misc: mic: SCIF header file and IOCTL interface

[mirror_ubuntu-zesty-kernel.git] / Documentation / mic / scif_overview.txt

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+The Symmetric Communication Interface (SCIF (pronounced as skiff)) is a low
+level communications API across PCIe currently implemented for MIC. Currently
+SCIF provides inter-node communication within a single host platform, where a
+node is a MIC Coprocessor or Xeon based host. SCIF abstracts the details of
+communicating over the PCIe bus while providing an API that is symmetric
+across all the nodes in the PCIe network. An important design objective for SCIF
+is to deliver the maximum possible performance given the communication
+abilities of the hardware. SCIF has been used to implement an offload compiler
+runtime and OFED support for MPI implementations for MIC coprocessors.
+
+==== SCIF API Components ====
+The SCIF API has the following parts:
+1. Connection establishment using a client server model
+2. Byte stream messaging intended for short messages
+3. Node enumeration to determine online nodes
+4. Poll semantics for detection of incoming connections and messages
+5. Memory registration to pin down pages
+6. Remote memory mapping for low latency CPU accesses via mmap
+7. Remote DMA (RDMA) for high bandwidth DMA transfers
+8. Fence APIs for RDMA synchronization
+
+SCIF exposes the notion of a connection which can be used by peer processes on
+nodes in a SCIF PCIe "network" to share memory "windows" and to communicate. A
+process in a SCIF node initiates a SCIF connection to a peer process on a
+different node via a SCIF "endpoint". SCIF endpoints support messaging APIs
+which are similar to connection oriented socket APIs. Connected SCIF endpoints
+can also register local memory which is followed by data transfer using either
+DMA, CPU copies or remote memory mapping via mmap. SCIF supports both user and
+kernel mode clients which are functionally equivalent.
+
+==== SCIF Performance for MIC ====
+DMA bandwidth comparison between the TCP (over ethernet over PCIe) stack versus
+SCIF shows the performance advantages of SCIF for HPC applications and runtimes.
+
+             Comparison of TCP and SCIF based BW
+
+  Throughput (GB/sec)
+    8 +                                             PCIe Bandwidth ******
+      +                                                        TCP ######
+    7 +    **************************************             SCIF %%%%%%
+      |                       %%%%%%%%%%%%%%%%%%%
+    6 +                   %%%%
+      |                 %%
+      |               %%%
+    5 +              %%
+      |            %%
+    4 +           %%
+      |          %%
+    3 +         %%
+      |        %
+    2 +      %%
+      |     %%
+      |    %
+    1 +
+      +    ######################################
+    0 +++---+++--+--+-+--+--+-++-+--+-++-+--+-++-+-
+      1       10     100      1000   10000   100000
+                   Transfer Size (KBytes)
+
+SCIF allows memory sharing via mmap(..) between processes on different PCIe
+nodes and thus provides bare-metal PCIe latency. The round trip SCIF mmap
+latency from the host to an x100 MIC for an 8 byte message is 0.44 usecs.
+
+SCIF has a user space library which is a thin IOCTL wrapper providing a user
+space API similar to the kernel API in scif.h. The SCIF user space library
+is distributed @ https://software.intel.com/en-us/mic-developer
+
+Here is some pseudo code for an example of how two applications on two PCIe
+nodes would typically use the SCIF API:
+
+Process A (on node A)                  Process B (on node B)
+
+/* get online node information */
+scif_get_node_ids(..)                  scif_get_node_ids(..)
+scif_open(..)                          scif_open(..)
+scif_bind(..)                          scif_bind(..)
+scif_listen(..)
+scif_accept(..)                                scif_connect(..)
+/* SCIF connection established */
+
+/* Send and receive short messages */
+scif_send(..)/scif_recv(..)            scif_send(..)/scif_recv(..)
+
+/* Register memory */
+scif_register(..)                      scif_register(..)
+
+/* RDMA */
+scif_readfrom(..)/scif_writeto(..)     scif_readfrom(..)/scif_writeto(..)
+
+/* Fence DMAs */
+scif_fence_signal(..)                  scif_fence_signal(..)
+
+mmap(..)                               mmap(..)
+
+/* Access remote registered memory */
+
+/* Close the endpoints */
+scif_close(..)                         scif_close(..)