Re: Max connections reached without max connections reached

> Looking again into the back trace[1], it appeared that the backend is
> getting stuck while getting the subtransaction's parent information,
> and that can only happen if the snapshot is getting marked as
> overflowed. So it seems that some of the scripts are using a lot of
> sub-transaction (> 64) within a transaction and in such case, we can
> not cache the sub-transaction information and the snapshot will be
> marked overflowed. After that, for checking the visibility of any
> tuple we need to consult the SLRU to get TopMost transaction
> information and if the subtransaction SLRU cache is full then it will
> have to do the I/O. So in this case it appeared that a lot of
> parallel scans/new connections are trying to read the pg_subtrans and
> due to limited cache size that is not even able to load the data in
> the cache and getting frequently blocked. So I assume that when you
> say the startup is getting stuck forever, how long did you wait, maybe
> it is moving but getting stuck again for the next tuple, so to confirm
> that attach the process with gdb put breakpoint in XidInMVCCSnapshot()
> and check whether it is hitting that breakpoint again or not. Also,
> check whether you are seeing "SubtransControlLock" in
> pg_stat_activity. There is one nice blog[2] explaining this issue so
> you can refer to that as well.

Ah. Snap. This makes a lot of sense. The developers have a nasty habit of
using LOOP over rows which has never been stamped out. I bet they have
applied this to something with a lot of rows.

Is there a way of seeing which statements / backends / processes have
greater than 64 stxns? I'd like to monitor this.

- James

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