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