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From: Paul Szczepanek <paul.szczepanek@arm.com>
To: dev@dpdk.org
Cc: Paul Szczepanek <paul.szczepanek@arm.com>,
 Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Subject: [PATCH v6 3/4] docs: add pointer compression to the EAL guide
Date: Thu, 29 Feb 2024 16:03:07 +0000
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Documentation added in the EAL guide for the new
utility functions for pointer compression
showing example code and potential usecases.

Signed-off-by: Paul Szczepanek <paul.szczepanek@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
---
 .../prog_guide/env_abstraction_layer.rst      | 142 ++++++++++++++++++
 1 file changed, 142 insertions(+)

diff --git a/doc/guides/prog_guide/env_abstraction_layer.rst b/doc/guides/prog_guide/env_abstraction_layer.rst
index 6debf54efb..f04d032442 100644
--- a/doc/guides/prog_guide/env_abstraction_layer.rst
+++ b/doc/guides/prog_guide/env_abstraction_layer.rst
@@ -1192,3 +1192,145 @@ will not be deallocated.

 Any successful deallocation event will trigger a callback, for which user
 applications and other DPDK subsystems can register.
+
+.. _pointer_compression:
+
+Pointer Compression
+-------------------
+
+Use ``rte_ptr_compress_16()`` and ``rte_ptr_decompress_16()`` to compress and
+decompress pointers into 16-bit offsets. Use ``rte_ptr_compress_32()`` and
+``rte_ptr_decompress_32()`` to compress and decompress pointers into 32-bit
+offsets.
+
+Compression takes advantage of the fact that pointers are usually located in a
+limited memory region (like a mempool). By converting them to offsets from a
+base memory address they can be stored in fewer bytes. How many bytes are needed
+to store the offset is dictated by the memory region size and alignment of
+objects the pointers point to.
+
+For example, a pointer which is part of a 4GB memory pool can be stored as 32
+bit offset. If the pointer points to memory that is 8 bytes aligned then 3 bits
+can be dropped from the offset and a 32GB memory pool can now fit in 32 bits.
+
+For performance reasons these requirements are not enforced programmatically.
+The programmer is responsible for ensuring that the combination of distance
+from the base pointer and memory alignment allow for storing of the offset in
+the number of bits indicated by the function name (16 or 32). Start of mempool
+memory would be a good candidate for the base pointer. Otherwise any pointer
+that precedes all pointers, is close enough and has the same alignment as the
+pointers being compressed will work.
+
+.. note::
+
+    Performance gains depend on the batch size of pointers and CPU capabilities
+    such as vector extensions. It's important to measure the performance
+    increase on target hardware. A test called ``ring_perf_autotest`` in
+    ``dpdk-test`` can provide the measurements.
+
+Example usage
+~~~~~~~~~~~~~
+
+In this example we send pointers between two cores through a ring. While this
+is a realistic use case the code is simplified for demonstration purposes and
+does not have error handling.
+
+.. code-block:: c
+
+    #include <rte_launch.h>
+    #include <rte_ring.h>
+    #include <rte_ring_elem.h>
+    #include <rte_ptr_compress.h>
+
+    #define ITEMS_ARRAY_SIZE (1024)
+    #define BATCH_SIZE (128)
+    #define ALIGN_EXPONENT (3)
+    #define ITEM_ALIGN (1<<ALIGN_EXPONENT)
+    #define CORE_SEND (1)
+    #define CORE_RECV (2)
+
+    struct item {
+      int a;
+    } __rte_aligned(ITEM_ALIGN);
+
+    static struct item items[ITEMS_ARRAY_SIZE] = {0};
+    static struct rte_ring *ring = NULL;
+
+    static int
+    send_compressed(void *args)
+    {
+      struct item *ptrs_send[BATCH_SIZE] = {0};
+      unsigned int n_send = 0;
+      struct rte_ring_zc_data zcd = {0};
+
+      /* in this example we only fill the ptrs_send once and reuse */
+      for (;n_send < BATCH_SIZE; n_send++)
+        ptrs_send[n_send] = &items[n_send];
+
+      for(;;) {
+        n_send = rte_ring_enqueue_zc_burst_elem_start(
+          ring, sizeof(uint32_t), BATCH_SIZE, &zcd, NULL);
+
+        /* compress ptrs_send into offsets */
+        rte_ptr_compress_32(items, /* base pointer */
+          ptrs_send, /* source array to be compressed */
+          zcd.ptr1, /* destination array to store offsets */
+          zcd.n1, /* how many pointers to compress */
+          ALIGN_EXPONENT /* how many bits can we drop from the offset */);
+
+        if (zcd.ptr2 != NULL)
+          rte_ptr_compress_32(items, ptrs_send + zcd.n1,
+            zcd.ptr2, n_send - zcd.n1, ALIGN_EXPONENT);
+
+        rte_ring_enqueue_zc_finish(ring, n_send);
+      }
+      return 1;
+    }
+
+    static int
+    recv_compressed(void *args)
+    {
+      struct item *ptrs_recv[BATCH_SIZE] = {0};
+      unsigned int n_recv;
+      struct rte_ring_zc_data zcd = {0};
+
+      for(;;) {
+        /* receive compressed pointers from the ring */
+        n_recv = rte_ring_dequeue_zc_burst_elem_start(
+          ring, sizeof(uint32_t), BATCH_SIZE, &zcd, NULL);
+
+        rte_ptr_decompress_32(items, /* base pointer */
+          zcd.ptr1, /* source array to decompress */
+          ptrs_recv, /* destination array to store pointers */
+          zcd.n1, /* how many pointers to decompress */
+          ALIGN_EXPONENT /* how many bits were dropped from the offset */);
+
+        /* handle the potential secondary buffer (caused by ring boundary) */
+        if (zcd.ptr2 != NULL)
+          rte_ptr_decompress_32(items,
+            zcd.ptr2,
+            ptrs_recv + zcd.n1,
+            n_recv - zcd.n1,
+            ALIGN_EXPONENT);
+
+        rte_ring_dequeue_zc_finish(ring, n_recv);
+
+        /* ptrs_recv contains what ptrs_send contained in the other thread */
+        /* (...) */
+      }
+      return 1;
+    }
+
+    void
+    compression_example(void)
+    {
+      ring = rte_ring_create_elem(
+        "COMPR_PTRS", sizeof(uint32_t),
+        1024, rte_socket_id(),
+        RING_F_SP_ENQ | RING_F_SC_DEQ);
+
+      rte_eal_remote_launch(send_compressed, NULL, CORE_SEND);
+      rte_eal_remote_launch(recv_compressed, NULL, CORE_RECV);
+
+      for(;;) {}
+    }
--
2.25.1