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bufmgr: Introduce infrastructure for faster relation extension
The primary bottlenecks for relation extension are: 1) The extension lock is held while acquiring a victim buffer for the new page. Acquiring a victim buffer can require writing out the old page contents including possibly needing to flush WAL. 2) When extending via ReadBuffer() et al, we write a zero page during the extension, and then later write out the actual page contents. This can nearly double the write rate. 3) The existing bulk relation extension infrastructure in hio.c just amortized the cost of acquiring the relation extension lock, but none of the other costs. Unfortunately 1) cannot currently be addressed in a central manner as the callers to ReadBuffer() need to acquire the extension lock. To address that, this this commit moves the responsibility for acquiring the extension lock into bufmgr.c functions. That allows to acquire the relation extension lock for just the required time. This will also allow us to improve relation extension further, without changing callers. The reason we write all-zeroes pages during relation extension is that we hope to get ENOSPC errors earlier that way (largely works, except for CoW filesystems). It is easier to handle out-of-space errors gracefully if the page doesn't yet contain actual tuples. This commit addresses 2), by using the recently introduced smgrzeroextend(), which extends the relation, without dirtying the kernel page cache for all the extended pages. To address 3), this commit introduces a function to extend a relation by multiple blocks at a time. There are three new exposed functions: ExtendBufferedRel() for extending the relation by a single block, ExtendBufferedRelBy() to extend a relation by multiple blocks at once, and ExtendBufferedRelTo() for extending a relation up to a certain size. To avoid duplicating code between ReadBuffer(P_NEW) and the new functions, ReadBuffer(P_NEW) now implements relation extension with ExtendBufferedRel(), using a flag to tell ExtendBufferedRel() that the relation lock is already held. Note that this commit does not yet lead to a meaningful performance or scalability improvement - for that uses of ReadBuffer(P_NEW) will need to be converted to ExtendBuffered*(), which will be done in subsequent commits. Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi> Reviewed-by: Melanie Plageman <melanieplageman@gmail.com> Discussion: https://postgr.es/m/20221029025420.eplyow6k7tgu6he3@awork3.anarazel.de
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