On Tue, Apr 27, 2021 at 9:17 AM Masahiko Sawada <sawada.m...@gmail.com> wrote: > > On Tue, Apr 27, 2021 at 11:31 AM vignesh C <vignes...@gmail.com> wrote: > > > > > > And I think there is > > > > also a risk to increase shared memory when we want to add other > > > > statistics in the future. > > > > > > > > > > Yeah, so do you think it is not a good idea to store stats in > > > ReplicationSlot? Actually storing them in a slot makes it easier to > > > send them during ReplicationSlotRelease which is quite helpful if the > > > replication is interrupted due to some reason. Or the other idea was > > > that we send stats every time we stream or spill changes. > > > > We use around 64 bytes of shared memory to store the statistics > > information per slot, I'm not sure if this is a lot of memory. If this > > memory is fine, then I felt the approach to store stats seems fine. If > > that memory is too much then we could use the other approach to update > > stats when we stream or spill the changes as suggested by Amit. > > I agree that makes it easier to send slot stats during > ReplicationSlotRelease() but I'd prefer to avoid storing data that > doesn't need to be shared in the shared buffer if possible. >
Sounds reasonable and we might add some stats in the future so that will further increase the usage of shared memory. > And those > counters are not used by physical slots at all. If sending slot stats > every time we stream or spill changes doesn't affect the system much, > I think it's better than having slot stats in the shared memory. > As the minimum size of logical_decoding_work_mem is 64KB, so in the worst case, we will send stats after decoding that many changes. I don't think it would impact too much considering that we need to spill or stream those many changes. If it concerns any users they can always increase logical_decoding_work_mem. The default value is 64MB at which point, I don't think it will matter sending the stats. > Also, not sure it’s better but another idea would be to make the slot > stats a global variable like pgBufferUsage and use it during decoding. > Hmm, I think it is better to avoid global variables if possible. > Or we can set a proc-exit callback? But to be honest, I'm not sure > which approach we should go with. Those approaches have proc and cons. > I think we can try the first approach listed here which is to send stats each time we spill or stream. -- With Regards, Amit Kapila.
