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

On Thu, Sep 26, 2019 at 11:37 PM Tomas Vondra
<tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>
> Hi,
>
> Attached is an updated patch series, rebased on current master. It does
> fix one memory accounting bug in ReorderBufferToastReplace (the code was
> not properly updating the amount of memory).
>

Few comments on 0001
1.
I am getting below linking error in pgoutput when compiling the patch
on my windows system:
pgoutput.obj : error LNK2001: unresolved external symbol _logical_work_mem

You need to use PGDLLIMPORT for logical_work_mem.

2. After, I fixed above and tried some basic test, it fails with below
callstack:
postgres.exe!ExceptionalCondition(const char *
conditionName=0x00d92854, const char * errorType=0x00d928bc, const
char * fileName=0x00d92e60,
int lineNumber=2148) Line 55
postgres.exe!ReorderBufferChangeMemoryUpdate(ReorderBuffer *
rb=0x02693390, ReorderBufferChange * change=0x0269dd38, bool
addition=true) Line 2148
postgres.exe!ReorderBufferQueueChange(ReorderBuffer * rb=0x02693390,
unsigned int xid=525, unsigned __int64 lsn=36083720,
ReorderBufferChange
* change=0x0269dd38) Line 635
postgres.exe!DecodeInsert(LogicalDecodingContext * ctx=0x0268ef80,
XLogRecordBuffer * buf=0x012cf718) Line 716 + 0x24 bytes C
postgres.exe!DecodeHeapOp(LogicalDecodingContext * ctx=0x0268ef80,
XLogRecordBuffer * buf=0x012cf718) Line 437 + 0xd bytes C
postgres.exe!LogicalDecodingProcessRecord(LogicalDecodingContext *
ctx=0x0268ef80, XLogReaderState * record=0x0268f228) Line 129
postgres.exe!pg_logical_slot_get_changes_guts(FunctionCallInfoBaseData
* fcinfo=0x02688680, bool confirm=true, bool binary=false) Line 307
postgres.exe!pg_logical_slot_get_changes(FunctionCallInfoBaseData *
fcinfo=0x02688680) Line 376

Basically, the assert added by you in ReorderBufferChangeMemoryUpdate
failed. Then, I explored a bit and it seems that you have missed
assigning a value to txn, a new variable added by this patch in
structure ReorderBufferChange:
@@ -77,6 +82,9 @@ typedef struct ReorderBufferChange
/* The type of change. */
enum ReorderBufferChangeType action;

+ /* Transaction this change belongs to. */
+ struct ReorderBufferTXN *txn;

3.
@@ -206,6 +206,17 @@ CREATE SUBSCRIPTION <replaceable
class="parameter">subscription_name</replaceabl
</para>
</listitem>
</varlistentry>
+
+ <varlistentry>
+ <term><literal>work_mem</literal> (<type>integer</type>)</term>
+ <listitem>
+ <para>
+ Limits the amount of memory used to decode changes on the
+ publisher. If not specified, the publisher will use the default
+ specified by <varname>logical_work_mem</varname>.
+ </para>
+ </listitem>
+ </varlistentry>

I don't see any explanation of how this will be useful? How can a
subscriber predict the amount of memory required by a publisher for
decoding? This is more unpredictable because when initially the
changes are recorded in ReorderBuffer, it doesn't even filter
corresponding to any publisher. Do we really need this? I think
giving more knobs to the user is helpful when they can someway know
how to use it. In this case, it is not clear whether the user can
ever use this.

4. Can we some way expose the memory consumed by ReorderBuffer? If
so, we might be able to write some tests covering new functionality.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com