chore: Make snapshotting more responsive

[romange]

This should improve situation around #4787 -
maybe not solve it completely but improve it significantly.

On my tests when doing snapshotting under read traffic with master (memtier_benchmark --ratio 0:1 -d 256 --test-time=400 --distinct-client-seed --key-maximum=2000000 -c 5 -t 2 --pipeline=3) I got drop from 250K qps to 8K qps during the full sync phase.

With this PR, the throughput went up to 45K qps - 5x more but still much lower than 250K qps.

[Copilot]

Copilot reviewed 6 out of 7 changed files in this pull request and generated 2 comments.

Files not reviewed (1)

• helio: Language not supported

Comments suppressed due to low confidence (2)

src/server/snapshot.cc:176

• Removing the conditional PushSerialized calls and related yield logic may lead to less frequent flushing of the serialization buffer, potentially impacting memory usage and server responsiveness under heavy load; please verify that this change maintains the intended performance characteristics.

ThisFiber::Yield();

src/server/snapshot.cc:427

• Ignoring the return value in the OnJournalEntry method might suppress error conditions that could be critical; ensure that this behavior is safe for all execution scenarios.

std::ignore = serializer_->WriteJournalEntry(item.data);

[adiholden]

why do you pull helio in this PR? lets make it in a seprate PR

[adiholden]

please dont merge yet, wait for my review

[romange]

I pulled helio because of cycle timer that I added recently for this.

[Copilot]

Copilot reviewed 6 out of 7 changed files in this pull request and generated no comments.

Files not reviewed (1)

• helio: Language not supported

Comments suppressed due to low confidence (1)

src/server/rdb_save.cc:913

• Returning an empty buffer when mem_buf_.InputLen() is zero may lead to unexpected behavior if downstream code expects the original mem_buf_.InputBuffer() output. Consider reviewing the downstream usage and either preserving the original behavior or ensuring that returning an empty buffer is handled appropriately.

return {};

[romange]

@adiholden PTAL