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#36 in Profiling

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Resource Control Demo Hash Daemon

rd-hashd is a workload simulator for resctl-demo and resctl-bench. Its primary goal is simulating a latency-senstive and throttleable primary workload which can saturate the machine in all local resources.

Imagine a latency-sensitive user-request-servicing application which is load balanced and configured to use all available resources of the machine under full load. Under nominal load, it'd consume smaller amounts of resources and show tighter latency profile. As load gets close to full, it'll consume most of the machine and the latencies would increase but, hopefully, stay within the target envelope. If the application gets stalled for whatever reasons including resource conflicts, it'd experience latency spikes and the load balancer would allocate it less requests until it can catch up.

rd-hashd simulates such workload in a self-contained manner. It sets up test files and memory heap with random contents and keeps calculating SHA1s using multiple worker threads. The concurrency level is modulated so that the RPS converges on the target while not exceeding the latency target. The RPS and latency targets can be dynamically modified. The memory access patterns follow normal distributions, small random sleeps are injected to emulate network interactions, and it also generates log writes.

Many aspects of rd-hashd's resource consumption behaviors are configureable and are tuned, by default, to behave similarly to a popular Facebook web workload, especially under memory and IO contentions.

rd-hashd is used as the primary workload for both resctl-demo and resctl-bench. While a single workload cannot possibly represent the many different ways systems are used, it can successfully capture many aspects of typical human-interactive workloads, reproduce a wide variety of resource conflict issues and verify their remedies, and be used as an approximate measure in evaluating hardware and operating system behaviors.

While rd-hashd is usually used a part of resctl-demo and resctl-bench, it can be useful as a stand-alone pseudo workload too. For more information on the containing projects, visit:


Configuration, Report and Log Files

There are two configuration channels - command line arguments and runtime parameters. The former can be specified as command line options or with the --args file. The latter can be specified using the --params file and dynamically updated while rd-hashd is running - just edit and save, the changes will be applied immediately.

If the specified --args and/or --params files don't exist, they will be created with the default values. Any configurations in the --args file can be overridden on the command line and the file will be updated accordingly. Note that only the arguments which are listed above --args in the help message are saved. If --params is not specified, the defaults are used and the parameters can't be updated while rd-hashd is running.

rd-hashd reports the current status in the optional --report file and the hash results are saved in the optional log files in the --log-dir directory.

The following will create the --args and --params files and exit.

$ rd-hashd --testfiles ~/rd-hashd/testfiles --args ~/rd-hashd/args.json \
  --params ~/rd-hashd/params.json --report ~/rd-hashd/report.json \
  --log-dir ~/rd-hashd/logs --interval 1 --prepare-config

Afterwards, rd-hashd can be run with the same configurations with:

  $ rd-hashd --args ~/rd-hashd/args.json


It is challenging to find the right parameters to maximize resource utilization. To help determining the parameters, --bench runs a series of tests and records the determined parameters in the specified --args and --params files.

With the resulting parameters, rd-hashd should saturate CPU and memory and use some amount of IO at the full load. RPS will be sensitive to memory and thus IO availability and resource conflicts will lead to raised request processing latencies and lowered RPS.

--bench may take over ten minutes and the system should be idle otherwise. While it tries its best, due to long tail memory accesses and fluctuating hardware IO performance characteristics, there is a low chance that the resulting configuration might not be sustainable in extended runs. If rd-hashd fails to keep CPU saturated and deviates significantly from the target RPS, try lowering the runtime parameter mem_frac. If not enough IO is being generated, try raising.

--bench preserves the existing parameters in the configuration files as much as possible. If the benchmark behaves in an unexpected way, try removing the configuration files to start from a clean slate.

Usage Example

The following is an example workflow. It clears the existing configurations, performs a benchmark to determine the parameters and then starts a normal run.

  $ mkdir -p ~/rd-hashd
  $ rm -f ~/rd-hashd/*.json
  $ rd-hashd --args ~/rd-hashd/args.json --testfiles ~/rd-hashd/testfiles \
             --params ~/rd-hashd/params.json --report ~/rd-hashd/report.json \
             --log-dir ~/rd-hashd/logs --interval 1 --bench
  $ rd-hashd --args ~/rd-hashd/args.json


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