X-Git-Url: http://git.droids-corp.org/?a=blobdiff_plain;f=doc%2Fguides%2Fprog_guide%2Fring_lib.rst;h=895484d9597bdfcfb49733c6a575cbe864791de6;hb=6abee736abe62e9555672c87e8d1659024489818;hp=d1ac47597945b35324bcf8bef721b500a04a5293;hpb=ebff988d0c70f13eb40973f81c0939842f41abb2;p=dpdk.git

diff --git a/doc/guides/prog_guide/ring_lib.rst b/doc/guides/prog_guide/ring_lib.rst
index d1ac475979..895484d959 100644
--- a/doc/guides/prog_guide/ring_lib.rst
+++ b/doc/guides/prog_guide/ring_lib.rst
@@ -359,6 +359,8 @@ That should help users to configure ring in the most suitable way for his
 specific usage scenarios.
 Currently supported modes:
 
+.. _Ring_Library_MPMC_Mode:
+
 MP/MC (default one)
 ~~~~~~~~~~~~~~~~~~~
 
@@ -369,11 +371,87 @@ per core) this is usually the most suitable and fastest synchronization mode.
 As a well known limitation - it can perform quite pure on some overcommitted
 scenarios.
 
+.. _Ring_Library_SPSC_Mode:
+
 SP/SC
 ~~~~~
 Single-producer (/single-consumer) mode. In this mode only one thread at a time
 is allowed to enqueue (/dequeue) objects to (/from) the ring.
 
+.. _Ring_Library_MT_RTS_Mode:
+
+MP_RTS/MC_RTS
+~~~~~~~~~~~~~
+
+Multi-producer (/multi-consumer) with Relaxed Tail Sync (RTS) mode.
+The main difference from the original MP/MC algorithm is that
+tail value is increased not by every thread that finished enqueue/dequeue,
+but only by the last one.
+That allows threads to avoid spinning on ring tail value,
+leaving actual tail value change to the last thread at a given instance.
+That technique helps to avoid the Lock-Waiter-Preemption (LWP) problem on tail
+update and improves average enqueue/dequeue times on overcommitted systems.
+To achieve that RTS requires 2 64-bit CAS for each enqueue(/dequeue) operation:
+one for head update, second for tail update.
+In comparison the original MP/MC algorithm requires one 32-bit CAS
+for head update and waiting/spinning on tail value.
+
+.. _Ring_Library_MT_HTS_Mode:
+
+MP_HTS/MC_HTS
+~~~~~~~~~~~~~
+
+Multi-producer (/multi-consumer) with Head/Tail Sync (HTS) mode.
+In that mode enqueue/dequeue operation is fully serialized:
+at any given moment only one enqueue/dequeue operation can proceed.
+This is achieved by allowing a thread to proceed with changing ``head.value``
+only when ``head.value == tail.value``.
+Both head and tail values are updated atomically (as one 64-bit value).
+To achieve that 64-bit CAS is used by head update routine.
+That technique also avoids the Lock-Waiter-Preemption (LWP) problem on tail
+update and helps to improve ring enqueue/dequeue behavior in overcommitted
+scenarios. Another advantage of fully serialized producer/consumer -
+it provides the ability to implement MT safe peek API for rte_ring.
+
+Ring Peek API
+-------------
+
+For ring with serialized producer/consumer (HTS sync mode) it is possible
+to split public enqueue/dequeue API into two phases:
+
+*   enqueue/dequeue start
+
+*   enqueue/dequeue finish
+
+That allows user to inspect objects in the ring without removing them
+from it (aka MT safe peek) and reserve space for the objects in the ring
+before actual enqueue.
+Note that this API is available only for two sync modes:
+
+*   Single Producer/Single Consumer (SP/SC)
+
+*   Multi-producer/Multi-consumer with Head/Tail Sync (HTS)
+
+It is a user responsibility to create/init ring with appropriate sync modes
+selected. As an example of usage:
+
+.. code-block:: c
+
+    /* read 1 elem from the ring: */
+    uint32_t n = rte_ring_dequeue_bulk_start(ring, &obj, 1, NULL);
+    if (n != 0) {
+        /* examine object */
+        if (object_examine(obj) == KEEP)
+            /* decided to keep it in the ring. */
+            rte_ring_dequeue_finish(ring, 0);
+        else
+            /* decided to remove it from the ring. */
+            rte_ring_dequeue_finish(ring, n);
+    }
+
+Note that between ``_start_`` and ``_finish_`` none other thread can proceed
+with enqueue(/dequeue) operation till ``_finish_`` completes.
+
 References
 ----------