Re: [HACKERS] Restricting maximum keep segments by repslots

At Wed, 08 Apr 2020 14:19:56 +0900 (JST), Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> wrote in
> I saw another issue, the following sequence on the primary freezes
> when invalidation happens.
>
> =# create table tt(); drop table tt; select pg_switch_wal();create table tt(); drop table tt; select pg_switch_wal();create table tt(); drop table tt; select pg_switch_wal(); checkpoint;
>
> The last checkpoint command is waiting for CV on
> CheckpointerShmem->start_cv in RequestCheckpoint(), while Checkpointer
> is waiting for the next latch at the end of
> CheckpointerMain. new_started doesn't move but it is the same value
> with old_started.
>
> That freeze didn't happen when I removed
> ConditionVariableSleep(&s->active_cv) in
> InvalidateObsoleteReplicationSlots.
>
> I continue investigating it.

I understand how it happens.

The latch triggered by checkpoint request by CHECKPOINT command has
been absorbed by ConditionVariableSleep() in
InvalidateObsoleteReplicationSlots. The attached allows checkpointer
use MyLatch for other than checkpoint request while a checkpoint is
running.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center