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Previously we would allocate blocks to parallel workers during a parallel sequential scan 1 block at a time. Since other workers were likely to request a block before a worker returns for another block number to work on, this could lead to non-sequential I/O patterns in each worker which could cause the operating system's readahead to perform poorly or not at all. Here we change things so that we allocate consecutive "chunks" of blocks to workers and have them work on those until they're done, at which time we allocate another chunk for the worker. The size of these chunks is based on the size of the relation. Initial patch here was by Thomas Munro which showed some good improvements just having a fixed chunk size of 64 blocks with a simple ramp-down near the end of the scan. The revisions of the patch to make the chunk size based on the relation size and the adjusted ramp-down in powers of two was done by me, along with quite extensive benchmarking to determine the optimal chunk sizes. For the most part, benchmarks have shown significant performance improvements for large parallel sequential scans on Linux, FreeBSD and Windows using SSDs. It's less clear how this affects the performance of cloud providers. Tests done so far are unable to obtain stable enough performance to provide meaningful benchmark results. It is possible that this could cause some performance regressions on more obscure filesystems, so we may need to later provide users with some ability to get something closer to the old behavior. For now, let's leave that until we see that it's really required. Author: Thomas Munro, David Rowley Reviewed-by: Ranier Vilela, Soumyadeep Chakraborty, Robert Haas Reviewed-by: Amit Kapila, Kirk Jamison Discussion: https://postgr.es/m/CA+hUKGJ_EErDv41YycXcbMbCBkztA34+z1ts9VQH+ACRuvpxig@mail.gmail.com |
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README
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.