Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 The Symmetric Communication Interface (SCIF (pronounced as skiff)) is a low 2 level communications API across PCIe currently implemented for MIC. Currently 3 SCIF provides inter-node communication within a single host platform, where a 4 node is a MIC Coprocessor or Xeon based host. SCIF abstracts the details of 5 communicating over the PCIe bus while providing an API that is symmetric 6 across all the nodes in the PCIe network. An important design objective for SCIF 7 is to deliver the maximum possible performance given the communication 8 abilities of the hardware. SCIF has been used to implement an offload compiler 9 runtime and OFED support for MPI implementations for MIC coprocessors. 10 11 ==== SCIF API Components ==== 12 The SCIF API has the following parts: 13 1. Connection establishment using a client server model 14 2. Byte stream messaging intended for short messages 15 3. Node enumeration to determine online nodes 16 4. Poll semantics for detection of incoming connections and messages 17 5. Memory registration to pin down pages 18 6. Remote memory mapping for low latency CPU accesses via mmap 19 7. Remote DMA (RDMA) for high bandwidth DMA transfers 20 8. Fence APIs for RDMA synchronization 21 22 SCIF exposes the notion of a connection which can be used by peer processes on 23 nodes in a SCIF PCIe "network" to share memory "windows" and to communicate. A 24 process in a SCIF node initiates a SCIF connection to a peer process on a 25 different node via a SCIF "endpoint". SCIF endpoints support messaging APIs 26 which are similar to connection oriented socket APIs. Connected SCIF endpoints 27 can also register local memory which is followed by data transfer using either 28 DMA, CPU copies or remote memory mapping via mmap. SCIF supports both user and 29 kernel mode clients which are functionally equivalent. 30 31 ==== SCIF Performance for MIC ==== 32 DMA bandwidth comparison between the TCP (over ethernet over PCIe) stack versus 33 SCIF shows the performance advantages of SCIF for HPC applications and runtimes. 34 35 Comparison of TCP and SCIF based BW 36 37 Throughput (GB/sec) 38 8 + PCIe Bandwidth ****** 39 + TCP ###### 40 7 + ************************************** SCIF %%%%%% 41 | %%%%%%%%%%%%%%%%%%% 42 6 + %%%% 43 | %% 44 | %%% 45 5 + %% 46 | %% 47 4 + %% 48 | %% 49 3 + %% 50 | % 51 2 + %% 52 | %% 53 | % 54 1 + 55 + ###################################### 56 0 +++---+++--+--+-+--+--+-++-+--+-++-+--+-++-+- 57 1 10 100 1000 10000 100000 58 Transfer Size (KBytes) 59 60 SCIF allows memory sharing via mmap(..) between processes on different PCIe 61 nodes and thus provides bare-metal PCIe latency. The round trip SCIF mmap 62 latency from the host to an x100 MIC for an 8 byte message is 0.44 usecs. 63 64 SCIF has a user space library which is a thin IOCTL wrapper providing a user 65 space API similar to the kernel API in scif.h. The SCIF user space library 66 is distributed @ https://software.intel.com/en-us/mic-developer 67 68 Here is some pseudo code for an example of how two applications on two PCIe 69 nodes would typically use the SCIF API: 70 71 Process A (on node A) Process B (on node B) 72 73 /* get online node information */ 74 scif_get_node_ids(..) scif_get_node_ids(..) 75 scif_open(..) scif_open(..) 76 scif_bind(..) scif_bind(..) 77 scif_listen(..) 78 scif_accept(..) scif_connect(..) 79 /* SCIF connection established */ 80 81 /* Send and receive short messages */ 82 scif_send(..)/scif_recv(..) scif_send(..)/scif_recv(..) 83 84 /* Register memory */ 85 scif_register(..) scif_register(..) 86 87 /* RDMA */ 88 scif_readfrom(..)/scif_writeto(..) scif_readfrom(..)/scif_writeto(..) 89 90 /* Fence DMAs */ 91 scif_fence_signal(..) scif_fence_signal(..) 92 93 mmap(..) mmap(..) 94 95 /* Access remote registered memory */ 96 97 /* Close the endpoints */ 98 scif_close(..) scif_close(..)