Add option to limit max prefetching in MultiScan #13920
Closed
+294
−16
Conversation
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
xingbowang
reviewed
Sep 4, 2025
Comment on lines
985
to
986
| Status s; | ||
| s.PermitUncheckedError(); |
There was a problem hiding this comment.
Seems all the usage of Status is local. I would recommend to just define the status on the place we use it, instead of define one at the top of the function.
xingbowang
approved these changes
Sep 5, 2025
anand1976
reviewed
Sep 5, 2025
| s = table_->LookupAndPinBlocksInCache<Block_kData>( | ||
|
|
||
| // Check if we would exceed the prefetch size limit with this block | ||
| uint64_t block_size = data_block_handle.size(); |
There was a problem hiding this comment.
Nit: This doesn't account for the trailer. Maybe use BlockSizeWithTrailer()?
include/rocksdb/options.h
Outdated
| // prefetch size. When the limit is exceeded, iterator will return | ||
| // Status::PrefetchLimitReached(). | ||
| // | ||
| // Note that this limit is per file and is on compressed block size. |
There was a problem hiding this comment.
Clarify in the comment that the prefetch happens only once, to distinguish it from ReadOptions::readahead_size which applies anytime the iterator does IO.
|
|
||
| // Check if we would exceed the prefetch size limit with this block | ||
| uint64_t block_size = data_block_handle.size(); | ||
| total_prefetch_size += block_size; |
There was a problem hiding this comment.
Shouldn't this be done only if the block doesn't exist in the block cache?
There was a problem hiding this comment.
As discussed, we want to limit memory usage of pinned blocks too.
xingbowang
approved these changes
Sep 5, 2025
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
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.
Summary: Add a new option
MultiScanArgs::max_prefetch_sizethat limits the memory usage of per file pinning of prefetched blocks. Note that this only accounts for compressed block size. This is intended to be a stopgap until we implement some kind of global prefetch manager that limits the global multiscan memory usage.Test plan: new unit test
./block_based_table_reader_test --gtest_filter="*MultiScanPrefetchSizeLimit/*"