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A Fully Buffered Memory System Simulator. FBsim 1.0. Rami Nasr -M.S. Thesis, and ENEE 759H Course Project Thursday May 12 th , 2005. Another Simulator?. Sim-DRAM exists and supports FB-DIMM. Why write another simulator? .

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a fully buffered memory system simulator

A Fully Buffered Memory System Simulator

FBsim 1.0

Rami Nasr

-M.S. Thesis, and ENEE 759H Course Project

Thursday May 12th, 2005

another simulator
Another Simulator?

Sim-DRAM exists and supports FB-DIMM. Why write another simulator?

  • Sim-DRAM still had a few unworkable bugs in its FB-DIMM model when I began my study.
  • FB-DIMM is radically different than other memory architectures. New simulator => fresh start.
  • FBsim is made exclusively for simulating and studying the FB-DIMM architecture. Easier to study FB-DIMM with an exclusive simulator.
  • Different scheduler, mapping algorithm, approach, style, section of study in the FB-DIMM design space.
  • FBsim is ideal for simulating ‘unreasonably’ high memory request rates and studying channel saturation effects.
  • The two simulators can be used to validate each other’s results in FB-DIMM studies.
  • Writing a memory simulator was a great experience for me.
fbsim overview
FBsim Overview
  • All code written from scratch.
  • Standalone product. Does not currently interface with CPU simulators or memory traces. Instead probabilistically models memory transactions according to user specifications.
  • => Does not actually store memory data
  • Written in ANSI C. ~5000 lines of code. Code organized into header files, commented, quite easy to hack.
  • Fast. For each memory channel, 1 second simulates ~10ms (or ~1ms during channel saturation) on a 2.4 GHz Pentium 4.
  • Supports Open & Closed Page Mode, Fixed & Variable Latency Mode.
  • Supports output of macro and micro (frame by frame) simulation data
  • Does not model channel init, maintenance, sync. overhead.
  • Does not model memory refresh.
  • Does not model power consumption, and power timing limitations (tFAW etc.).
  • The above options can be incorporated readily into future versions.
fbsim overview 2
FBsim Overview 2

Channel Scheduler 0

Channel Scheduler 1

Input Transaction Generator

Address Mapper

  • A Frame Iteration
  • Try to generate transactions
  • Map any generated transactions to its channel scheduler.
  • Fire each scheduler once.

Channel Scheduler 7

input transaction model
Input Transaction Model
  • Step Distributions
  • Normal (Gaussian) Distributions
address mapping

Closed Page Mode

Open Page Mode

Address Mapping
  • Physical address must be mapped somehow to the right channel, DIMM, rank, bank, row, and column.
  • FBsim built to support different DIMM capacities, different channel capacities, even unbalanced configurations
  • => Algorithm needed to map incoming transaction to DIMM

WHILE (a non zero row sum exists)


WHILE (visit each channel with a non zero

row sum exactly once)


The next 'result' is channel DIMM with the

highest number.

Decrement that DIMM's number by 1.

Decrement the row sum by 1.



Modulus = 4+2+1+2 = 9

fb dimm frame format review
FB-DIMM Frame Format Review
  • SouthBound (SB) Frame could be a:
    • Channel Frame (not modeled in FBsim)
    • Command Frame (up to three DRAM commands, with only one command possible to each DIMM in the channel)
    • Command + Wdata Frame (holds one DRAM command, plus one DDR beat of write data)
  • NorthBound (NB) Frame could be a:
    • Channel Frame (not modeled in FBsim)
    • Read Response Frame (holds two DDR beats of returned read data)
some of my results

1x8 achieved 7.9 GBps before saturating (82%)

  • 2x4 achieved 15.6 GBps (82%)
  • 4x2 achieved 31.3 GBps (82%)
  • 8x1 achieved 45.2 GBps (59%!)
Some of my Results
  • Case Study Conclusion
  • With at least two DIMMs on each channel, performance scales very well in FB-DIMM
  • More than two DIMMs only increases capacity, not throughput
  • Adding each DIMM adds ~5ns average channel latency in FLM, and slightly over half that in VLM
  • In closed page mode, only 82% of peak theoretical throughput of a channel can be reached.
some of my results 2
Some of my Results 2
  • In Closed Page Mode with 2:1 read/write ratio, a reordering window of size ~12 transactions achieves best possible performance (channel saturation) for a FB-DIMM channel scheduler. Increasing window-size over this has no benefit.
  • The more skewed the read/write ratio, the bigger the scheduling window needs to be (at 4:1, its ~18).
  • In Variable Latency Mode, a reordering window of size ~20 achieves best possible performance.
some of my results 3
Some of my Results 3

Micro-study shows that in Closed Page Mode, the FB channel can at most reach ~93% write data utilization on the SB, and ~84% read data utilization on the NB.

Micro-study showed that FBsim channel utilization was slightly worse for non 2:1 read/write ratios (it was 2% worse for 4:1). FBsim scheduler can quite straightforwardly be made more adaptive to read/write ratio of transactions in scheduler.

future ideas with fbsim
Future Ideas with FBsim
  • I’m graduating this semester (if Dr Jacob and Mr (Dr?) Wang so please), and escaping to the corporate world.
  • => Writing a guide for FBsim along with some ideas for future work. Anyone who wishes to take over development is eagerly encouraged to.
  • If so, I would be happy to help get things rolling by email or in person. Feel free to access & use anything in FBsim or my thesis paper.
  • I strongly believe a very interesting paper or three can quite quickly come out of this research area


future ideas with fbsim 2
Future Ideas with FBsim 2
  • For credibility in a paper, add an interface between FBsim and a CPU simulator or memory traces. Run real benchmarks through FBsim. Compare and contrast these results with the transaction modeling results.
  • AND/OR add more functionality and provable realism to the transaction modeler. Study this.
  • Best yet, integrate FBsim into the Sim-DRAM package as an added option.
  • Add modeling for channel overhead, memory refresh overhead, error simulation and error handling, power consumption constraints and metrics.
  • Enhance adaptivity of FBsim scheduler to non 2:1 read/write ratios.
  • Experiment with address mapping algorithm and load balancing.
  • Experiment with different type scheduler implementations (eg. ones not based on pattern matching). *involved*
  • Study hardware constraints in FB-DIMM channel scheduling.
more possible fb dimm studies
More Possible FB-DIMM Studies
  • Channel utilization and configuration trade-offs for Open Page Mode
  • Performance degradation of shrinking scheduler reorder window size
  • Relaxation on critical DRAM device parameters (density, nBanks, timing constraints, clock frequency) allowed by FB-DIMM architecture
  • OR optimizing the FB-DIMM architecture by increasing the SB and NB channel widths (adding lines) or bitrates, and maybe modifying the frame protocol
  • AMB is a logic device on a memory module!! Can add buffers, arithmetic units, processing power, etc…..
special thanks to
Special Thanks to..
  • Dr Jacob for introducing me to the field and guiding my progress
  • David Wang for the course lectures and material