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

On Fri, May 22, 2020 at 4:46 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, May 22, 2020 at 11:54 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > v22-0006-Add-support-for-streaming-to-built-in-replicatio
> > ----------------------------------------------------------------------------
> >
> Few more comments on v22-0006 patch:
>
> 1.
> +stream_cleanup_files(Oid subid, TransactionId xid, bool missing_ok)
> +{
> + int i;
> + char path[MAXPGPATH];
> + bool found = false;
> +
> + subxact_filename(path, subid, xid);
> +
> + if ((unlink(path) < 0) && (errno != ENOENT) && !missing_ok)
> + ereport(ERROR,
> + (errcode_for_file_access(),
> + errmsg("could not remove file \"%s\": %m", path)));
>
> Here, we have unlinked the files containing information of subxacts
> but don't we need to free the corresponding memory (memory for
> subxacts) as well?

Basically, stream_cleanup_files, is used for
1) cleanup file on worker exit
2) while writing the first segment of the xid we clean up to ensure
there are no orphaned file with same xid.
3) After apply commit we clean up the file.

Whereas subxacts memory is used between the stream start and stream
stop as soon stream stop we write the subxacts changes to file and
free the memory. So there is no case that we can have subxact memory
at stream_cleanup_files, except on worker exit but there we are
already exiting the worker. IMHO we don't need to free memory there.

> 2.
> apply_handle_stream_abort()
> {
> ..
> + subxact_filename(path, MyLogicalRepWorker->subid, xid);
> +
> + if (unlink(path) < 0)
> + ereport(ERROR,
> + (errcode_for_file_access(),
> + errmsg("could not remove file \"%s\": %m", path)));
> +
> + return;
> ..
> }
>
> Like the previous comment, it seems here also we need to free subxacts
> memory and additionally we forgot to adjust the xids array as well.

In this, we are allocating memory in subxact_info_read, but we are
again calling subxact_info_write which will free the memory.

> 3.
> apply_handle_stream_abort()
> {
> ..
> + /* XXX optimize the search by bsearch on sorted data */
> + for (i = nsubxacts; i > 0; i--)
> + {
> + if (subxacts[i - 1].xid == subxid)
> + {
> + subidx = (i - 1);
> + found = true;
> + break;
> + }
> + }
> +
> + if (!found)
> + return;
> ..
> }
>
> Is it possible that we didn't find the xid in subxacts array? If so,
> I think we should mention the same in comments, otherwise, we should
> have an assert for found.

We may not find due to the empty transaction, I have changed the comments.

> 4.
> apply_handle_stream_abort()
> {
> ..
> + changes_filename(path, MyLogicalRepWorker->subid, xid);
> +
> + if (truncate(path, subxacts[subidx].offset))
> + ereport(ERROR,
> + (errcode_for_file_access(),
> + errmsg("could not truncate file \"%s\": %m", path)));
> ..
> }
>
> Will truncate works on Windows? I see in the code we ftruncate which
> is defined as chsize in win32.h and win32_port.h. I have not tested
> this so I am not very sure about this. I got a below warning when I
> tried to compile this code on Windows. I think it is better to
> ftruncate as it is used at other places in the code as well.
>
> worker.c(798): warning C4013: 'truncate' undefined; assuming extern
> returning int

I have changed to the ftruncate.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com