Remove file prefetching from FileStream#20916
Open
Dandandan wants to merge 6 commits intoapache:mainfrom
Open
Conversation
Simplify the FileStream state machine by removing the mechanism that opens the next file in parallel while scanning the current one. Files are now opened sequentially (Scan -> Idle -> Open) instead of prefetching. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Dandandan
changed the title
Contributor
Author
|
run benchmarks |
|
Benchmark job started for this request (job |
|
Benchmark job started for this request (job |
|
Benchmark job started for this request (job |
This reverts commit 38fe60a.
|
🤖 Benchmark completed (GKE) | trigger Details
Resource Usagetpch — base (merge-base)
tpch — branch
|
|
🤖 Benchmark completed (GKE) | trigger Details
Resource Usageclickbench_partitioned — base (merge-base)
clickbench_partitioned — branch
|
|
🤖 Benchmark completed (GKE) | trigger Details
Resource Usagetpcds — base (merge-base)
tpcds — branch
|
Dandandan
commented
Mar 12, 2026
| .metrics | ||
| .clone(); | ||
| let _timer = scanning_total_metric.timer(); | ||
| self.start_next_file().transpose() |
Contributor
Author
There was a problem hiding this comment.
So this @alamb is what I was mostly talking about. It will read the footer (what we want) but AFAIK also:
- build the pruning predicate (I think this is suboptimal, too early)
- prune row groups
- optionally load the page index
- return the stream (without driving that forward)
We should be able to do this much better with the IO / CPU separation.
Contributor
There was a problem hiding this comment.
This makes sense to me 👍🏻
|
🤖 |
|
🤖: Benchmark completed Details
|
Dandandan
changed the title
Contributor
Author
|
run benchmarks |
|
Benchmark job started for this request (job |
|
Benchmark job started for this request (job |
|
Benchmark job started for this request (job |
|
🤖 Benchmark completed (GKE) | trigger Details
Resource Usageclickbench_pushdown — base (merge-base)
clickbench_pushdown — branch
|
Contributor
Author
|
The benchmarks with S3-like latency show that it is pretty ineffective (or even counterproductive, see query 6 of ClickBench) at this point. Let's remove this to simplify this code and make room for the better CPU/IO pipelining? |
adriangb
approved these changes
Mar 17, 2026
Contributor
adriangb
left a comment
adriangb
left a comment
There was a problem hiding this comment.
Yep agreed let’s move forward with this, the logic and data both support it.
Contributor
|
I think we should ask @thinkharderdev about this one as he put in the original prefetch as I recall Maybe we could also put out a call to some other downstream users (via the mailing list or something) to ask them to test What I worry about is that if we remove this feature and someone was relying on it downstream they will hit a regression without any way to revert / go back |
Contributor
|
Yeah good call. I posted on discord and the mailing list. |
Contributor
Author
Agreed - let's bake it a bit more to gather feedback thanks @adriangb @alamb |
thinkharderdev
approved these changes
Mar 17, 2026
Contributor
thinkharderdev
left a comment
thinkharderdev
left a comment
There was a problem hiding this comment.
Makes sense. The original goal here was to pipeline the IO but current file opening blends too much IO and CPU work for this to be effective I think
Contributor
Author
|
run benchmark supported? |
|
Hi @Dandandan, thanks for the request (#20916 (comment)). Supported benchmarks:
Usage: Per-side configuration ( env:
SHARED_SETTING: enabled
baseline:
ref: v45.0.0
env:
DATAFUSION_RUNTIME_MEMORY_LIMIT: 1G
changed:
ref: v46.0.0
env:
DATAFUSION_RUNTIME_MEMORY_LIMIT: 2G |
Contributor
Author
|
run benchmark tpch10 clickbench_partitioned clickbench_1 clickbench_extended |
Contributor
Author
|
Thanks all, let's wait a couple of days for further feedback - I launched some more benchmarks to check them all |
Contributor
Author
|
run benchmark tpch10 clickbench_partitioned clickbench_1 clickbench_extended |
|
🤖 Benchmark completed (GKE) | trigger Details
Resource Usagetpch10 — base (merge-base)
tpch10 — branch
|
|
🤖 Benchmark completed (GKE) | trigger Details
Resource Usageclickbench_extended — base (merge-base)
clickbench_extended — branch
|
|
🤖 Benchmark completed (GKE) | trigger Details
Resource Usageclickbench_partitioned — base (merge-base)
clickbench_partitioned — branch
|
|
🤖 Benchmark completed (GKE) | trigger Details
Resource Usageclickbench_extended — base (merge-base)
clickbench_extended — branch
|
|
🤖 Benchmark completed (GKE) | trigger Details
Resource Usageclickbench_1 — base (merge-base)
clickbench_1 — branch
|
|
🤖 Benchmark completed (GKE) | trigger Details
Resource Usageclickbench_1 — base (merge-base)
clickbench_1 — branch
|
|
🤖 Benchmark completed (GKE) | trigger Details
Resource Usageclickbench_partitioned — base (merge-base)
clickbench_partitioned — branch
|
|
🤖 Benchmark completed (GKE) | trigger Details
Resource Usagetpch10 — base (merge-base)
tpch10 — branch
|
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
6 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Simplify the FileStream state machine by removing the mechanism that opens the next file in parallel while scanning the current one. Files are now opened sequentially (Scan -> Idle -> Open) instead of prefetching.
Which issue does this PR close?
Rationale for this change
Current prefetching is ineffective or even counterproductive, see e.g. query 6 when testing against simulated S3 latency:
Let's remove it so we can make room for better prefetching taking care of pruning / late materilization better, cache behavior and IO/CPU split.
What changes are included in this PR?
Remove this API / prefetching to make room for more impactful prefetching,
Are these changes tested?
Existing tests.
Are there any user-facing changes?
Yes,
NextOpennext: Option<NextOpen>,are removed ofpubmembers.