552c9ca5a0
WHAT: Open our low-level communication infrastructure by moving all necessary components (btl/rcache/allocator/mpool) down in OPAL All the components required for inter-process communications are currently deeply integrated in the OMPI layer. Several groups/institutions have express interest in having a more generic communication infrastructure, without all the OMPI layer dependencies. This communication layer should be made available at a different software level, available to all layers in the Open MPI software stack. As an example, our ORTE layer could replace the current OOB and instead use the BTL directly, gaining access to more reactive network interfaces than TCP. Similarly, external software libraries could take advantage of our highly optimized AM (active message) communication layer for their own purpose. UTK with support from Sandia, developped a version of Open MPI where the entire communication infrastucture has been moved down to OPAL (btl/rcache/allocator/mpool). Most of the moved components have been updated to match the new schema, with few exceptions (mainly BTLs where I have no way of compiling/testing them). Thus, the completion of this RFC is tied to being able to completing this move for all BTLs. For this we need help from the rest of the Open MPI community, especially those supporting some of the BTLs. A non-exhaustive list of BTLs that qualify here is: mx, portals4, scif, udapl, ugni, usnic. This commit was SVN r32317.
250 строки
10 KiB
Plaintext
250 строки
10 KiB
Plaintext
Design notes on usnic BTL
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nomenclature
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fragment - something the PML asks us to send or put, any size
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segment - something we can put on the wire in a single packet
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chunk - a piece of a fragment that fits into one segment
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a segment can contain either an entire fragment or a chunk of a fragment
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each segment and fragment has associated descriptor.
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Each segment data structure has a block of registered memory associated with
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it which matches MTU for that segment
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ACK - acks get special small segments with only enough memory for an ACK
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non-ACK segments always have a parent fragment
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fragments are either large (> MTU) or small (<= MTU)
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a small fragment has a segment descriptor embedded within it since it
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always needs exactly one.
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a large fragment has no permanently associated segments, but allocates them
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as needed.
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channels
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a channel is a queue pair with an associated completion queue
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each channel has its own MTU and r/w queue entry counts
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There are 2 channels, command and data
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command queue is generally for higher priority fragments
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data queue is for standard data traffic
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command queue should possibly be called "priority" queue
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command queue is shorter and has a smaller MTU that the data queue
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this makes the command queue a lot faster than the data queue, so we
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hijack it for sending very small fragments (<= tiny_mtu, currently 768 bytes)
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command queue is used for ACKs and tiny fragments
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data queue is used for everything else
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PML fragments marked priority should perhaps use command queue
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sending
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Normally, all send requests are simply enqueued and then actually posted
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to the NIC by the routine opal_btl_usnic_module_progress_sends().
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"fastpath" tiny sends are the exception.
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Each module maintains a queue of endpoints that are ready to send.
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An endpoint is ready to send if all of the following are met:
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- the endpoint has fragments to send
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- the endpoint has send credits
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- the endpoint's send window is "open" (not full of un-ACKed segments)
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Each module also maintains a list of segments that need to be retransmitted.
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Note that the list of pending retrans is per-module, not per-endpoint.
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send progression first posts any pending retransmissions, always using the
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data channel. (reason is that if we start getting heavy congestion and
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there are lots of retransmits, it becomes more important than ever to
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prioritize ACKs, clogging command channel with retrans data makes things worse,
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not better)
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Next, progression loops sending segments to the endpoint at the top of
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the "endpoints_with_sends" queue. When an endpoint exhausts its send
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credits or fills its send window or runs out of segments to send, it removes
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itself from the endpoint_with_sends list. Any pending ACKs will be
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picked up and piggy-backed on these sends.
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Finally, any endpoints that still need ACKs whose timer has expired will
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be sent explicit ACK packets.
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[double-click fragment sending]
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The middle part of the progression loop handles both small (single-segment)
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and large (multi-segment) sends.
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For small fragments, the verbs descriptor within the embedded segment is
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updated with length, BTL header is updated, then we call
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opal_btl_usnic_endpoint_send_segment() to send the segment.
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After posting, we make a PML callback if needed.
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For large fragments, a little more is needed. segments froma large
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fragment have a slightly larger BTL header which contains a fragment ID,
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and offset, and a size. The fragment ID is allocated when the first chunk
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the fragment is sent. A segment gets allocated, next blob of data is
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copied into this segment, segment is posted. If last chunk of fragment
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sent, perform callback if needed, then remove fragment from endpoint
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send queue.
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[double-click opal_btl_usnic_endpoint_send_segment()]
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This is common posting code for large or small segments. It assigns a
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sequence number to a segment, checks for an ACK to piggy-back,
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posts the segment to the NIC, and then starts the retransmit timer
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by checking the segment into hotel. Send credits are consumed here.
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send dataflow
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PML control messages with no user data are sent via:
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desc = usnic_alloc(size)
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usnic_send(desc)
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user messages less than eager limit and 1st part of larger
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messages are sent via:
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desc = usnic_prepare_src(convertor, size)
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usnic_send(desc)
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larger msgs
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desc = usnic_prepare_src(convertor, size)
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usnic_put(desc)
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usnic_alloc() currently asserts the length is "small", allocates and
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fills in a small fragment. src pointer will point to start of
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associated registered mem + sizeof BTL header, and PML will put its
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data there.
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usnic_prepare_src() allocated either a large or small fragment based on size
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The fragment descriptor is filled in to have 2 SG entries, 1st pointing to
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place where PML should construct its header. If the data convertor says
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data is contiguous, 2nd SG entry points to user buffer, else it is null and
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sf_convertor is filled in with address of convertor.
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usnic_send()
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If the fragment being sent is small enough, has contiguous data, and
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"very few" command queue send WQEs have been consumed, usnic_send() does
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a fastpath send. This means it posts the segment immediately to the NIC
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with INLINE flag set.
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If all of the conditions for fastpath send are not met, and this is a small
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fragment, the user data is copied into the associated registered memory at this
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time and the SG list in the descriptor is collapsed to one entry.
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After the checks above are done, the fragment is enqueued to be sent
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via opal_btl_usnic_endpoint_enqueue_frag()
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usnic_put()
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PML will have filled in destination address in descriptor. This is saved
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and the fragment is enqueued for processing.
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opal_btl_usnic_endpoint_enqueue_frag()
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This appends the fragment to the "to be sent" list of the endpoint and
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conditionally adds the endpoint to the list of endpoints with data to send
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via opal_btl_usnic_check_rts()
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receive dataflow
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BTL packets has one of 3 types in header: frag, chunk, or ack.
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A frag packet is a full PML fragment.
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A chunk packet is a piece of a fragment that needs to be reassembled.
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An ack packet is header only with a sequence number being ACKed.
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Both frag and chunk packets go through some of the same processing.
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Both may carry piggy-backed ACKs which may need to be processed.
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Both have sequence numbers which must be processed and may result in
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dropping the packet and/or queueing an ACK to the sender.
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frag packets may be either regular PML fragments or PUT segments.
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If the "put_addr" field of the BTL header is set, this is a PUT and
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the data is copied directly to the user buffer. If this field is NULL,
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the segment is passed up to the PML. The PML is expected to do everything
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it needs with this packet in the callback, including copying data out if
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needed. Once the callback is complete, the receive buffer is recycled.
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chunk packets are parts of a larger fragment. If an active fragment receive
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for the matching fragment ID cannot be found, and new fragment info
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descriptor is allocated. If this is not a PUT (put_addr == NULL), we
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malloc() data to reassemble the fragment into. Each subsequent chunk
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is copied either into this reassembly buffer or directly into user memory.
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When the last chunk of a fragment arrives, a PML callback is made for non-PUTs,
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then the fragment info descriptor is released.
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======================================
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fast receive optimization
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In order to optimize latency of small packets, the component progress routine
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implements a fast path for receives. If the first completion is a receive on
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the priority queue, then it is handled by a routine called
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opal_btl_usnic_recv_fast() which does nothing but validates that the packet
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is OK to be received (sequence number OK and not a DUP) and then delivers it
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to the PML. This packet is recorded in the channel structure, and all
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bookeeping for the packet is deferred until the next time component_progress
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is called again.
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This fast path cannot be taken every time we pass through component_progress
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because there will be other completions that need processing, and the receive
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bookeeping for one fast receive must be complete before allowing another fast
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receive to occur, as only one recv segment can be saved for deferred
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processing at a time. This is handled by maintaining a variable in
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opal_btl_usnic_recv_fast() called fastpath_ok which is set to false every time
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the fastpath is taken. A call into the regular progress routine will set this
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flag back to true.
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======================================
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reliability:
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every packet has sequence #
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each endpoint has a "send window" , currently 4096 entries.
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once a segment is sent, it is saved in window array until ACK is received
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ACKs acknowledge all packets <= specified sequence #
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rcvr only ACKs a sequence # when all packets up to that sequence have arrived
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each pkt has dflt retrans timer of 100ms
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packet will be scheduled for retrans if timer expires
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Once a segment is sent, it always has its retransmit timer started.
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This is accomplished by opal_hotel_checkin()
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Any time a segment is posted to the NIC for retransmit, it is checked out
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of the hotel (timer stopped).
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So, a send segment is always in one of 4 states:
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- on free list, unallocated
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- on endpoint to-send list in the case of segment associated with small fragment
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- posted to NIC and in hotel awaiting ACK
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- on module re-send list awaiting retransmission
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rcvr:
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- if a pkt with seq >= expected seq is received, schedule ack of largest
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in-order sequence received if not already scheduled. dflt time is 50us
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- if a packet with seq < expected seq arrives, we send an ACK immediately,
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as this indicates a lost ACK
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sender:
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duplicate ACK triggers immediate retrans if one is not pending for that segment
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Reordering induced by two queues and piggy-backing:
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ACKs can be reordered-
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not an issue at all, old ACKs are simply ignored
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Sends can be reordered-
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(small send can jump far ahead of large sends)
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large send followed by lots of small sends could trigger many retrans
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of the large sends. smalls would have to be paced pretty precisely to
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keep command queue empty enough and also beat out the large sends.
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send credits limit how many larges can be queued on the sender, but there
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could be many on the receiver
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