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docs: Update outdated compute budget docs (#24737) 

* docs: Update outdated compute budget docs

* docs: Add section about reducing tx fees
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Justin Starry 2022-04-30 03:38:59 +08:00 committed by GitHub
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@ -69,14 +69,13 @@ parent. If an invoked program consumes the budget or exceeds a bound, the entire
invocation chain and the parent are halted.
The current [compute
budget](https://github.com/solana-labs/solana/blob/0224a8b127ace4c6453dd6492a38c66cb999abd2/sdk/src/compute_budget.rs#L102)
can be found in the Solana SDK.
budget](https://github.com/solana-labs/solana/blob/db32549c00a1b5370fcaf128981ad3323bbd9570/program-runtime/src/compute_budget.rs)
can be found in the Solana Program Runtime.
For example, if the current budget is:
```rust
max_units: 200,000,
log_units: 100,
log_u64_units: 100,
create_program address units: 1500,
invoke_units: 1000,
@ -90,7 +89,6 @@ log_pubkey_units: 100,
Then the program
- Could execute 200,000 BPF instructions, if it does nothing else.
- Could log 2,000 log messages.
- Cannot exceed 4k of stack usage.
- Cannot exceed a BPF call depth of 64.
- Cannot exceed 4 levels of cross-program invocations.
@ -115,21 +113,24 @@ Budget](#compute-budget).
With a transaction-wide compute budget the `max_units` cap is applied to the
entire transaction rather than to each instruction within the transaction. The
default number of maximum units remains at 200k which means the sum of the
default number of maximum units will be 1.4M which means the sum of the
compute units used by each instruction in the transaction must not exceed that
value. The number of maximum units allows is intentionally kept small to
facilitate optimized programs and form the bases for a minimum fee level.
value. The default number of maximum units allows is intentionally kept large to
avoid breaking existing client behavior.
There are a lot of uses cases that require more than 200k units
transaction-wide. To enable these uses cases transactions can include a
[``ComputeBudgetInstruction`](https://github.com/solana-labs/solana/blob/0224a8b127ace4c6453dd6492a38c66cb999abd2/sdk/src/compute_budget.rs#L44)
requesting a higher compute unit cap. Higher compute caps will be charged
higher fees.
### Reduce transaction fees
_Note: At the time of writing, transaction fees are still charged by the number of
signatures but will eventually be calculated from requested compute unit cap._
Most transactions won't use the default number of compute units so they can include a
[``ComputeBudgetInstruction`](https://github.com/solana-labs/solana/blob/db32549c00a1b5370fcaf128981ad3323bbd9570/sdk/src/compute_budget.rs#L39)
to lower the compute unit cap. **Important: Lower compute caps will be charged lower fees.**
Compute Budget instructions don't require any accounts and must lie in the first
3 instructions of a transaction otherwise they will be ignored.
The `ComputeBudgetInstruction::request_units` function can be used to crate
The `ComputeBudgetInstruction::request_units` function can be used to create
these instructions:
```rust