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Minor: Remove memory reservation in JoinLeftData used in HashJoin #13751

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Dec 13, 2024
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Minor: Remove memory reservation in JoinLeftData used in HashJoin #13751

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14c7ec5
Refactor JoinLeftData structure by removing unused memory reservation…
jayzhan-synnada Dec 13, 2024
bde5edb
Add Debug and Clone derives for HashJoinStreamState and ProcessProbeB…
jayzhan-synnada Dec 13, 2024
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8 changes: 2 additions & 6 deletions datafusion/physical-plan/src/joins/hash_join.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -90,9 +90,6 @@ struct JoinLeftData {
/// Counter of running probe-threads, potentially
/// able to update `visited_indices_bitmap`
probe_threads_counter: AtomicUsize,
/// Memory reservation that tracks memory used by `hash_map` hash table
/// `batch`. Cleared on drop.
_reservation: MemoryReservation,
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@korowa korowa Dec 14, 2024 •
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In case reservation is not stored in JoinLeftData, it'll be dropped right after collect_left_input due to being unneeded anymore. And due to MemoryReservation calls free on its drop, it looks like that with this patch, during the whole join execution build side data will be untracked by memory pool.

So the initial idea of this attribute was to make reservation to live as long as JoinLeftData exists.

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UPD: some "logs" (additional prints 😞 ) just to check join execution before/after this patch:

// Before
---- joins::hash_join::tests::join_inner_one_no_shared_column_names stdout ----
Waiting build side
Reserving 324 bytes
Reserving 124 bytes
Fetching probe batch
Processing probe batch
Fetching probe batch
Processing unmatched build side
Freeing 448 bytes

// After
---- joins::hash_join::tests::join_inner_one_no_shared_column_names stdout ----
Waiting build side
Reserving 324 bytes
Reserving 124 bytes
Freeing 448 bytes
Fetching probe batch
Processing probe batch
Fetching probe batch
Processing unmatched build side

so yes, the memory is "freed" before join completes its execution, which doesn't seem to be an expected behavior.

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@jayzhan211 jayzhan211 Dec 14, 2024 •
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The reason I think this is not an issue is because it only matters when we run 'collect_left_join', we track the memory and free after the build side is done. Although, the memory is clean while the hash map is not freed. BUT, since we don't have expected memory change in the probe stage, the memory is not helpful for the probe stage. That's why I think this will not cause any issues if we drop the reservation early.

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Although, the memory is clean while the hash map is not freed

That actually is the issue, because there is single memory pool for a RuntimeEnv, and it's intended to be used by multiple sessions or at least by the multiple operators within the same query. Even in case of single query like select * from a join b order by a.field untracked build side may cause OOM due to SortExec will consider MemoryPool as being empty, while there is a build side data in memory.

}

impl JoinLeftData {
Expand All @@ -102,14 +99,12 @@ impl JoinLeftData {
batch: RecordBatch,
visited_indices_bitmap: SharedBitmapBuilder,
probe_threads_counter: AtomicUsize,
reservation: MemoryReservation,
) -> Self {
Self {
hash_map,
batch,
visited_indices_bitmap,
probe_threads_counter,
_reservation: reservation,
}
}

Expand Down Expand Up @@ -902,7 +897,6 @@ async fn collect_left_input(
single_batch,
Mutex::new(visited_indices_bitmap),
AtomicUsize::new(probe_threads_count),
reservation,
);

Ok(data)
Expand Down Expand Up @@ -1019,6 +1013,7 @@ impl BuildSide {
/// └─ ProcessProbeBatch
///
/// ```
#[derive(Debug, Clone)]
enum HashJoinStreamState {
/// Initial state for HashJoinStream indicating that build-side data not collected yet
WaitBuildSide,
Expand All @@ -1044,6 +1039,7 @@ impl HashJoinStreamState {
}

/// Container for HashJoinStreamState::ProcessProbeBatch related data
#[derive(Debug, Clone)]
struct ProcessProbeBatchState {
/// Current probe-side batch
batch: RecordBatch,
Expand Down
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