid state

  • RTE_COMP_OP_STATUS_OUT_OF_SPACE_TERMINATED,

    Output buffer ran out of space during processing. Error case, PMD cannot continue from here.

  • RTE_COMP_OP_STATUS_OUT_OF_SPACE_RECOVERABLE,

    Output buffer ran out of space before operation completed, but this is not an error case. Output data up to op.produced can be used and the next op in the stream should continue on from op.consumed+1.

17.3.5. Operation status after enqueue / dequeue

Some of the above values may arise in the op after an rte_compressdev_enqueue_burst(). If the number of ops enqueued < the number of ops requested then the app should check the op.status of nb_enqd+1. If the status is RTE_COMP_OP_STATUS_NOT_PROCESSED, it likely indicates a full-queue case for a hardware device, and a retry after dequeuing some ops is likely to be successful. If the op holds any other status, e.g. RTE_COMP_OP_STATUS_INVALID_ARGS, a retry with the same op is unlikely to be successful.

17.3.6. Produced, Consumed And Operation Status

  • If the status is RTE_COMP_OP_STATUS_SUCCESS,

    consumed = amount of data read from input buffer, and produced = amount of data written in destination buffer

  • If status is RTE_COMP_OP_STATUS_ERROR,

    consumed = produced = undefined

  • If status is RTE_COMP_OP_STATUS_OUT_OF_SPACE_TERMINATED,

    consumed = 0 and produced = usually 0, but in decompression cases a PMD may return > 0 i.e. amount of data successfully produced until out of space condition hit. Application can consume output data in this case, if required.

  • If status is RTE_COMP_OP_STATUS_OUT_OF_SPACE_RECOVERABLE,

    consumed = amount of data read, and produced = amount of data successfully produced until out of space condition hit. The PMD has ability to recover from here, so an application can submit the next op from consumed+1, and a destination buffer with available space.

17.4. Transforms

Compression transforms (rte_comp_xform) are the mechanism to specify the details of the compression operation such as algorithm, window size, and checksum.

17.5. Compression API Hash support

The compression API allows an application to enable digest calculation alongside compression and decompression of data. A PMD reflects its support for hash algorithms via capability algo feature flags. If supported, the PMD always calculates the digest on plaintext i.e. before compression and after decompression.

Currently supported list of hash algos are SHA-1 and SHA2 family SHA256.

See the DPDK API Reference for details.

If required, the application should set the valid hash algo in compress or decompress xforms during rte_compressdev_stream_create() or rte_compressdev_private_xform_create(), and pass a valid output buffer in rte_comp_op hash field struct to store the resulting digest. The buffer passed should be contiguous and large enough to store digest, which is 20 bytes for SHA-1 and 32 bytes for SHA2-256.

17.6. Compression API Stateless operation

An op is processed stateless if it has - op_type set to RTE_COMP_OP_STATELESS - flush value set to RTE_COMP_FLUSH_FULL or RTE_COMP_FLUSH_FINAL (required only on compression side), - All required input in source buffer

When all of the above conditions are met, the PMD initiates stateless processing and releases acquired resources after processing of current operation is complete. The application can enqueue multiple stateless ops in a single burst and must attach priv_xform handle to such ops.

17.6.1. priv_xform in Stateless operation

A priv_xform is private data managed internally by the PMD to do stateless processing. A priv_xform is initialized by an application providing a generic xform structure to rte_compressdev_private_xform_create, which returns an opaque priv_xform reference. If the PMD supports SHAREABLE priv_xform, indicated via algorithm feature flag, then the application can attach the same priv_xform with many stateless ops at a time. If not, then the application needs to create as many priv_xforms as it expects to have stateless operations in-flight.

../_images/stateless-op.svg

Fig. 17.3 Stateless Ops using Non-Shareable priv_xform

../_images/stateless-op-shared.svg

Fig. 17.4 Stateless Ops using Shareable priv_xform

The application should call rte_compressdev_private_xform_create() and attach it to a stateless op before enqueuing them for processing and free via rte_compressdev_private_xform_free() during termination.

An example pseudocode to setup and process NUM_OPS stateless ops with each of length OP_LEN using priv_xform would look like:

/*
 * pseudocode for stateless compression
 */

uint8_t cdev_id = rte_compressdev_get_dev_id(<PMD name>);

/* configure the device. */
if (rte_compressdev_configure(cdev_id, &conf) < 0)
    rte_exit(EXIT_FAILURE, "Failed to configure compressdev %u", cdev_id);

if (rte_compressdev_queue_pair_setup(cdev_id, 0, NUM_MAX_INFLIGHT_OPS,
                        socket_id()) < 0)
    rte_exit(EXIT_FAILURE, "Failed to setup queue pair\n");

if (rte_compressdev_start(cdev_id) < 0)
    rte_exit(EXIT_FAILURE, "Failed to start device\n");

/* setup compress transform */
struct rte_comp_xform compress_xform = {
    .type = RTE_COMP_COMPRESS,
    .compress = {
        .algo = RTE_COMP_ALGO_DEFLATE,
        .deflate = {
            .huffman = RTE_COMP_HUFFMAN_DEFAULT
        },
        .level = RTE_COMP_LEVEL_PMD_DEFAULT,
        .chksum = RTE_COMP_CHECKSUM_NONE,
        .window_size = DEFAULT_WINDOW_SIZE,
        .hash_algo = RTE_COMP_HASH_ALGO_NONE
    }
};

/* create priv_xform and initialize it for the compression device. */
rte_compressdev_info dev_info;
void *priv_xform = NULL;
int shareable = 1;
rte_compressdev_info_get(cdev_id, &dev_info);
if (dev_info.capabilities->comp_feature_flags & RTE_COMP_FF_SHAREABLE_PRIV_XFORM) {
    rte_compressdev_private_xform_create(cdev_id, &compress_xform, &priv_xform);
} else {
    shareable = 0;
}

/* create operation pool via call to rte_comp_op_pool_create and alloc ops */
struct rte_comp_op *comp_ops[NUM_OPS];
rte_comp_op_bulk_alloc(op_pool, comp_ops, NUM_OPS);

/* prepare ops for compression operations */
for (i = 0; i < NUM_OPS; i++) {
    struct rte_comp_op *op = comp_ops[i];
    if (!shareable)
        rte_compressdev_private_xform_create(cdev_id, &compress_xform, &op->priv_xform)
    else
        op->private_xform = priv_xform;
    op->op_type = RTE_COMP_OP_STATELESS;
    op->flush_flag = RTE_COMP_FLUSH_FINAL;

    op->src.offset = 0;
    op->dst.offset = 0;
    op->src.length = OP_LEN;
    op->input_chksum = 0;
    setup op->m_src and op->m_dst;
}
num_enqd = rte_compressdev_enqueue_burst(cdev_id, 0, comp_ops, NUM_OPS);
/* wait for this to complete before enqueuing next*/
do {
    num_deque = rte_compressdev_dequeue_burst(cdev_id, 0 , &processed_ops, NUM_OPS);
} while (num_dqud < num_enqd);

17.6.2. Stateless and OUT_OF_SPACE

OUT_OF_SPACE is a condition when the output buffer runs out of space and where the PMD still has more data to produce. If the PMD runs into such condition, then the PMD returns RTE_COMP_OP_OUT_OF_SPACE_TERMINATED error. In such case, the PMD resets itself and can set consumed=0 and produced=amount of output it could produce before hitting out_of_space. The application would need to resubmit the whole input with a larger output buffer, if it wants the operation to be completed.

17.6.3. Hash in Stateless

If hash is enabled, the digest buffer will contain valid data after an op is successfully processed i.e. dequeued with status = RTE_COMP_OP_STATUS_SUCCESS.

17.6.4. Checksum in Stateless

If checksum is enabled, checksum will only be available after an op is successfully processed i.e. dequeued with status = RTE_COMP_OP_STATUS_SUCCESS.

17.7. Compression API Stateful operation

The compression API provides RTE_COMP_FF_STATEFUL_COMPRESSION and RTE_COMP_FF_STATEFUL_DECOMPRESSION feature flag for the PMD to reflect its support for Stateful operations.

A Stateful operation in DPDK compression means the application invokes enqueue burst() multiple times to process a related chunk of data because the application broke the data into several ops.

In such cases - ops are setup with op_type RTE_COMP_OP_STATEFUL, - all ops except the last are set with flush value = RTE_COMP_FLUSH_NONE/SYNC and the last is set with flush value RTE_COMP_FLUSH_FULL/FINAL.

In case of either one or all of the above conditions, the PMD initiates stateful processing and releases acquired resources after processing the operation with flush value = RTE_COMP_FLUSH_FULL/FINAL is complete. Unlike stateless, the application can enqueue only one stateful op from a particular stream at a time and must attach a stream handle to each op.

17.7.1. Stream in Stateful operation

A stream in DPDK compression is a logical entity which identifies a related set of ops. For example, one large file broken into multiple chunks, then the file is represented by a stream, and each chunk of that file is represented by a compression op rte_comp_op. Whenever an application wants stateful processing of such data, then it must get a stream handle via making call to rte_compressdev_stream_create() with an xform, which will return an opaque stream handle to attach to all of the ops carrying data of that stream. In stateful processing, every op requires previous op data for compression/decompression. A PMD allocates and sets up resources such as history, states, etc. within a stream, which are maintained during the processing of related ops.

Unlike priv_xforms, a stream is always a NON_SHAREABLE entity. One stream handle must be attached to only one set of related ops and cannot be reused until all of them are processed with a success/failure status.

../_images/stateful-op.svg

Fig. 17.5 Stateful Ops

An application should call rte_compressdev_stream_create() and attach it to the op before enqueuing them for processing and free via rte_compressdev_stream_free() during termination. All ops that are to be processed statefully should carry the same stream.

See the DPDK API Reference for details.

An example pseudocode to set up and process a stream having NUM_CHUNKS, with each chunk size of CHUNK_LEN, would look like:

/*
 * pseudocode for stateful compression
 */

uint8_t cdev_id = rte_compressdev_get_dev_id(<PMD name>);

/* configure the  device. */
if (rte_compressdev_configure(cdev_id, &conf) < 0)
    rte_exit(EXIT_FAILURE, "Failed to configure compressdev %u", cdev_id);

if (rte_compressdev_queue_pair_setup(cdev_id, 0, NUM_MAX_INFLIGHT_OPS,
                                socket_id()) < 0)
    rte_exit(EXIT_FAILURE, "Failed to setup queue pair\n");

if (rte_compressdev_start(cdev_id) < 0)
    rte_exit(EXIT_FAILURE, "Failed to start device\n");

/* setup compress transform. */
struct rte_comp_xform compress_xform = {
    .type = RTE_COMP_COMPRESS,
    .compress = {
        .algo = RTE_COMP_ALGO_DEFLATE,
        .deflate = {
            .huffman = RTE_COMP_HUFFMAN_DEFAULT
        },
        .level = RTE_COMP_LEVEL_PMD_DEFAULT,
        .chksum = RTE_COMP_CHECKSUM_NONE,
        .window_size = DEFAULT_WINDOW_SIZE,
        .hash_algo = RTE_COMP_HASH_ALGO_NONE
    }
};

/* create stream */
void *stream;
rte_compressdev_stream_create(cdev_id, &compress_xform, &stream);

/* create an op pool and allocate ops */
rte_comp_op_bulk_alloc(op_pool, comp_ops, NUM_CHUNKS);

/* Prepare source and destination mbufs for compression operations */
unsigned int i;
for (i = 0; i < NUM_CHUNKS; i++) {
    if (rte_pktmbuf_append(mbufs[i], CHUNK_LEN) == NULL)
        rte_exit(EXIT_FAILURE, "Not enough room in the mbuf\n");
    comp_ops[i]->m_src = mbufs[i];
    if (rte_pktmbuf_append(dst_mbufs[i], CHUNK_LEN) == NULL)
        rte_exit(EXIT_FAILURE, "Not enough room in the mbuf\n");
    comp_ops[i]->m_dst = dst_mbufs[i];
}

/* Set up the compress operations. */
for (i = 0; i < NUM_CHUNKS; i++) {
    struct rte_comp_op *op = comp_ops[i];
    op->stream = stream;
    op->m_src = src_buf[i];
    op->m_dst = dst_buf[i];
    op->op_type = RTE_COMP_OP_STATEFUL;
    if (i == NUM_CHUNKS-1) {
        /* set to final, if last chunk*/
        op->flush_flag = RTE_COMP_FLUSH_FINAL;
    } else {
        /* set to NONE, for all intermediary ops */
        op->flush_flag = RTE_COMP_FLUSH_NONE;
    }
    op->src.offset = 0;
    op->dst.offset = 0;
    op->src.length = CHUNK_LEN;
    op->input_chksum = 0;
    num_enqd = rte_compressdev_enqueue_burst(cdev_id, 0, &op[i], 1);
    /* wait for this to complete before enqueuing next*/
    do {
        num_deqd = rte_compressdev_dequeue_burst(cdev_id, 0 , &processed_ops, 1);
    } while (num_deqd < num_enqd);
    /* analyze the amount of consumed and produced data before pushing next op*/
}

17.7.2. Stateful and OUT_OF_SPACE

If a PMD supports stateful operation, then an OUT_OF_SPACE status is not an actual error for the PMD. In such a case, the PMD returns with status RTE_COMP_OP_STATUS_OUT_OF_SPACE_RECOVERABLE with consumed = number of input bytes read, and produced = length of complete output buffer. The application should enqueue the next op with source starting at consumed+1, and an output buffer with available space.

17.7.3. Hash in Stateful

If enabled, the digest buffer will contain valid digest after the last op in a stream (having flush = RTE_COMP_FLUSH_FINAL) is successfully processed i.e. dequeued with status = RTE_COMP_OP_STATUS_SUCCESS.

17.7.4. Checksum in Stateful

If enabled, the checksum will only be available after the last op in a stream (having flush = RTE_COMP_FLUSH_FINAL) is successfully processed i.e. dequeued with status = RTE_COMP_OP_STATUS_SUCCESS.

17.8. Burst in compression API

Scheduling of compression operations on DPDK’s application data path is performed using a burst oriented asynchronous API set. A queue pair on a compression device accepts a burst of compression operations using the enqueue burst API. On physical devices the enqueue burst API will place the operations to be processed on the device’s hardware input queue, for virtual devices the processing of the operations is usually completed during the enqueue call to the compression device. The dequeue burst API will retrieve any processed operations available from the queue pair on the compression device, from physical devices this is usually directly from the devices processed queue, and for virtual device’s from an rte_ring where processed operations are placed after being processed on the enqueue call.

A burst in DPDK compression can be a combination of stateless and stateful operations with a condition that for stateful ops only one op at a time should be enqueued from a particular stream i.e. two ops should never belong to the same stream in a single burst. However, a burst may contain multiple stateful ops, as long as each op is attached to a different stream, i.e. a burst can look like:

enqueue_burst op1.no_flush op2.no_flush op3.flush_final op4.no_flush op5.no_flush

Where, op1 .. op5 all belong to different independent data units. op1, op2, op4, op5 must be stateful as stateless ops can only use flush full or final and op3 can be of type stateless or stateful. Every op with type set to RTE_COMP_OP_STATELESS must be attached to priv_xform and every op with type set to RTE_COMP_OP_STATEFUL must be attached to stream.

Since each operation in a burst is independent and thus can be completed out of order, applications which need ordering should setup a per-op user data area, with reordering information so that it can determine enqueue order at dequeue.

Also, if multiple threads calls enqueue_burst() on the same queue pair then it’s the application’s responsibility to use a proper locking mechanism to ensure exclusive enqueuing of operations.

17.8.1. Enqueue / Dequeue Burst APIs

The burst enqueue API uses a compression device identifier and a queue pair identifier to specify the compression device queue pair to schedule the processing on. The nb_ops parameter is the number of operations to process which are supplied in the ops array of rte_comp_op structures. The enqueue function returns the number of operations it actually enqueued for processing, a return value equal to nb_ops means that all packets have been enqueued.

The dequeue API uses the same format as the enqueue API but the nb_ops and ops parameters are now used to specify the max processed operations the user wishes to retrieve and the location in which to store them. The API call returns the actual number of processed operations returned, this can never be larger than nb_ops.

17.9. Sample code

There are unit test applications that show how to use the compressdev library inside app/test/test_compressdev.c

17.9.1. Compression Device API

The compressdev Library API is described in the DPDK API Reference.