Andrzej Jackowski 98f431dd81 auth: utils: add yielding to crypt_sha512 ...

This change allows yielding during hashing computations to prevent
stalls.

The performance of this solution was compared with the previous
implementation that used one alien thread and the implementation
after the alien thread was reverted. The results (median) of
`perf-cql-raw` with `--connection-per-request 1 --smp 10` parameters
are as follows:
 - Alien thread: 41.5 new connections/s per shard
 - Reverted alien thread: 244.1 new connections/s per shard
 - This commit (yielding in hashing): 198.4 new connections/s per shard

The alien thread is limited by a single-core hashing throughput,
which is roughly 400-500 hashes/s in the test environment. Therefore,
with smp=10, the throughput is below 50 hashes/s, and the difference
between the alien thread and other solutions further increases with
higer smp.

The roughly 20% performance deterioration compared to
the old implementation without the alien thread comes from the fact
that the new hashing algorithm implemented in `utils/crypt_sha512.cc`
performs an expensive self-verification and stack cleanup.

On the other hand, with smp=10 the current implementation achieves
roughly 5x higher throughput than the alien thread. In addition,
due to yielding added in this commit, the algorithm is expected
to provide similar protection from stalls as the alien thread did.
In a test that in parallel started a cassandra-stress workload and
created thousands of new connections using python-driver, the values of
`scylla_reactor_stalls_count` metric were as follows:
 - Alien thread: 109 stalls/shard total
 - Reverted alien thread: 13186 stalls/shard total
 - This commit (yielding in hashing): 149 stalls/shard total

Similarly, the `scylla_scheduler_time_spent_on_task_quota_violations_ms`
values were:
 - Alien thread: 1087 ms/shard total
 - Reverted alien thread: 72839 ms/shard total
 - This commit (yielding in hashing): 1623 ms/shard total

To summarize, yielding during hashing computations achieves similar
throughput to the old solution without the alien thread but also
prevents stalls similarly to the alien thread.

Fixes: scylladb/scylladb#26859
Refs: scylladb/scylla-enterprise#5711

2025-12-10 15:36:18 +01:00

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