Ubuntu Manpage: fi_rxm - The RxM (RDM over MSG) Utility Provider

Provided by: libfabric-dev_1.11.0-3_amd64

NAME

       fi_rxm - The RxM (RDM over MSG) Utility Provider

OVERVIEW

       The  RxM  provider  (ofi_rxm) is an utility provider that supports FI_EP_RDM type endpoint
       emulated over FI_EP_MSG type  endpoint(s)  of  an  underlying  core  provider.   FI_EP_RDM
       endpoints  have  a  reliable  datagram  interface  and  RxM  emulates  this  by hiding the
       connection management of underlying FI_EP_MSG endpoints from the user.  Additionally,  RxM
       can hide memory registration requirement from a core provider like verbs if the apps don't
       support it.

REQUIREMENTS

   Requirements for core provider
       RxM provider requires the core provider to support the following features:

       · MSG endpoints (FI_EP_MSG)

       · RMA read/write (FI_RMA) - Used for implementing rendezvous protocol for large messages.

       · FI_OPT_CM_DATA_SIZE of at least 24 bytes.

   Requirements for applications
       Since RxM emulates RDM endpoints by  hiding  connection  management  and  connections  are
       established  only on-demand (when app tries to send data), the first several data transfer
       calls would return EAGAIN.  Applications should be aware  of  this  and  retry  until  the
       operation succeeds.

       If an application has chosen manual progress for data progress, it should also read the CQ
       so that the connection establishment progresses.  Not doing so would result  in  a  stall.
       See also the ERRORS section in fi_msg(3).

SUPPORTED FEATURES

       The RxM provider currently supports FI_MSG, FI_TAGGED, FI_RMA and FI_ATOMIC capabilities.

       Endpoint types
              The provider supports only FI_EP_RDM.

       Endpoint capabilities
              The  following  data  transfer  interface  is supported: FI_MSG, FI_TAGGED, FI_RMA,
              FI_ATOMIC.

       Progress
              The RxM provider supports both  FI_PROGRESS_MANUAL  and  FI_PROGRESS_AUTO.   Manual
              progress  in general has better connection scale-up and lower CPU utilization since
              there's no separate auto-progress thread.

       Addressing Formats
              FI_SOCKADDR, FI_SOCKADDR_IN

       Memory Region
              FI_MR_VIRT_ADDR, FI_MR_ALLOCATED, FI_MR_PROV_KEY MR mode  bits  would  be  required
              from the app in case the core provider requires it.

LIMITATIONS

When using RxM provider, some limitations from the underlying MSG provider could also show
up. Please refer to the corresponding MSG provider man pages to find about those
limitations.

Unsupported features
RxM provider does not support the following features:

· op_flags: FI_FENCE.

· Scalable endpoints

· Shared contexts

· FABRIC_DIRECT

· FI_MR_SCALABLE

· Authorization keys

· Application error data buffers

· Multicast

· FI_SYNC_ERR

· Reporting unknown source addr data as part of completions

· Triggered operations

Progress limitations
When sending large messages, an app doing an sread or waiting on the CQ file descriptor
may not get a completion when reading the CQ after being woken up from the wait. The app
has to do sread or wait on the file descriptor again. This is needed because RxM uses a
rendezvous protocol for large message sends. An app would get woken up from waiting on CQ
fd when rendezvous protocol request completes but it would have to wait again to get an
ACK from the receiver indicating completion of large message transfer by remote RMA read.

FI_ATOMIC limitations
The FI_ATOMIC capability will only be listed in the fi_info if the fi_info hints parameter
specifies FI_ATOMIC. If FI_ATOMIC is requested, message order FI_ORDER_RAR, FI_ORDER_RAW,
FI_ORDER_WAR, FI_ORDER_WAW, FI_ORDER_SAR, and FI_ORDER_SAW can not be supported.

Miscellaneous limitations
· RxM protocol peers should have same endian-ness otherwise connections won't successfully
complete. This enables better performance at run-time as byte order translations are
avoided.

RUNTIME PARAMETERS

The ofi_rxm provider checks for the following environment variables.

FI_OFI_RXM_BUFFER_SIZE
Defines the transmit buffer size / inject size. Messages of size less than this
would be transmitted via an eager protocol and those above would be transmitted via
a rendezvous or SAR (Segmentation And Reassembly) protocol. Transmit data would be
copied up to this size (default: ~16k).

FI_OFI_RXM_COMP_PER_PROGRESS
Defines the maximum number of MSG provider CQ entries (default: 1) that would be
read per progress (RxM CQ read).

FI_OFI_RXM_SAR_LIMIT
Set this environment variable to control the RxM SAR (Segmentation And Reassembly)
protocol. Messages of size greater than this (default: 128 Kb) would be
transmitted via rendezvous protocol.

FI_OFI_RXM_USE_SRX
Set this to 1 to use shared receive context from MSG provider, or 0 to disable
using shared receive context. Shared receive contexts reduce overall memory usage,
but may increase in message latency. If not set, verbs will not use shared receive
contexts by default, but the tcp provider will.

FI_OFI_RXM_TX_SIZE
Defines default TX context size (default: 1024)

FI_OFI_RXM_RX_SIZE
Defines default RX context size (default: 1024)

FI_OFI_RXM_MSG_TX_SIZE
Defines FI_EP_MSG TX size that would be requested (default: 128).

FI_OFI_RXM_MSG_RX_SIZE
Defines FI_EP_MSG RX size that would be requested (default: 128).

FI_UNIVERSE_SIZE
Defines the expected number of ranks / peers an endpoint would communicate with
(default: 256).

FI_OFI_RXM_CM_PROGRESS_INTERVAL
Defines the duration of time in microseconds between calls to RxM CM progression
functions when using manual progress. Higher values may provide less noise for
calls to fi_cq read functions, but may increase connection setup time (default:
10000)

FI_OFI_RXM_CQ_EQ_FAIRNESS
Defines the maximum number of message provider CQ entries that can be consecutively
read across progress calls without checking to see if the CM progress interval has
been reached (default: 128)

Tuning

   Bandwidth
       To   optimize   for   bandwidth,   ensure   you   use   higher  values  than  default  for
       FI_OFI_RXM_TX_SIZE,  FI_OFI_RXM_RX_SIZE,  FI_OFI_RXM_MSG_TX_SIZE,   FI_OFI_RXM_MSG_RX_SIZE
       subject  to  memory  limits  of  the  system  and the tx and rx sizes supported by the MSG
       provider.

       FI_OFI_RXM_SAR_LIMIT is another  knob  that  can  be  experimented  with  to  optimze  for
       bandwidth.

   Memory
       To conserve memory, ensure FI_UNIVERSE_SIZE set to what is required.  Similarly check that
       FI_OFI_RXM_TX_SIZE, FI_OFI_RXM_RX_SIZE, FI_OFI_RXM_MSG_TX_SIZE and  FI_OFI_RXM_MSG_RX_SIZE
       env variables are set to only required values.

NOTES

       The  data transfer API may return -FI_EAGAIN during on-demand connection setup of the core
       provider FI_MSG_EP.  See fi_msg(3) for a detailed description of handling FI_EAGAIN.

Troubleshooting / Known issues

       If an RxM endpoint is expected to communicate with more peers than the  default  value  of
       FI_UNIVERSE_SIZE  (256)  CQ  overruns  can  happen.   To avoid this set a higher value for
       FI_UNIVERSE_SIZE.  CQ overrun can make a MSG endpoint unusable.

       At higher # of ranks, there may be connection errors due to a node running out of  memory.
       The   workaround   is   to   use   shared   receive   contexts   for   the   MSG  provider
       (FI_OFI_RXM_USE_SRX=1) or reduce  eager  message  size  (FI_OFI_RXM_BUFFER_SIZE)  and  MSG
       provider TX/RX queue sizes (FI_OFI_RXM_MSG_TX_SIZE / FI_OFI_RXM_MSG_RX_SIZE).

AUTHORS

       OpenFabrics.