Page Comparison

...

• MSR's Monsoon vs. UCSD's Fat-Tree commodity system
  • Want network connecting many 1GigE nodes with no oversubscription
  • MSR uses a hierarchical configuration (10GigE aggregation and core switches, 1GigE TOR switches)
  • UCSD's uses identical, 24-port, commodity 1GigE switches (i.e. k = 48)
  • Both theoretically capable of 1:1 oversubscription (i.e. no oversubscription)

• 	Hierarchical	Fat-tree
  # hosts	25,920	27,648
  # switches	108 x 144-port 10GigE 1,296 x 20-port 1GigE w/ 2x10GigE uplinks	2,880 x 48-port 1GigE
  # wires	57,024 (~91% GigE, ~9% 10GigE)	82,944
  # unique paths	144 (36 via core with 2x dual uplinks in each subtree)	572

  • Notes:
    • 48-port 1GigE switches cost ~$2.5-3k
      • 2,880 * $2500 = $7M
    • 20-port 1GigE switches w/ 10GigE uplinks probably cost about the same (~2.5-3k) [uplinks not commodity]
      • 1,296 * $2500 = $3.24M
    • 144-port 10GigE switches advertised as $1500/port ($216k/switch) in mid-2007
      • to be competitive with fat-tree on per-port cost, price per port must drop 6.25x to $241.76 ($34.8k/switch)
      • 6.25x drop seems pretty close to the common 2-year drop in price

Alternative Network Topologies

• Ideas from supercomputing:
  • Hypercubes
  • Torus's (?Tori?)
    • IBM Blue Gene connects tens of thousands of CPUs with high bandwidth (e.g. 380MB/sec with 4.5usec avg. ping-pongs - link)
• Hosts connect to n neighbours and route amongst themselves
  • Requires hosts to route frames
    • => higher latencies, unless we can do it on the NIC (NetFPGA?)
  • High wiring complexity
    • no idea how this compares to already high complexity of hierarchical and, especially, fat-tree topologies
  • May impose greater constraints on cluster geometry to appropriately establish links??
  • No dedicated switching elements, simpler (electrically) point-to-point links

RAMCloud Requirements

• latency
  • Arista 48-port 10GigE switches advertise a minimum of 600nsec latency (no idea what the distribution looks like)
    • across 6 hops, that's > 3.6 usec
  • Woven System's 144-port 10GigE switches advertise 1.6usec port-to-port latency (twice Arista's minimum)
    • => > 3.2usec in first two levels of hierarchy
  • take away: sub-5usec is probably not currently possible
• bandwidth
  • 128 bytes / object * 1.0e6 objects/second = 122MBytes/sec (not including any packet overhead)
    • this is gigabit range... 10GigE vs. GigE may be a significant question:
      • Arista 48-port 10GigE's not commodity (~$20k/switch, vs. $2-3k/switch of commodity 1GigE)
      • But what if we have much bigger, hot objects on a machine?
        • Do we want to assume a single machine can always handle requests?
          • e.g. 10KByte object => max. ~12,500 requests/sec on gigabit

Misc. Thoughts

• If networking costs only small part of total DC cost, why is there oversubscription currently?
  • it's possible to pay more and reduce oversubscription - cost doesn't seem the major factor
  • but people argue that oversubscription leads to significant bottlenecks in real DCs
    • but, then, why aren't they reducing oversubscription from the get go?