EmilyMatt commented on PR #19695: URL: https://github.com/apache/datafusion/pull/19695#issuecomment-3723707998
> I have a question: let's say we have only 100MB of memory left, and there is a 1GB batch arriving at the `SortExec`, and this PR makes it possible to sort this batch in memory and write it to a single spill file. > > Sorting it in memory and incrementally appending it to the spill file still needs extra memory. The amount should be the memory usage of the sort columns in the original large batch, so in the worst case it is also around 1GB. This should not be possible. Are we trying to ignore the memory limit and sort and spill anyway in this PR? > > I believe that, for internal operators, outputting batches in `batch_size` is a convention. This convention can greatly simplify operator implementation; otherwise, all operators have to consider extremely large or extremely small input batches, which would make long-term maintenance very hard. > > The root cause of this issue, I think, is that `AggregateExec` is not respecting this convention and can potentially output batches that are much larger than `batch_size`. What do you think about moving this fix to `AggregateExec` instead, so it has internal spilling to ensure it does not output large batches? This seems like an issue that should be addressed outside `SortExec`. @2010YOUY01 I agree, this PR does not exactly resolve the issue I opened. While it makes sense to do this in aggregate exec, and in every other operator that does Emit, it does not resolve the underlying problem. @Nachiket-Roy sort_batch_chunked is eager, meaning, it will still peak at 2x the batch memory. The idea is to calculate the sort indices, then everytime do a take on the first batch_size indices, and spill them immediately, that way we only need the original batch size reservation, + an additional smaller chunk for the batch_size rows we do a take on. We can use some estimation method on how much memory that is, but generally we're talking about a few megabytes of extra memory, nothing compared to the practically unlimited size of the original batch. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
