On Fri, Feb 11, 2022 at 7:56 PM Yura Sokolov <y.soko...@postgrespro.ru> wrote: > > В Сб, 16/10/2021 в 16:37 +0530, Bharath Rupireddy пишет: > > On Thu, Oct 14, 2021 at 10:56 AM Fujii Masao > > <masao.fu...@oss.nttdata.com> wrote: > > > On 2021/10/12 15:46, Bharath Rupireddy wrote: > > > > On Tue, Oct 12, 2021 at 5:37 AM Fujii Masao > > > > <masao.fu...@oss.nttdata.com> wrote: > > > > > On 2021/10/12 4:07, Bharath Rupireddy wrote: > > > > > > Hi, > > > > > > > > > > > > While working on [1], it is found that currently the ProcState array > > > > > > doesn't have entries for auxiliary processes, it does have entries > > > > > > for > > > > > > MaxBackends. But the startup process is eating up one slot from > > > > > > MaxBackends. We need to increase the size of the ProcState array by > > > > > > 1 > > > > > > at least for the startup process. The startup process uses ProcState > > > > > > slot via InitRecoveryTransactionEnvironment->SharedInvalBackendInit. > > > > > > The procState array size is initialized to MaxBackends in > > > > > > SInvalShmemSize. > > > > > > > > > > > > The consequence of not fixing this issue is that the database may > > > > > > hit > > > > > > the error "sorry, too many clients already" soon in > > > > > > SharedInvalBackendInit. > > > > > > On second thought, I wonder if this error could not happen in practice. > > > No? > > > Because autovacuum doesn't work during recovery and the startup process > > > can safely use the ProcState entry for autovacuum worker process. > > > Also since the minimal allowed value of autovacuum_max_workers is one, > > > the ProcState array guarantees to have at least one entry for autovacuum > > > worker. > > > > > > If this understanding is right, we don't need to enlarge the array and > > > can just update the comment. I don't strongly oppose to enlarge > > > the array in the master, but I'm not sure it's worth doing that > > > in back branches if the issue can cause no actual error. > > > > Yes, the issue can't happen. The comment in the SInvalShmemSize, > > mentioning about the startup process always having an extra slot > > because the autovacuum worker is not active during recovery, looks > > okay. But, is it safe to assume that always? Do we have a way to > > specify that in the form an Assert(when_i_am_startup_proc && > > autovacuum_not_running) (this looks a bit dirty though)? Instead, we > > can just enlarge the array in the master and be confident about the > > fact that the startup process always has one dedicated slot. > > But this slot wont be used for most of cluster life. It will be just > waste.
Correct. In the standby autovacuum launcher and worker are not started so, the startup process will always have a slot free for it to use. > And `Assert(there_is_startup_proc && autovacuum_not_running)` has > value on its own, hasn't it? So why doesn't add it with comment. Assertion doesn't make sense to me now. Because the postmaster ensures that the autovacuum launcher/workers will not get started in standby mode and we can't reliably know in InitRecoveryTransactionEnvironment (startup process) whether or not autovacuum launcher process has been started. FWIW, here's a patch just adding a comment on how the startup process can get a free procState array slot even when SInvalShmemSize hasn't accounted for it. Regards, Bharath Rupireddy.
v1-0001-Add-comment-about-startup-process-getting-procSta.patch
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