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authorAndres Freund <andres@anarazel.de>2020-08-13 16:25:21 -0700
committerAndres Freund <andres@anarazel.de>2020-08-13 16:25:21 -0700
commit1f51c17c68d05c28d5b9294d8013cb9e7e653160 (patch)
treebb2dc0ab3009ced3ccfe819aba406f1f5810d4e4 /src/backend/access/transam/clog.c
parenta811ea5bde2fbf450095994b5726dcbf64d68668 (diff)
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snapshot scalability: Move PGXACT->xmin back to PGPROC.
Now that xmin isn't needed for GetSnapshotData() anymore, it leads to unnecessary cacheline ping-pong to have it in PGXACT, as it is updated considerably more frequently than the other PGXACT members. After the changes in dc7420c2c92, this is a very straight-forward change. For highly concurrent, snapshot acquisition heavy, workloads this change alone can significantly increase scalability. E.g. plain pgbench on a smaller 2 socket machine gains 1.07x for read-only pgbench, 1.22x for read-only pgbench when submitting queries in batches of 100, and 2.85x for batches of 100 'SELECT';. The latter numbers are obviously not to be expected in the real-world, but micro-benchmark the snapshot computation scalability (previously spending ~80% of the time in GetSnapshotData()). Author: Andres Freund <andres@anarazel.de> Reviewed-By: Robert Haas <robertmhaas@gmail.com> Reviewed-By: Thomas Munro <thomas.munro@gmail.com> Reviewed-By: David Rowley <dgrowleyml@gmail.com> Discussion: https://postgr.es/m/20200301083601.ews6hz5dduc3w2se@alap3.anarazel.de
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