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Commit06fb561

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Increase io_combine_limit range to 1MB.
The default of 128kB is unchanged, but the upper limit is changed from32 blocks to 128 blocks, unless the operating system's IOV_MAX is toolow. Some other RDBMSes seem to cap their multi-block buffer pool I/Oaround this number, and it seems useful to allow experimentation.The concrete change is to our definition of PG_IOV_MAX, which providesthe maximum for io_combine_limit and io_max_combine_limit. It alsoaffects a couple of other places that work with arrays of struct iovecor smaller objects on the stack, so we still don't want to use thesystem IOV_MAX directly without a clamp: it is not under our control andlikely to be 1024. 128 seems acceptable for our current usage.For Windows, we can't use real scatter/gather yet, so we continue todefine our own IOV_MAX value of 16 and emulate preadv()/pwritev() withloops. Someone would need to research the trade-offs of raising thatnumber.NB if trying to see this working: you might temporarily need to hackBAS_BULKREAD to be bigger, since otherwise the obvious way of "a verybig 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
1 parent10f6646 commit06fb561

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4 files changed

+16
-7
lines changed

4 files changed

+16
-7
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‎doc/src/sgml/config.sgml

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@@ -2638,6 +2638,8 @@ include_dir 'conf.d'
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This parameter can only be set in
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the <filename>postgresql.conf</filename> file or on the server
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command line.
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The maximum possible size depends on the operating system and block
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size, but is typically 1MB on Unix and 128kB on Windows.
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The default is 128kB.
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</para>
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</listitem>
@@ -2655,6 +2657,8 @@ include_dir 'conf.d'
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higher than the <varname>io_max_combine_limit</varname> parameter, the
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lower value will silently be used instead, so both may need to be raised
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to increase the I/O size.
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The maximum possible size depends on the operating system and block
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size, but is typically 1MB on Unix and 128kB on Windows.
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The default is 128kB.
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</para>
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</listitem>

‎src/backend/storage/aio/read_stream.c

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@@ -515,9 +515,10 @@ read_stream_begin_impl(int flags,
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* finishes we don't want to have to wait for its buffers to be consumed
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* before starting a new one.
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*
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* Be careful not to allow int16 to overflow (even though that's not
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* possible with the current GUC range limits), allowing also for the
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* spare entry and the overflow space.
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* Be careful not to allow int16 to overflow. That is possible with the
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* current GUC range limits, so this is an artificial limit of ~32k
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* buffers and we'd need to adjust the types to exceed that. We also have
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* to allow for the spare entry and the overflow space.
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*/
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max_pinned_buffers= (max_ios+1)*io_combine_limit;
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max_pinned_buffers=Min(max_pinned_buffers,

‎src/backend/utils/misc/postgresql.conf.sample

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@@ -200,9 +200,9 @@
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#backend_flush_after = 0# measured in pages, 0 disables
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#effective_io_concurrency = 16# 1-1000; 0 disables prefetching
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#maintenance_io_concurrency = 16# 1-1000; 0 disables prefetching
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#io_max_combine_limit = 128kB# usually 1-32 blocks (depends on OS)
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#io_max_combine_limit = 128kB# usually 1-128 blocks (depends on OS)
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# (change requires restart)
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#io_combine_limit = 128kB# usually 1-32 blocks (depends on OS)
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#io_combine_limit = 128kB# usually 1-128 blocks (depends on OS)
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#io_method = worker# worker, sync (change requires restart)
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#io_max_concurrency = -1# Max number of IOs that one process

‎src/include/port/pg_iovec.h

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@@ -33,8 +33,12 @@ struct iovec
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#endif
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/* Define a reasonable maximum that is safe to use on the stack. */
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#definePG_IOV_MAX Min(IOV_MAX, 32)
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/*
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* Define a reasonable maximum that is safe to use on the stack in arrays of
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* struct iovec and other small types. The operating system could limit us to
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* a number as low as 16, but most systems have 1024.
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*/
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#definePG_IOV_MAX Min(IOV_MAX, 128)
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/*
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* Like preadv(), but with a prefix to remind us of a side-effect: on Windows

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