Hi, On 2019-07-31 23:51:40 +0200, Julien Rouhaud wrote: > On Wed, Jul 31, 2019 at 10:55 AM Evgeny Efimkin <efim...@yandex-team.ru> > wrote: > > What reason to use pg_atomic_uint64? > > The queryid is read and written without holding any lock on the PGPROC > entry, so the pg_atomic_uint64 will guarantee that we get a consistent > value in pg_stat_get_activity(). Other reads shouldn't be a problem > as far as I remember.
Hm, I don't think that's necessary in this case. That's what the st_changecount protocol is trying to ensure, no? /* * To avoid locking overhead, we use the following protocol: a backend * increments st_changecount before modifying its entry, and again after * finishing a modification. A would-be reader should note the value of * st_changecount, copy the entry into private memory, then check * st_changecount again. If the value hasn't changed, and if it's even, * the copy is valid; otherwise start over. This makes updates cheap * while reads are potentially expensive, but that's the tradeoff we want. * * The above protocol needs memory barriers to ensure that the apparent * order of execution is as it desires. Otherwise, for example, the CPU * might rearrange the code so that st_changecount is incremented twice * before the modification on a machine with weak memory ordering. Hence, * use the macros defined below for manipulating st_changecount, rather * than touching it directly. */ int st_changecount; And if it were necessary, why wouldn't any of the other fields in PgBackendStatus need it? There's plenty of other fields written to without a lock, and several of those are also 8 bytes (so it's not a case of assuming that 8 byte reads might not be atomic, but for byte reads are). Greetings, Andres Freund