55 lines
2.7 KiB
Markdown
55 lines
2.7 KiB
Markdown
## Gas Balancing
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The gas balancing process targets to set gas costs of syscalls to be in line with
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10 gas per nanosecond on reference hardware.
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The process can be either performed for all syscalls based on existing messages and chains or targeted
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at a single syscall.
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#### Reference hardware
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The reference hardware is TR3970x with 128GB of RAM. This is what was available at the time and
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may be subject to change.
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### Complete gas balancing
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Complete gas balancing is performed using a `lotus-bench` the process is based on importing a chain export
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and collecting gas traces which are later aggregated.
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Before building `lotus-bench` make sure `EnableDetailedTracing` in `chain/vm/runtime.go` is set to `true`.
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The process can be started using `./lotus-bench import` with `--car` flag set to the location of
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CAR chain export. `--start-epoch` and `--end-epoch` can be used to limit the range of epochs to run
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the benchmark. Note that the state tree of `start-epoch` needs to be in the CAR file or has to be previously computed
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to work.
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The output will be a `bench.json` file containing information about every syscall invoked
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and the time taken by these invocations. This file can grow to be quite big in size so make sure you have
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spare space.
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After the bench run is complete the `bench.json` file can be analyzed with `./lotus-bench import analyze bench.json`.
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It will compute means, standard deviations and co-variances (when applicable) of syscall runtimes.
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The output is in nanoseconds, so the gas values for syscalls should be 10x that. In cases where the co-variance of
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execution time to some parameter is evaluated, the strength of the correlation should be taken into account.
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#### Special cases
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OnImplPut compute gas is based on the flush time to disk of objects created,
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during block execution (when gas traces are formed) objects are only written to memory. Use `vm/flush_copy_ms` and `vm/flush_copy_count` to estimate OnIpldPut compute cost.
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### Targeted gas balancing
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In some cases complete gas balancing is infeasible, either a new syscall gets introduced or
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complete balancing is too time consuming.
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In these cases, the recommended way to estimate gas for a given syscall is to perform an `in-vivo` benchmark.
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In the past `in-vitro` as in standalone benchmarks were found to be highly inaccurate when compared to results
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of real execution.
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An in-vivo benchmark can be performed by running an example of such a syscall during block execution.
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The best place to hook-in such a benchmark is the message execution loop in
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`chain/stmgr/stmgr.go` in `ApplyBlocks()`. Depending on the time required to complete the syscall it might be
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advisable to run the execution only once every few messages.
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