Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions

On Wed, Mar 04, 2020 at 09:13:49AM +0530, Dilip Kumar wrote:
>On Wed, Mar 4, 2020 at 3:16 AM Tomas Vondra
><tomas.vondra@2ndquadrant.com> wrote:
>>
>> Hi,
>>
>> I started looking at this patch series again, hoping to get it moving
>> for PG13.
>
>Nice.
>
> There's been a tremendous amount of work done since I last
>> worked on it, and a lot was discussed on this thread, so it'll take a
>> while to get familiar with the new code ...
>>
>> The first thing I realized that WAL-logging of assignments in v12 does
>> both the "old" logging (using dedicated message) and "new" with
>> toplevel-XID embedded in the first message. Yes, the patch was wrong,
>> because it eliminated all calls to ProcArrayApplyXidAssignment() and so
>> it was trivial to crash the replica due to KnownAssignedXids overflow.
>> But I don't think re-introducing XLOG_XACT_ASSIGNMENT message is the
>> right fix.
>>
>> I actually proposed doing this (having both ways to log assignments) so
>> that there's no regression risk with (wal_level < logical). But IIRC
>> Andres objected to it, argumenting that we should not log the same piece
>> of information in two very different ways at the same time (IIRC it was
>> discussed on the FOSDEM dev meeting, so I don't have a link to share).
>> And I do agree with him ...
>>
>> The question is, why couldn't the replica use the same assignment info
>> we already write for logical decoding? The main challenge is that now
>> the assignment can be sent in many different xlog messages, from a bunch
>> of resource managers (essentially, any xlog message with a xid can have
>> embedded XID of the toplevel xact). So the handling would either need to
>> happen in every rmgr, or we need to move it before we call the rmgr.
>>
>> For exampple, we might do this e.g. in StartupXLOG() I think, per the
>> attached patch (FWIW this particular fix was written by Masahiko Sawada,
>> not me). This does the trick for me - I'm no longer able to reproduce
>> the KnownAssignedXids overflow.
>>
>> The one difference is that we used to call ProcArrayApplyXidAssignment
>> for larger groups of XIDs, as sent in the assignment message. Now we
>> call it for each individual assignment. I don't know if this is an
>> issue, but I suppose we might introduce some sort of local caching
>> (accumulate the assignments into a local array, call the function only
>> when we have enough of them).
>
>Thanks for the pointers,  I will think over these points.
>
>>
>> Aside from that, I think there's a minor bug in xact.c - the patch adds
>> a "assigned" field to TransactionStateData, but then it fails to add a
>> default value into TopTransactionStateData. We probably interpret NULL
>> as false, but then there's nothing for the pointer. I suspect it might
>> leave some random garbage there, leading to strange things later.
>
>Actually, we will never access that field for the
>TopTransactionStateData, right?
>See below code,  we have a check that only if IsSubTransaction(), then
>we access the "assigned" filed.
>
>+bool
>+IsSubTransactionAssignmentPending(void)
>+{
>+ if (!XLogLogicalInfoActive())
>+ return false;
>+
>+ /* we need to be in a transaction state */
>+ if (!IsTransactionState())
>+ return false;
>+
>+ /* it has to be a subtransaction */
>+ if (!IsSubTransaction())
>+ return false;
>+
>+ /* the subtransaction has to have a XID assigned */
>+ if (!TransactionIdIsValid(GetCurrentTransactionIdIfAny()))
>+ return false;
>+
>+ /* and it needs to have 'assigned' */
>+ return !CurrentTransactionState->assigned;
>+
>+}
>

The problem is not with the "assigned" field, really. AFAICS we probably
initialize it to false because we interpret NULL as false. My concern
was that we essentially leave the last pointer not initialized. That
seems like a bug, not sure if it breaks something in practice.

>>
>> Another thing I noticed is LogicalDecodingProcessRecord() extracts the
>> toplevel XID using a macro
>>
>>    txid = XLogRecGetTopXid(record);
>>
>> but then it just starts accessing the fields directly again in the
>> ReorderBufferAssignChild call. I think we should do this instead:
>>
>>      ReorderBufferAssignChild(ctx->reorder,
>>                               txid,
>>                              XLogRecGetXid(record),
>>                               buf.origptr);
>
>Make sense.  I will change this in the patch.
>

+1, thanks


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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