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When a backend attempts to start a read IO and finds the first buffer already has I/O in progress, previously it waited for that I/O to complete before initiating reads for any of the subsequent buffers. Although it must wait for the I/O to finish when acquiring the buffer, there's no reason for it to wait when setting up the read operation. Waiting at this point prevents starting I/O on subsequent buffers and can significantly reduce concurrency. This matters in two workloads: 1) When multiple backends scan the same relation concurrently. 2) When a single backend requests the same block multiple times within the readahead distance. Waiting each time an in-progress read is encountered effectively degenerates the access pattern into synchronous I/O. To fix this, when encountering an already in-progress IO for the head buffer, the wait reference is now recorded and waiting is deferred until WaitReadBuffers(), when the buffer actually needs to be acquired. In rare cases, a backend may still need to wait synchronously at IO start time: If another backend has set BM_IO_IN_PROGRESS on the buffer but has not yet set the wait reference. Such windows should be brief and uncommon. Author: Melanie Plageman <melanieplageman@gmail.com> Author: Andres Freund <andres@anarazel.de> Reviewed-by: Andres Freund <andres@anarazel.de> Reviewed-by: Melanie Plageman <melanieplageman@gmail.com> Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com> Discussion: https://postgr.es/m/flat/zljergweqti7x67lg5ije2rzjusie37nslsnkjkkby4laqqbfw%403p3zu522yykv |
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