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shm_mq: Update mq_bytes_written less often.

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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
This commit is contained in:
Robert Haas
2021-10-14 16:06:43 -04:00
parent 7821a0bf20
commit 46846433a0
6 changed files with 69 additions and 19 deletions
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7
src/backend/libpq/pqmq.c
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@@ -154,7 +154,12 @@ mq_putmessage(char msgtype, const char *s, size_t len)
for (;;)
{
result = shm_mq_sendv(pq_mq_handle, iov, 2, true);
/*
* Immediately notify the receiver by passing force_flush as true so
* that the shared memory value is updated before we send the parallel
* message signal right after this.
*/
result = shm_mq_sendv(pq_mq_handle, iov, 2, true, true);
if (pq_mq_parallel_leader_pid != 0)
SendProcSignal(pq_mq_parallel_leader_pid,