ATOLL - Technology Overview
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Overview
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ATOLL is a high-performance interconnect, which can be used as System Area Network (SAN) to build Clusters of PC's or workstations (NOW, COW). The unique feature of ATOLL is its architecture, which consists not of a single NI, as all other interconnects. Instead, the ATOLL chip is build out of 4 replicated NIs. To eliminate any need for additional hardware, we've put also the switch and 4 links onto the die. That's the reason why we call ATOLL A Network on a Chip.
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Architecture
- ATOLL integrates 4 independent network interfaces (NIs), an 8x8 crossbar switch and 4 link interfaces in one single chip! This tight integration of all necessary network components on one ASIC enables users to build clusters without any external switching hardware at a yet unmatched price/performance ratio.
An optimized 64 bit PCI-X interface, which can also run as a 66 Mhz interface, enables ATOLL to be used as interconnect for all kind of node architectures, from PC's to server or SMP systems.
The 4 independent NIs make ATOLL an ideal interconnect for SMP nodes, since each processor has its own NI, eliminating any software overhead to multiplex the NI. The PCI-X interface is multiplexed between all NIs and is capable to fully satorate all four outgoing links (each with 250 Mbyte/s peak) with its 1 Gbyte/s peak bandwidth.
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Technology
- The ATOLL chip uses the latest ASIC technology to deliver a new level of communication performance to cluster users: the chip is implemented in a 0.18 um process and runs at 250 Mhz. So each of the four bidirectional, byte-wide links offers a bandwidth of 250 Mbyte/s in each direction, totalling in a bisection bandwidth of 2 Gbyte/s for a single ATOLL card!
The links drive LVDS signals over 5 m cables with standard SCSI-3 connectors.
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Performance
- ATOLL is a highly pipelined and optimized architecture, which is capable to deliver nearly peak performance even for small messages. System simulations indicate that one-way latency will start at about 4 µs (ATOLL-API). Link utilization goes up to 90% of the peak bandwidth, resulting in more than 220 Mbyte/s for larger messages. With the utilization of multiple NIs in parallel one can reach a total data rate of more than 400 Mbyte/s out of each node!
Considering these performance estimations and the cost-effective implementation, ATOLL offers a price/performance ratio yet unavailable on the SAN market:
The diagram depicts the 'per node' costs, including switching hardware (per port costs):
Fast Ethernet: $50 NIC + $50 per port
ATOLL: $600 NIC +$0 per port
todays SANs: $1200 NIC + $200 per port
The bandwidth value represents the bidirectional 'per NIC' bandwidth.
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Flexibility
- Data transfer is available via DMA or Programmed I/O, which enables the user to achieve high performance for small and! large message sizes. The PIO mode is an ideal candidate for synchronization and exchange of small messages between nodes, since it offers true zero-copy data transfer without any intermediate data copies.
In DMA mode, the ATOLL chip writes the incoming data to the hosts main memory without any CPU intervention. Also the notification mechanism to signal the CPU received messages is implemented in a very efficient way. All relevant status information is periodically written to main memory. Therefore, the CPU only polls on cache-coherent memory, not waisting any I/O bus cycles.
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Reliability
- The ATOLL network not only checks transferred data for transmission errors, it also retransmits the erroneous data in case of errors! This is achieved through a segmentation of the message data into smaller packets, which are subject to a link-to-link retransmission protocol. This whole mechanism is absolutely transparent to the user, simplifying the task of the software layers on top of the ATOLL hardware.
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Software
- ATOLL API and ATOLL device drivers will be available for Linux and later Solaris operating systems. MPI(CH) and PVM ports will be available for message passing applications. All software will be available as open source.
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Photos
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5.7 x 5.7 mm ATOLL-Chip 
ATOLL under test 
ATOLL-DIE with wire body 
ATOLL PCI board 
Bond Pads 
more photos
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