pgsql: snapshot scalability: Move PGXACT->xmin back to PGPROC.

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(at)anarazel(dot)de>
Reviewed-By: Robert Haas <robertmhaas(at)gmail(dot)com>
Reviewed-By: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Reviewed-By: David Rowley <dgrowleyml(at)gmail(dot)com>
Discussion: https://postgr.es/m/20200301083601.ews6hz5dduc3w2se@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1f51c17c68d05c28d5b9294d8013cb9e7e653160

Modified Files
--------------
src/backend/access/gist/gistxlog.c | 2 +-
src/backend/access/nbtree/nbtpage.c | 2 +-
src/backend/access/transam/README | 4 ++--
src/backend/access/transam/twophase.c | 2 +-
src/backend/commands/indexcmds.c | 2 +-
src/backend/replication/logical/snapbuild.c | 6 ++---
src/backend/replication/walsender.c | 10 ++++----
src/backend/storage/ipc/procarray.c | 36 +++++++++++++----------------
src/backend/storage/ipc/sinvaladt.c | 2 +-
src/backend/storage/lmgr/proc.c | 4 ++--
src/backend/utils/time/snapmgr.c | 28 +++++++++++-----------
src/include/storage/proc.h | 10 ++++----
12 files changed, 52 insertions(+), 56 deletions(-)