Fast Buffer Memory with Deterministic Packet Departures Mayank Kabra, Siddhartha Saha, Bill Lin University of California, San Diego

IEEE Hot Interconnects XIV, August 23-25, Packet Buffer in Routers Router Core: Scheduler and Packet Buffers In Out  Incoming linecards have = 8ns to read and write a packet.  The routers need to store the packets to deal with congestion.  Bandwidth X RTT = 40Gb/s*250ms = 1Gb buffer.  Too big to store in SRAM, hence need to use DRAM.  Problem: DRAM access time ~40ns. So, there is roughly 10x speed difference. Linecards

IEEE Hot Interconnects XIV, August 23-25, Parallel and Interleaved DRAM banks DRAMs  Assume the speed difference is 3x P SRAM PP PPP

IEEE Hot Interconnects XIV, August 23-25, Problems with Parallelism  The access pattern can create problems.  If we try to access 3, 6, 9 and 11 one after another, it is possible to issue interleaved read requests and read those packets out at Line Speed. DRAMs

IEEE Hot Interconnects XIV, August 23-25, Problems with Parallelism  But, accessing 2 & 3 or 10 & 11 in succession is problematic.  This is an example of a Bank Conflict DRAMs

IEEE Hot Interconnects XIV, August 23-25, Use The Packet Departure Time  Wide classes of routers (Crossbar Routers) where the packets departures are determined by the scheduler on the fly.  Packet buffers which cater to these routers exist but are complex  There are other high performance routers such as Switch-Memory-Switch, Load Balance Routers for which packet departure time can be calculated when the packet is inserted in the buffer. Solution Idea: We will use the known departure times of the packets to schedule them to different DRAM banks such that there won’t be any conflicts. Solution Idea: We will use the known departure times of the packets to schedule them to different DRAM banks such that there won’t be any conflicts.

IEEE Hot Interconnects XIV, August 23-25, Packet Buffer Abstraction  Fixed sized packets, time is slotted (Example: 40Gb/s, 40 byte packet => 8ns).  The buffer may contain arbitrary large number of logical queues, but with deterministic access.  Single-write Single-read time-deterministic packet buffer model.

IEEE Hot Interconnects XIV, August 23-25, Packet Buffer Architecture  Interleaved memory architecture with multiple slower DRAM banks.  K slower DRAM banks.  b time slots to complete a single memory read or write operation.  b consecutive time slots is a frame.  A time slot t belongs to frame [t/b]

IEEE Hot Interconnects XIV, August 23-25, Packet Buffer Operation DRAMs... 12K-1K b packets …… aggregatede-aggregate SRAM Bypass Buffer arriving packetsdeparting packets

IEEE Hot Interconnects XIV, August 23-25, Packet Arrival [Frame 1]  Frame 1:  Assume b = 3  Packets P 1, P 2 & P 3 arrive in time slot 1, 2 and 3 respectively.  They are aggregated before writing to the DRAM. P1P1 P1P1 P2P2 P2P2 P3P3 P3P DRAMs

IEEE Hot Interconnects XIV, August 23-25, Packet Arrival [Frame 2]  Frame 2:  Packets P 1, P 2 & P 3 are being written to the DRAM banks (1, 2 & 3) during Frame 2.  New packets P 4, P 5, P 6 comes, which are stored in the buffer. P4P4 P4P4 P5P5 P5P5 P6P6 P6P DRAMs P1P1 P1P1 P2P2 P2P2 P3P3 P3P3

IEEE Hot Interconnects XIV, August 23-25, Packet Departure [Frame 19]  Packets P 58, P 59 & P 60 are scheduled to depart at time slots 58, 59 and 60 respectively (frame 20).  They will be read from the DRAM banks one frame slot before their departure frame slot (frame 19) DRAMs P 59 P 60 P 58

IEEE Hot Interconnects XIV, August 23-25, Packet Departure [Frame 20]  Packets P 58, P 59 & P 60 are read from the buffer and are output from the switch at time slot 58, 59 and 60 respectively DRAMs P 60 P 58 P 59

IEEE Hot Interconnects XIV, August 23-25, SRAM Bypass Buffer  The operational model dictates that the minimum round trip latency to write and read a packet from one of the DRAM banks is 4 frames.  Thus, a packet with a departure time less than 4b-1 time slots away cannot be stored into DRAM.  A small amount of SRAM (size 4b) is used as a bypass buffer.

IEEE Hot Interconnects XIV, August 23-25, Number of DRAM banks  Arrival Write Conflicts: P P P P P P At any current frame f, there can be at most b packets that will be written to the DRAM banks (including the current packet). Hence, for each packet, there will be maximum of b-1 “Arrival Write Conflicts” DRAMs

IEEE Hot Interconnects XIV, August 23-25, Number of DRAM banks  Arrival Read Conflicts: P P P P P P At any current frame f, there can be at most b packets that will be read from the DRAM banks. Those b banks will be busy in the current time frame and will be unavailable. Hence, for each packet, there will be maximum of b “Arrival Read Conflicts” DRAMs

IEEE Hot Interconnects XIV, August 23-25, Number of DRAM banks  Departure Read Conflicts: P P P P P P Any packet that is written in the current frame f, it will eventually need to be read in a future frame d for departure. At that future frame d, there are b-1 other departing packets. Hence, for each packet, there will be maximum of b-1 “Departure Read Conflicts” DRAMs

IEEE Hot Interconnects XIV, August 23-25, 2006 How Many DRAM Banks? P P DRAMs  Total Conflicts:  Arrival Write: (b-1)  Arrival Read: b  Departure Read: (b-1)  Hence, total (3b-2) conflicts.  If the number of banks is more than (3b-2), we will always have a free bank for all the packets.

IEEE Hot Interconnects XIV, August 23-25, DRAM Bank Selection  To find a compatible memory, maintain a two dimensional read-transaction bitmap R.  Each row corresponds to a frame slot.  Each column corresponds to a DRAM bank (hence 3b – 1 columns).  R(f, m) denotes whether m th DRAM bank has an already stored packet that must be read at the f th frame slot.

IEEE Hot Interconnects XIV, August 23-25, DRAM Bank Selection  Write-reservation bitmap W of size (3b – 1)  W(m) denotes that in current frame, m th memory bank has been assigned an arriving packet.

IEEE Hot Interconnects XIV, August 23-25, DRAM Bank Selection Logic

IEEE Hot Interconnects XIV, August 23-25, DRAM Bank Selection  Approach: Greedy solution avoiding the three types of conflicts.  To check if a memory bank is compatible for a packet p arriving at timeframe f, and having a departure timeframe d:  Check NOT(W(m) | R(f,m) | R(d, m))  Instead of checking one memory bank at a time, we can check all of them at once: V = NOT(W | R(f) | R(d)), where R(f) and R(d) are the row vectors. From V, get the index of the first compatible memory.  If n is the bank selected for p, then set W(n) = 1 and R(d,n) = 1.

IEEE Hot Interconnects XIV, August 23-25, Size of the Bitmap  Size of the packet buffer is T packets i.e., T is the farthest departure time slot relative to the current time slot.  Farthest departure frame:  Each row in the bitmap is (3b – 1) bits, then the size of the bitmap is:  Assuming a RTT of 250ms and a line rate of 40Gb/s, the packet buffer would correspond to a memory requirement of T = 3 x 10 7 packets, which makes the bitmap size close to 11MB.

IEEE Hot Interconnects XIV, August 23-25, Additional Details  Location of a packet in the DRAM:  Once a bank has been selected, need a way to assign the actual memory location to write, and later, read the packet.  Determine the memory location based on the departure frame using a circular indexing to map a frame to a packet location in the memory.  How to reorder/de-aggregate the packets?  Store the timestamp in the DRAM with the packet.

IEEE Hot Interconnects XIV, August 23-25, Conclusion  Developed a simple packet buffer architecture when the packet departure times are known e.g., Switch-Memory- Switch and Load-Balanced Routers.  Can support arbitrary large number of logical queues.  Number of DRAM banks and SRAM bypass buffer depend only on the physical parameters.

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