zhuqi-lucas commented on PR #14644: URL: https://github.com/apache/datafusion/pull/14644#issuecomment-2661372383
> > This PR can't solve > > select * from t consumes 2GB memory to fully materialize the output, in the best case select * from t order by c1 consumes around 2GB memory, given sorting is an O(1) space operation. The current situation is it consumes 4GB+ (more than 2 times). > > I think this is by design. The physical operators should not reserve memory for the batches it produces. It is the parent operator's duty to reserve memory for the batches fetched from children operators if it needs to hold the batches in memory for a long time. > > > Problem > > I tried one query and this PR is not working as expected, I specified one query to run under 5GB memory (select * without order requires 7GB) but it's still consuming around 10GB, could you double check? I suspect we missed some details. > > It is a problem of datafusion-cli. If datafusion-cli decides to hold all the result batches in memory, it should create a memory consumer for itself and reserve memory for the result batches. > > datafusion-cli [calls collect](https://github.com/apache/datafusion/blob/45.0.0/datafusion-cli/src/exec.rs#L249-L254) to hold the entire result set in memory before displaying it, this is unnecessary when maxrows is not unlimited. I've tried the following code to replace the collect call, and the maximum resident set size has reduced to 4.8 GB: > > ```rust > // Bounded stream; collected results are printed after all input consumed. > let schema = physical_plan.schema(); > let mut stream = execute_stream(physical_plan, task_ctx.clone())?; > let mut results = vec![]; > while let Some(batch) = stream.next().await { > let batch = batch?; > results.push(batch); > if let MaxRows::Limited(max_rows) = print_options.maxrows { > if results.len() >= max_rows { > break; > } > } > } > adjusted.into_inner().print_batches(schema, &results, now)?; > ``` Good finding, may be we can file a issue to improve the datafusion-cli memory usage and reservation. -- 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: github-unsubscr...@datafusion.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org --------------------------------------------------------------------- To unsubscribe, e-mail: github-unsubscr...@datafusion.apache.org For additional commands, e-mail: github-h...@datafusion.apache.org
