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Christian Schwarz
4825b0fec3
part of https://github.com/neondatabase/neon/issues/8184 # Problem We want to bypass PS PageCache for all data block reads, but `compact_level0_phase1` currently uses `ValueRef::load` to load the WAL records from delta layers. Internally, that maps to `FileBlockReader:read_blk` which hits the PageCache [here](https://github.com/neondatabase/neon/blob/e78341e1c220625d9bfa3f08632bd5cfb8e6a876/pageserver/src/tenant/block_io.rs#L229-L236). # Solution This PR adds a mode for `compact_level0_phase1` that uses the `MergeIterator` for reading the `Value`s from the delta layer files. `MergeIterator` is a streaming k-merge that uses vectored blob_io under the hood, which bypasses the PS PageCache for data blocks. Other notable changes: * change the `DiskBtreeReader::into_stream` to buffer the node, instead of holding a `PageCache` `PageReadGuard`. * Without this, we run out of page cache slots in `test_pageserver_compaction_smoke`. * Generally, `PageReadGuard`s aren't supposed to be held across await points, so, this is a general bugfix. # Testing / Validation / Performance `MergeIterator` has not yet been used in production; it's being developed as part of * https://github.com/neondatabase/neon/issues/8002 Therefore, this PR adds a validation mode that compares the existing approach's value iterator with the new approach's stream output, item by item. If they're not identical, we log a warning / fail the unit/regression test. To avoid flooding the logs, we apply a global rate limit of once per 10 seconds. In any case, we use the existing approach's value. Expected performance impact that will be monitored in staging / nightly benchmarks / eventually pre-prod: * with validation: * increased CPU usage * ~doubled VirtualFile read bytes/second metric * no change in disk IO usage because the kernel page cache will likely have the pages buffered on the second read * without validation: * slightly higher DRAM usage because each iterator participating in the k-merge has a dedicated buffer (as opposed to before, where compactions would rely on the PS PageCaceh as a shared evicting buffer) * less disk IO if previously there were repeat PageCache misses (likely case on a busy production Pageserver) * lower CPU usage: PageCache out of the picture, fewer syscalls are made (vectored blob io batches reads) # Rollout The new code is used with validation mode enabled-by-default. This gets us validation everywhere by default, specifically in - Rust unit tests - Python tests - Nightly pagebench (shouldn't really matter) - Staging Before the next release, I'll merge the following aws.git PR that configures prod to continue using the existing behavior: * https://github.com/neondatabase/aws/pull/1663 # Interactions With Other Features This work & rollout should complete before Direct IO is enabled because Direct IO would double the IOPS & latency for each compaction read (#8240). # Future Work The streaming k-merge's memory usage is proportional to the amount of memory per participating layer. But `compact_level0_phase1` still loads all keys into memory for `all_keys_iter`. Thus, it continues to have active memory usage proportional to the number of keys involved in the compaction. Future work should replace `all_keys_iter` with a streaming keys iterator. This PR has a draft in its first commit, which I later reverted because it's not necessary to achieve the goal of this PR / issue #8184.