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Debugging Guide

Debugging Guide

This section describes how to debug your kernel code. See Working With Code Samples for how to compile and simulate (run the program).

To debug, you can use the following tools:

  • csdb debugger for interactive debugging on hardware.

  • sdk_debug_shell visualize, which launches the SDK GUI to look at all simulation results such as timeline and traces. See SDK GUI for more information.

  • sim.log simulator log file, which records a cycle-by-cycle log of wavelets or instructions executed on each PE.

CSDB debugger

CSDB is the Cerebras Software Language Debugger for the Wafer-Scale Engine. CSDB can be run on the host machine for interactive debugging with the Wafer-Scale Engine on issues such as hangs and functional failures. CSDB can also be used to inspect and debug coredumps produced from a simulator run.

Note

For debugging on hardware, csdb is not supported on legacy CS-2 systems running Cerebras software version 1.6 or lower.

Additionally, csdb cannot be run via appliance mode on Wafer-Scale Clusters.

Below is a tutorial on how to use csdb to inspect a coredump from a simulator run.

Tutorial

We will use the GEMV with Checkerboard Pattern example program for this tutorial. First, to produce corefiles, we will need to add the following line to run.py right before runner.stop() is called:

runner.dump_core("corefile.cs1")

Note that the specified filename for the coredump MUST be corefile.cs1 to produce the correct file types for csdb.

Run commands.sh to compile and execute the program and produce the corefiles. The run will produce four files: corefile.cs1_0, corefile.cs1_1, corefile.cs1_2, and corefile.cs1_3.

We are now ready to use csdb. Start csdb from the current working directory:

$ csdb .
INFO:csdb: . contains more than one CSL compile directory.
Starting debug shell...

csdb reports that we have multiple compile directories: this is because the top level compile directory, out, contains subdirectories containing compile output for the memcpy infrastructure.

Select out as our compile context, and target the produced corefiles:

(csdb) context select out
(csdb) target create --core-file=corefile.cs1

Run settings to see the current working directory, compile context, and target, along with the fabric rectangle dimensions:

(csdb) settings
INFO:csdb: Workdir: .
INFO:csdb: Compile context: gemv-checkerboard-pattern/out/
INFO:csdb: Target (core file): corefile.cs1
INFO:csdb: Rectangle(s):
INFO:csdb:   Rect (x = 0, y = 0, width = 11, height = 6) selected
INFO:csdb: Trace: no selected.

Run help to take a look at the available options:

(csdb) help

Documented commands (type help <topic>):
========================================
context  memory     register  target  wavelet
image    rectangle  settings  trace   workdir

Undocumented commands:
======================
exit  help  quit

Select a new subrectangle of PEs, containing only a single PE, and deselect the default rectangle containing the whole fabric. Show the current rectangle(s) with rectangle show:

(csdb) rectangle show
INFO:csdb: Rectangle(s):
INFO:csdb:   Rect (x = 0, y = 0, width = 11, height = 6) selected
(csdb) rectangle select 4,1,1,1
(csdb) rectangle show
INFO:csdb: Rectangle(s):
INFO:csdb:   Rect (x = 0, y = 0, width = 11, height = 6) selected
INFO:csdb:   Rect (x = 4, y = 1, width = 1, height = 1) selected
(csdb) rectangle deselect 0,0,11,6
INFO:csdb: Removing ('', Rect (x = 0, y = 0, width = 11, height = 6))
(csdb) rectangle show
INFO:csdb: Rectangle(s):
INFO:csdb:   Rect (x = 4, y = 1, width = 1, height = 1) selected

We can read memory values of the PE in the rectangle by using the memory command:

(csdb) memory read --address 0x9e0  --length 4
MSGS155 21:48:27 GMT  Output will be directed to file 'memory-x4y1w1h1_09e0_09e4.log'
MSGS155 21:48:27 GMT  Log file: 'memory-x4y1w1h1_09e0_09e4.log'

The memory values will be written to the log file specified above.

Terminology

  • Compilation context: The directory generated by cslc. By default, the name is out.

  • Trace: The directory generated after simulation is ran. By default, the name is simfab_traces.

  • Working directory: Also known as workdir, this is the directory to which the debugger writes its output.

Commands

Context command

The context command is used to select or change the compile context created by CSL compiler. Once a context is selected, a debug session can be started by creating a target.

Usage: context [OPTIONS] COMMAND [ARGS]...

  Set compile context.

Options:
  --help  Show this message and exit.

Commands:
  list    List all the compile context in workdir.
  select  Select the directory that contains the ELF binaries as compile...
  show    Show the selected compile context.

Example: list all the contexts and select one

Note

The “.” after “[2]” in example below means current directory.

(csdb) context list
INFO:csdb: [0] orig_hw2/out
INFO:csdb: [1] orig_hw/out
INFO:csdb: [2] .
(csdb) context select orig_hw/out
# is same as
(csdb) context select [1]

Memory command

To read from the memory, the user must first specify a rectangle and a target. When memory read is called, CSDB will read from a core file or a device. The output of the read is piped into a log file with name beginning with “memory”. All address and lenght are in units of 16-bits (2-bytes).

Usage: memory [OPTIONS] COMMAND [ARGS]...

  Read and write to memory locations in PEs.

Options:
  --help  Show this message and exit.

Commands:
  read   Read memory from a core file or a device.
  write  Write memory to a device in units of 2-bytes.

Example: output from reading tile (4,1) on address 0x09e0, length 4

(4,1) 09e0: 06af 8af0 06af 8060

Rectangle command

The purpose of rectangle command is allow the user to select a rectangle within the fabric. By default, the selected rectangle is the entire fabric. The context must be selected before you can use the rectangle commands.

Usage: rectangle [OPTIONS] COMMAND [ARGS]...

  Select rectangle

Options:
  --help  Show this message and exit.

Commands:
  reset   Resets the current rectangle to fabric dimension.
  select  Selects a rectangle.
  show    Shows the current rectangle.

Example: select a rectangle on (1,2) w3 h4

(csdb) rectangle select 1,2,3,4

Settings command

The settings command is used to see the work directory, compile context, target, rectangle and trace.

Target command

The purpose of the target command is to create a debug session. It is similar to attaching gdb to a process. You can create an interactive debug session by connecting to a CM IP address, or perform a post-mortem debugging by examining a core file. During an interactive debug session, you can use save-core to save a core file for examination later.

(csdb) target
Usage: target [OPTIONS] COMMAND [ARGS]...

  Connect to CM for interactive debugging or examine a core file.

Options:
  --help  Show this message and exit.

Commands:
  create     Connects to CM or read core file as target.
  list       List all the core files.
  save-core  Save a core file after connecting to a CM target
  show       Show selected core file.

Example: create an interactive debug session

(csdb) target create --cmaddr 12.34.56.78:9000

Example: list and load the core file

(csdb) target list
INFO:csdb: [0] core-ckpt
INFO:csdb: [1] my_try1-ckpt
(csdb) target create --core-file core-ckpt

Trace command

The purpose of the trace command is to specify a directory in which a simfab_traces has been generated, so that the simfab_traces can be read for wavelet trace information.

(csdb) trace --help
Usage: trace [OPTIONS] COMMAND [ARGS]...

  Select trace

Options:
  --help  Show this message and exit.

Commands:
  list    List all the valid post-run generated directory.
  select  Select the directory that contains post-run traces.
  show    Show the post-run directory that is set.

Example: select current run directory with simfab_traces

(csdb) trace select .

At this point, you can use the wavelet command to inspect the wavelet traces of this run.

Workdir command

The purpose of the workdir command is to specify a directory for output files to be written.

(csdb) workdir --help
Usage: workdir [OPTIONS] COMMAND [ARGS]...

  workdir is the directory for debug session.

Options:
  --help  Show this message and exit.

Commands:
  select  Select a workdir.

Example: select a workdir

(csdb) workdir select path/to/workdir

sdk_debug_shell

The sdk_debug_shell tool is used to run a smoke test or launch the SDK GUI visualizer.

$ sdk_debug_shell --help
Usage: sdk_debug_shell [OPTIONS] COMMAND [ARGS]

  Debugger tool for the Cerebras WSE kernel code.

Options:
  --help  Show this message and exit.

Commands:
  smoke          Run the smoke tests.
  visualize      Invokes the visualization tool csviz.

Smoke test

The smoke option runs the smoke tests in the specified directory.

Usage: sdk_debug_shell smoke [OPTIONS] [CSL_EXAMPLES_DIR]...

  Run the smoke tests.

Options:
  --help  Show this message and exit.

Visualizer

When you use the visualize option, the debugger will invoke the SDK GUI and you can visually inspect the debugging information in a web browser. The default artifact_dir is the current directory.

$ sdk_debug_shell visualize --help
Usage: sdk_debug_shell visualize [OPTIONS]

  Visualize routing between PEs, post-simulation run results. For example,
  wavelet trace and instruction trace in a web browser.

Options:
  --artifact_dir PATH
  --help               Show this message and exit.

Simulator Logs

When running in the simulator, you can produce a simulator log file sim.log with cycle-by-cycle information about wavelets or instructions. The SINGULARITYENV_SIMFABRIC_DEBUG environment variable is used to control the output of sim.log.

Landing Logs

SINGULARITYENV_SIMFABRIC_DEBUG=landing produces a log of wavelet landings on each PE’s router, giving the cycle and color on which the wavelet lands, the direction from which it landed, its data, and its identity.

An example landing log looks like this:

@53 P6.1 (hwtile) landing C3 from link R, ctrl=0, idx=0000, data=0000 (+0.000(-15)), half=0, ident=00000E0300000000, lf=0
@55 P5.1 (hwtile) landing C3 from link E, ctrl=0, idx=0000, data=0000 (+0.000(-15)), half=0, ident=00000E0300000000, lf=0

The first line says that on cycle 53, the router of the PE at X=6, Y=1 received a wavelet of color 3 (C3) from the ramp (i.e., sent by the CE). The second line says that on cycle 55, the router of the PE at X=5, Y=1, received a wavelet of color 3 from the EAST link (i.e., from the PE at X=6, Y=1).

Let’s take a closer look at each entry on the first line and its meaning:

  • @53: The cycle when the landing occurs. The first cycle of the simulation is zero.

  • @P6.1: The coordinates of the PE on which the landing occurs. The coordinates take the format @PX.Y, where X=0, Y=0 is the top left corner of the fabric rectangle.

  • (hwtile): The type of tile implementation. hwtile is the full Cerebras microcode execution engine. iotile are the links by which data enters or exits the wafer, along the EAST or WEST edges.

  • C3: The color of the landing wavelet.

  • link R: The link from which the wavelet arrived. There are five links: EAST (E), WEST (W), NORTH (N), SOUTH (S), and RAMP (R). The four cardinal directions refer to the four neighboring PEs, while the RAMP refers to the CE of this PE.

  • ctrl=0: The control bit is not set. If 0, the wavelet is a data wavelet. If 1, a control wavelet.

  • idx=0000, data=0000 (+0.000(-15)): The wavelet interpreted as 16-bit index and data fields. The data field is shown again in fp16 representation.

  • half=0: The wavelet is not a half-wavelet. On WSE-3, wavelets can be interpreted as 16-bit “half-wavelets.” This field is not present on WSE-2.

  • ident=00000E0300000000: Unique identifier for this wavelet. Notice that in the small example above, both lines in the landing log have the same identifier. The same wavelet that leaves the PE at X=6, Y=1 arrives on the EAST link of the PE at X=5, Y=1 two cycles later. Thus, the ident field allows you to trace the flow of wavelets across the fabric.

  • lf=0: The local flip bit is not set. The local flip bit is set by the CE to signal that the switch be advanced (from the RAMP direction to one of the cardinal directions).

Instruction Trace Logs

SINGULARITYENV_SIMFABRIC_DEBUG=inst_trace produces an instruction trace that shows which instruction a PE is executing at each cycle.

An example instruction trace looks like this:

@497 P4.1: Id: 12, Instr: 225, Seq: 0, Pipe: 3, Msg: [IS OP] 0x021c: T01                  FMACS          Dest:[DDS1]       Src0:[S0DS1]       Src1:[S1DS1]       Src2:R13,R12
@500 P4.1: Id: 12, Instr: 225, Seq: 0, Pipe: 6, Msg: [EX OP] 0x021c: T01                  FMACS          Dest:3f800000               Src0:00000000      Src1:3f800000      Src2:3f800000                                   [     ] U0
@501 P4.1: Id: 12, Instr: 225, Seq: 1, Pipe: 6, Msg: [EX OP] 0x021c: T01                  FMACS          Dest:41500000               Src0:40c00000      Src1:40e00000      Src2:3f800000                                   [     ] U0
@502 P4.1: Id: 12, Instr: 225, Seq: 2, Pipe: 6, Msg: [EX OP] 0x021c: T01                  FMACS          Dest:41c80000               Src0:41400000      Src1:41500000      Src2:3f800000                                   [     ] U0
@503 P4.1: Id: 12, Msg: [EX OP] IDLE
@504 P4.1: Id: 12, Instr: 225, Seq: 3, Pipe: 6, Msg: [EX OP] 0x021c: T01                  FMACS          Dest:42140000               Src0:41900000      Src1:41980000      Src2:3f800000                                   [     ] U0

The first line says that on cycle 497, an FMACS instruction was decoded [IS OP] by the PE at X=4, Y=1. The next four lines show this same instruction executing, save for an idle cycle at cycle 503.

Let’s take a closer look at each entry on the first line and its meaning:

  • @493: The cycle tow hich this line refers. The first cycle of the simulation is zero.

  • @P4.1: The coordinates of the PE on which the landing occurs. The coordinates take the format @PX.Y, where X=0, Y=0 is the top left corner of the fabric rectangle.

  • Id: 12: The position of the PE in a 1D array. This simulator log comes from a simulation of an 8 x 3 fabric, so the position X=4, Y=1 corresponds to PE 12.

  • Instr: 225: A unique instruction ID. The instruction ID stays with the instruction from beginning to end. Notice that the instruction ID is the same for all instructions in the above simulator log excerpt.

  • Seq: The sequence number of the instruction. For a vector instruction, the sequence number will increase as we step through the elements of the vector. Thus, for a vector instruction that is 100 elements long, the sequence number will go from 0 to 99.

  • Pipe: 3: The execution pipeline stage. On WSE-3, stage 3 is instruction decode, and stage 6 is instruction execution. On WSE-2, stage 2 is instruction decode, and stage 4 is instruction execution.

  • Msg: [IS OP]: The name of the pipeline stage. [IS OP] is instruction decode, and [EX OP] is instruction execution.

  • 0x021c: The address of the instruction in memory.

  • T01: The task ID of the task in which the instruction is executing. On WSE-3, data tasks are bound to input queues, so T00 to T07 refer to data tasks and the ID of the input queue to which they are bound. On WSE-2, data tasks are bound to colors, so the ID of a data task can be in the range T00 to T23, and refers to the color to which the task is bound. The task number can also be appended with a microthread ID. For example T01.UT4 would mean this current instruction is running on microthread 4.

  • FMACS: The name of disassembled instruction.

  • Dest:[DDS1] Src0:[S0DS1] Src1:[S1DS1] Src2:R13,R12: The instruction operands. For the instruction decode pipeline stage, the operand registers are given. FMACS has one destination operand and three source operands. The destination operand is in DDS1, or destination data structure register (DSR) 1. The first two source operands are also DSR operands, in src0 DSR 1 and src1 DSR 1 respectively. The third source operand uses general purpose registers (GPR) 12 and 13. This is a 32-bit operation, so the 32-bit scalar operand used for the third source operand uses two GPRs.

For instruction execution [EX OP], additional fields are present. In the second line above, we see:

  • [     ]: Error flags. In the above example, no error flags are set. There are five error cases, one for each position between the square brackets:

    • u = underflow

    • o = overflow

    • x = inexact

    • i = invalid op

    • z = divide by zero

    For example, [ o   ] means the instruction encountered an overflow.

  • U0: The SIMD unit(s) involved in the instruction. In a single cycle, the CE can run up to SIMD-4 for WSE-2, and SIMD-8 for WSE-3, depending on the instruction. Because this instruction is a single precision FMAC, the instruction can only run in SIMD-1, and thus only one SIMD unit is used.

The [EX OP] entry above with IDLE means that P4.1 was idle on cycle 503.

Interpreting Logs for Programs with memcpy

Most of the SDK example programs use the memcpy library, which, in conjunction with the host runtime SdkRuntime, can copy or stream data to and from PEs in your program rectangle, and launch functions in your program rectangle.

When looking at the simulator logs, you may be surprised to see colors, resources, and PEs that your program does not explicitly use. These are memcpy resources. We note here a few things to look out for:

  • Programs using memcpy are typically compiled with fabric offsets 4,1, though additional East and West buffers can be introduced to reduce I/O latency. Without buffers, the top left-most PE of your program rectangle is at P4.1.

  • memcpy uses colors 21, 22, 23, local task IDs 27, 28, 30, and control task IDs 33, 34, 35, 36, and 37. It also used microthread 0 (UT0), input queue 0, and output queue 0. On WSE-3, memcpy additionally uses input queue 1.

  • Color 21 is used for device-to-host data transfers, color 23 is used for host-to-device data transfers, and color 22 is used for the memcpy command sequence.

  • Functions launched via the memcpy kernel launch mechanism will execute within task 22 (T22) on WSE-2, since the task which calls these functions is bound to color 22. Funcitons will execute within task 1 (T01) on WSE-3, since the task is bound to input queue 1.

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