Trip Report from IOM-2009

Last weekend I participated at IOM-2009, a workshop on The Influence of I/O on Microprocessor Architecture, co-located with the High Performance Computer Architectures (HPCA) conference-2009 at Raleigh, NC. Ram Huggahalli, Principal Engineer, and Platform Architect in Communication Technology Lab, Intel served as the workshop chair. I must say that this workshop was very well organized. There were 8 presentations, 45 minutes each, leaving ample time for valuable discussions and feedback.

Challenges
The workshop addressed the challenge of how to provide more I/O to systems, particularly with 40 GigE and 100 GigE getting standardized very soon. Here are the challenges as listed by the workshop chair.

1) Making I/O an integral part of chip/system design instead of having it as a peripheral device. Networking and other I/O needs to be integrated with microprocessor design instead of being thought as a peripheral device.
2) Making some kind of revolutionary change in the way network I/O is performed because (i) Memory access wont become faster, (ii) Demand for I/O (networking) will only increase and the current way will probably not scale.

Summary of viewpoints:
Main viewpoints expressed in the workshop:
1) A presentation from Sandia National Labs demonstrated experimental data on pre-release Nehalem systems. Main points: (i) Nehalem showing much improved network I/O because of reduced local memory latency,
(ii) NUMAness in Nehalem plays major role, many benchmarks show great difference using local memory than using remote memory. a list of commercial benchmarks for which performance varies a lot depending on whether memory is located locally or remotely. Memory is controlled in linux with numactl.

2) Main stumbling bottleneck for Network I/O: Avoiding copy required from user space to kernel space for transmit and vice-versa for receive. Strategies presented:
(i) Cache injection (2 papers: 1 from IBM Labs, Zurich, and 1 from Univ. of Victoria): Strategy is to inject data received directly into cache so that when receive() is issued, there is no cache-miss and data is readily available. Challenge is that data that is displaced in cache will cause cache-misses, and therefore it is hard to come up with an algorithm which is suited to all workloads. Presenters from Univ. of Victoria presented strategies in the context of MPI running on a IBM Cell processor.

(ii) IOMMU to drive hardware accelerators (1 paper from Univ. Pittsburgh, Intel): Using IOMMU hardware to access physical memory by supplying virtual address so that a hardware accelerator device can directly access memory. The presenter demonstrated this approach in the context of a USB drive.

(iii) Creating DMA Cache (Chinese Academy of Sciences) : Having separate cache to keep I/O data before it can be read by application. As a result primary cache is not affected.

(iv) Intelligent NICs (Virginia Tech): NICs which can interact with the CPU and transfer data when required.

(3) Other Interesting papers:
(i) Using network processors to create virtual NICs. (Univ Massachussetts, Lowell)
(ii) Active end-system analysis for finding bottleneck rate for receive network I/O: This work is mainly from UCDavis, while I have contributed to the theoretical part. This work demonstrates the importance of pacing on the transmit side, and illustrates how to compute the bottleneck at the receiver using a stochastic model. Slides are available here.

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This blog discusses my work as a performance engineer at Sun Microsystems. It touches upon key topics of performance issues in operating systems and the Solaris Networking stack.

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