pgsql: Move "hot" members of PGPROC into a separate PGXACT array.

Move "hot" members of PGPROC into a separate PGXACT array.

This speeds up snapshot-taking and reduces ProcArrayLock contention.
Also, the PGPROC (and PGXACT) structures used by two-phase commit are
now allocated as part of the main array, rather than in a separate
array, and we keep ProcArray sorted in pointer order.  These changes
are intended to minimize the number of cache lines that must be pulled
in to take a snapshot, and testing shows a substantial increase in
performance on both read and write workloads at high concurrencies.

Pavan Deolasee, Heikki Linnakangas, Robert Haas

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ed0b409d22346b1b027a4c2099ca66984d94b6dd

Modified Files
--------------
src/backend/access/transam/twophase.c |  115 ++++++++------
src/backend/access/transam/varsup.c   |   11 +-
src/backend/access/transam/xact.c     |    4 +-
src/backend/commands/analyze.c        |    4 +-
src/backend/commands/vacuum.c         |    4 +-
src/backend/postmaster/postmaster.c   |    3 +
src/backend/replication/walsender.c   |    2 +-
src/backend/storage/ipc/ipci.c        |    2 +-
src/backend/storage/ipc/procarray.c   |  276 ++++++++++++++++++++-------------
src/backend/storage/lmgr/deadlock.c   |    4 +-
src/backend/storage/lmgr/lock.c       |    3 +-
src/backend/storage/lmgr/proc.c       |   87 ++++++++---
src/backend/utils/time/snapmgr.c      |    2 +-
src/include/storage/proc.h            |   47 ++++--
14 files changed, 356 insertions(+), 208 deletions(-)