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The default of 128kB is unchanged, but the upper limit is changed from 32 blocks to 128 blocks, unless the operating system's IOV_MAX is too low. Some other RDBMSes seem to cap their multi-block buffer pool I/O around this number, and it seems useful to allow experimentation. The concrete change is to our definition of PG_IOV_MAX, which provides the maximum for io_combine_limit and io_max_combine_limit. It also affects a couple of other places that work with arrays of struct iovec or smaller objects on the stack, so we still don't want to use the system IOV_MAX directly without a clamp: it is not under our control and likely to be 1024. 128 seems acceptable for our current usage. For Windows, we can't use real scatter/gather yet, so we continue to define our own IOV_MAX value of 16 and emulate preadv()/pwritev() with loops. Someone would need to research the trade-offs of raising that number. NB if trying to see this working: you might temporarily need to hack BAS_BULKREAD to be bigger, since otherwise the obvious way of "a very big SELECT" is limited by that for now. Suggested-by: Tomas Vondra <tomas@vondra.me> Discussion: https://postgr.es/m/CA%2BhUKG%2B2T9p-%2BzM6Eeou-RAJjTML6eit1qn26f9twznX59qtCA%40mail.gmail.com |
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