Make file prefetch configurable #17758
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Currently we prefetch 1 file. In bandwith limited systems (e.g. object storage) having wider IO than CPU tends to make sense. This makes the prefetch configurable so that we can open >2 files at a time. My main concern is memory use. For now users will have to tune this manually. Long term I think we should track how much prefetched data is 'waiting' using a memory pool + the newly added memory tracking in arrow -> you can set a pretty large prefetch value and it will be throttled by memory. This means that scans with very selective filters (that produce few rows per file) will benefit heavily from wide IO prefetching while scans that produce large amounts of data will do little prefetching if they hit memory limits.
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From my testing this works but doesn't seem to have much of a positive benefit (does not improve query performance). Maybe something is wrong with the implementation. But I think fundamentally the problem is that you want to go wide with IO but not really with other parts (e.g. CPU work applying filters in predicate pushdown), and this is a bit too high of a level for that. @alamb is working on changes to Parquet decoding which will allow prefetching at the byte range level, that sounds much more appropriate since we will be able to do IO-only prefetching. I will continue to do some investigations but I am going to close this for now to keep the PR backlog down. |
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Currently we prefetch 1 file. In bandwith limited systems (e.g. object storage) having wider IO than CPU tends to make sense. This makes the prefetch configurable so that we can open >2 files at a time. My main concern is memory use. For now users will have to tune this manually. Long term I think we should track how much prefetched data is 'waiting' using a memory pool + the newly added memory tracking in arrow -> you can set a pretty large prefetch value and it will be throttled by memory. This means that scans with very selective filters (that produce few rows per file) will benefit heavily from wide IO prefetching while scans that produce large amounts of data will do little prefetching if they hit memory limits.