pgsql: shm_mq: Update mq_bytes_written less often.

shm_mq: Update mq_bytes_written less often.

Do not update shm_mq's mq_bytes_written until we have written
an amount of data greater than 1/4th of the ring size, unless
the caller of shm_mq_send(v) requests a flush at the end of
the message. This reduces the number of calls to SetLatch(),
and also the number of CPU cache misses, considerably, and thus
makes shm_mq significantly faster.

Dilip Kumar, reviewed by Zhihong Yu and Tomas Vondra. Some
minor cosmetic changes by me.

Discussion: http://postgr.es/m/CAFiTN-tVXqn_OG7tHNeSkBbN+iiCZTiQ83uakax43y1sQb2OBA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/46846433a03dff4f2e08c8a161e54a842da360d6

Modified Files
--------------
src/backend/executor/tqueue.c | 2 +-
src/backend/libpq/pqmq.c | 7 +++-
src/backend/storage/ipc/shm_mq.c | 62 +++++++++++++++++++++++++++++------
src/include/storage/shm_mq.h | 8 +++--
src/test/modules/test_shm_mq/test.c | 7 ++--
src/test/modules/test_shm_mq/worker.c | 2 +-
6 files changed, 69 insertions(+), 19 deletions(-)