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

On Thu, Jun 4, 2020 at 2:05 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Wed, Jun 3, 2020 at 2:43 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Tue, Jun 2, 2020 at 7:53 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > >
> > > On Tue, Jun 2, 2020 at 4:56 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > >
> > > > On Tue, Jun 2, 2020 at 3:59 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > > >
> > > > > I thin for our use case BufFileCreateShared is more suitable. I think
> > > > > we need to do some modifications so that we can use these apps without
> > > > > SharedFileSet. Otherwise, we need to unnecessarily need to create
> > > > > SharedFileSet for each transaction and also need to maintain it in xid
> > > > > array or xid hash until transaction commit/abort. So I suggest
> > > > > following modifications in shared files set so that we can
> > > > > conveniently use it.
> > > > > 1. ChooseTablespace(const SharedFileSet fileset, const char name)
> > > > > if fileset is NULL then select the DEFAULTTABLESPACEOID
> > > > > 2. SharedFileSetPath(char path, SharedFileSet fileset, Oid tablespace)
> > > > > If fileset is NULL then in directory path we can use MyProcPID or
> > > > > something instead of fileset->creator_pid.
> > > > >
> > > >
> > > > Hmm, I find these modifications a bit ad-hoc. So, not sure if it is
> > > > better than the patch maintains sharedfileset information.
> > >
> > > I think we might do something better here, maybe by supplying function
> > > pointer or so, but maintaining sharedfileset which contains different
> > > tablespace/mutext which we don't need at all for our purpose also
> > > doesn't sound very appealing.
> > >
> >
> > I think we can say something similar for Relation (rel cache entry as
> > well) maintained in LogicalRepRelMapEntry. I think we only need a
> > pointer to that information.
>
> Yeah, I see.
>
> > > Let me see if I can not come up with
> > > some clean way of avoiding the need to shared-fileset then maybe we
> > > can go with the shared fileset idea.
> > >
> >
> > Fair enough.
>
> While evaluating it further I feel there are a few more problems to
> solve if we are using BufFile, First thing is that in subxact file we
> maintain the information of xid and its offset in the changes file.
> So now, we will also have to store 'fileno' but that we can find using
> BufFileTell. Yet another problem is that currently, we don't
> have the truncate option in the BufFile, but we need it if the
> sub-transaction gets aborted. I think we can implement an extra
> interface with the BufFile and should not be very hard as we already
> know the fileno and the offset. I will evaluate this part further and
> let you know about the same.

I have further evaluated this and also tested the concept with a POC
patch. Soon I will complete and share, here is the scatch of the
idea.

As discussed we will use SharedBufFile for changes files and subxact
files. There will be a separate LogicalStreamingResourceOwner, which
will be used to manage the VFD of the shared buf files. We can create
a per stream resource owner i.e. on stream start we will create the
resource owner and all the shared buffiles will be opened under that
resource owner, which will be deleted on stream stop. We need to
remember the SharedFileSet so that for subsequent stream for the same
transaction we can open the same file again, for this we will use a
hash table with xid as a key and in that, we will keep stream_fileset
and subxact_fileset's pointers as payload.

+typedef struct StreamXidHash
+{
+ TransactionId xid;
+ SharedFileSet *stream_fileset;
+ SharedFileSet *subxact_fileset;
+} StreamXidHash;

We have to do some extension to the buffile modules, some of them are
already discussed up-thread but still listing them all down here
- A new interface BufFileTruncateShared(BufFile *file, int fileno,
off_t offset), for truncating the subtransaction changes, if changes
are spread across multiple files those files will be deleted and we
will adjust the file count and current offset accordingly in BufFile.
- In BufFileOpenShared, we will have to implement a mode so that we
can open in write mode as well, current only read-only mode supported.
- In SharedFileSetInit, if dsm_segment is NULL then we will not
register the file deletion on on_dsm_detach.
- As usual, we will clean up the files on stream abort/commit, or on
the worker exit.

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