03.02.2025 10:11, Zixuan Fu пишет:
> Hi Hackers,
>
> While profiling a program with `perf`, I noticed that `scram_SaltedPassword`
> consumed more CPU time than expected. After some investigation, I found
> that the function performs many HMAC iterations (4096 rounds for
> SCRAM-SHA-256), and each iteration reinitializes the HMAC context, causing
> excessive overhead.
>
> OpenSSL has an optimization for this case: when the key remains the
> same, the HMAC context can be reused with a lightweight state reset by
> passing NULL as the key. To take advantage of this, I introduced
> `pg_hmac_reuse()`, which replaces the key with NULL when OpenSSL is used.
Good catch.
Since pg_hmac_reuse is not `static`, I'd add some checks that key is
exactly same. At least there should be
Assert(key == prev_key && len == prev_len && hash_bytes(key, len) ==
prev_hash);
Where `prev_key`, `prev_len` and `prev_hash` are static variables, filled
in `pg_hmac_init`.
I don't know, should it be `Assert`, or check that leads to `elog(ERROR)`.
`hash_bytes` is fast enough to not cause measurable slow down in production.
On the other hand, use cases are trivial enough to occasional misuses to be
caught using just `Assert`.
> With this change, the performance improves by approximately **4x** (reducing
> execution time from 4ms to 1ms). The built-in PostgreSQL HMAC implementation
> does not support context reuse, and modifying it would require substantial
> changes. Therefore, `pg_hmac_reuse()` simply calls `pg_hmac_init()` in that
> case, maintaining the existing logic.
-------
regards,
Yura
