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Chronos is an FPGA Acceleration framework for applications with speculative parallelism.

Chronos was published at ASPLOS 2020. Paper, Talk


  1. Directory Structure
  2. Getting started - A tutorial on configuring and running sssp
  3. Chronos software interface
  4. Debugging Chronos
  5. Pipelined cores

Directory Structure

Inside cl_chronos/,

  • build/
    This directory all synthesis scripts

  • design/

    All RTL design files go here. The top module is contained in

    design/ contains all generic configuration information (eg: number of tiles, queue sizes etc..)

    All individual applications are in design/apps/. Each app consist of a separate .sv file for each core type along with a config.vh file for applicaton-specific configuration (eg: how many of each core type per tile).

    To configure/change the currently running application, run:

    cd design ./scripts/ <app_name>

    This script would read the corresponding config.vh and generate several RTL files (gen_core_spec.vh and gen_core_spec_tile.vh) required for synthesis and simulation.

  • hls/

    HLS versions of astar and sssp

  • riscv_code/

    code for all applications with risc-v variants

  • software/

    Runtime code that will program the FPGA, transfer the data, and collect and analyze and performance results.

  • tools/

    Contains miscelleaneous tools. One such tool, 'graph_gen' is used to generate test inputs (and do format conversion of existing inputs) for graph algorithms.

  • verif/

    RTL verification code and scripts.

Getting started - A tutorial on configuring and running sssp.

Chronos depends on the Amazon AWS EC2 Hardware and Software Development kit. The script would automatically clone this repo and perform other necessary configurations.

  • Step 1: Configure number of tiles and other queue sizes in design/

    For this example, we will build a single tile system with the default parameters.

  • Step 2: Configure Chronos to use sssp

    We will build an 8 cores/tile sssp system. This is specified in design/apps/sssp/config.vh

    run the following to generate the sssp cores

    ( $CL_DIR = hdk/cl/developer_design/cl_chronos) cd $CL_DIR/design ./scripts/ sssp

  • Step 3: Test graph generation

    We will use the vivado RTL simulator to verify our design works correctly by running sssp on a small graph. First to generate such a graph we need to run the graph_gen tool.

    cd $CL_DIR/tools/graph_gen/ make ./graph_gen sssp grid 4

    This would generate a 4x4 grid graph with random weights. (grid_4x4.sssp).

  • Step 4: RTL Simulation

    4.1) First, compile the design with Vivado simulator. Our testbench is in (verif/tests/test_chronos)

    cd $CL_DIR/verif/scripts/ make TEST=test_chronos compile

    This would create a directory verif/sim/test_chronos.

    4.2) Now, we need to copy the input file into this directory before running the simulation.

    cp $CL_DIR/tools/graph_gen/grid_4x4.sssp $CL_DIR/verif/sim/test_chronos/input_graph

    (The testbench expects the input file be named 'input_graph')

    4.3) Now run the simulation

    cd $CL_DIR/verif/scripts/ make TEST=test_chronos run

    If all goes well, you will see the testbench completing with 0 errors

  • Step 5: Synthesis

    cd $CL_DIR/build/scripts/ ./

    This would launch a vivado synthesis/ place-and-route job. The output of this process is a placed-and-routed design placed in $CL_DIR/build/checkpoints/to_aws/.Developer_CL.tar

    Refer to the following document for a detailed account on how to generate a runnable FPGA image from the placed-and-routed design.

    However, I will breifly summarize the steps here

    First, copy the design file to a location in Amazon S3.

    aws s3 cp $CL_DIR/build/checkpoints/to_aws/.Developer_CL.tar <location_in_s3>.tar

    Create the FPGA image

    ~/bin/aws ec2 create-fpga-image --name --input-storage-location Bucket=<s3_bucket_name>,Key=<location_in_s3> --logs-storage-location Bucket=<s3_bucket_name> ,Key=<temp_location_in_s3>

    Running this command would generate an Amazon Image ID which we can load into the FPGA.

  • Step 6: Running sssp on the FPGA

    (Note this step has to be done on an f1.2x AWS instance)

    6.1) Setup the environment at repo-root source 6.2) Load the generated image into the FPGA sudo fpga-load-local-image -S 0 -I <fgpa_image_id> (I've noted that sometimes, this command needed to be run twice the first time after the instance is booted up)

    6.3) Build and run the runtime program that will transfer the input graph to the FPGA, collect the results and analyze performance cd $CL_DIR/software/runtime make ./test_chronos sssp grid_4x4.sssp

Notes on Chronos software interface

Software communicates with a Chronos FPGA instance through two main interfaces provided by the AWS Shell.

  1. OCL is a 32-bit register access interface.

    This interface is used to set runtime configuration values (eg. number of tiles, queue sizes) as well as read hardware counters.

    THe 32-bit OCL address has the follwing mapping (in verilog style): {8'h 0, 8'h{tile_id}, 8'h{component_id}, 8'h{component_register}.

    Refer to $CL_DIR/design/addr_map.vh for the complete list of component IDs and registers, but as an example the current task unit utilization of tile 2 can be read by reading: {8'h 0, 8'2, 8'h{ID_TASK_UNIT}, 8'h14}.

    (The ID_TASK_UNIT would depend on number of cores in the tile).

    In the software runtime code ($CL_DIR/software/runtime/test_chronos.c), the helper function pci_peek, pci_poke is to read and write to these registers.

    Each tile contains a special component, 'OCL_SLAVE' which exposes a register interface to do the following (among others): i) read current current cycle number ii) read hardware configuration parametes (specified at build time) iii) enqueue initial tasks iv) read arbitrary memory addresses. iV) check if the program has completed.

  2. A DMA interface to transfer data to and from the FPGA

    In the software runtime code ($CL_DIR/software/runtime/test_chronos.c), the helper function dma_write is used to write to the FPGA memory.

Debugging Chronos

Several Chronos components can be configured to log important events to an on-chip circular buffer (at-speed). For example the task queue logs task_enq, task_deq, task_commit, task_abort events. Refer to each component's .sv file for the full list.

This logging uses a lot of on-chip RAM, and therefore is not enabled by default. Refer to on how to enable.

The contents of this log can be read through the DMA interface by reading from a specific address that is beyond the 64GB DDR size.

Concretely, reading from the address ( (1<<36) | (t << 28) | (comp << 20 )) will read the log of tile 't' and component 'comp'.

Pipelined Cores

This is a template for specifying Chronos tasks in hardware. Tasks coded with this template are higher throughput, takes less area, and requires less lines of coding than writing specialized FSM-based cores for each task. All results in the paper use cores generated from this template.

All five existing applications have already been mapped to this template, each proving around ~2X speedup. To try it out, add the 'pipe' option to the gen_cores script. e.g,: To generate pipelined sssp cores, run './scripts/gen_cores sssp pipe' at step 2 in the tutorial above.

This template requires each task be mapped to a read-write (RW) portion followed by a read-only (RO) portion. The RW portion can only access the data belonging to the task's object, while the RO portion can read any arbitrary read-only data.

e.g,: For SSSP, the RW portion corresponds to reading current distance to the node, and then updating it. The RO portion reads the offsets, neighbors and creates new tasks. See 'design/apps/sssp/' for more documentation.


Chronos can be configured to prefetch a memory address when a tile receives a new task (see The cache ( issues prefetch requests from a FIFO, the capacity of which can configured using the L2_PREFETCH_CAPACITY OCL register. Setting this 0 disables prefetching.

The prefetch address computed by differently depending on the core type. For non-pipelined cores, it is computed base_address + (object_size * task_object_id) where base_address and object_size are configulable via the OCL interface. For pipelined cores, these are computed implictly from the RW reader module.