Re: PATCH: logical_work_mem and logical streaming of largein-progress transactions - Mailing list pgsql-hackers

On Wed, Jun 24, 2020 at 4:27 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
>  iOn Wed, Jun 24, 2020 at 4:04 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Tue, Jun 23, 2020 at 7:00 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > >
> > > Here is the POC patch to discuss the idea of a cleanup of shared
> > > fileset on proc exit.  As discussed offlist,  here I am maintaining
> > > the list of shared fileset.  First time when the list is NULL I am
> > > registering the cleanup function with on_proc_exit routine.  After
> > > that for subsequent fileset, I am just appending it to filesetlist.
> > > There is also an interface to unregister the shared file set from the
> > > cleanup list and that is done by the caller whenever we are deleting
> > > the shared fileset manually.  While explaining it here, I think there
> > > could be one issue if we delete all the element from the list will
> > > become NULL and on next SharedFileSetInit we will again register the
> > > function.  Maybe that is not a problem but we can avoid registering
> > > multiple times by using some flag in the file
> > >
> >
> > I don't understand what you mean by "using some flag in the file".
>
> Basically, in POC as shown in below code snippet,  We are checking
> that if the "filesetlist" is NULL then only register the on_proc_exit
> function.  But, as described above if all the items are deleted the
> list will be NULL.  So I told that instead of checking the filesetlist
> is NULL,  we can have just a boolean variable that if we have
> registered the callback then don't do it again.
>

Check if there is any precedent of the same in the code?

>
> >
> > Review comments on various patches.
> >
> > poc_shared_fileset_cleanup_on_procexit
> > =================================
> > 1.
> > - ent->subxact_fileset =
> > - MemoryContextAlloc(ApplyContext, sizeof(SharedFileSet));
> > + MemoryContext oldctx;
> >
> > + /* Shared fileset handle must be allocated in the persistent context */
> > + oldctx = MemoryContextSwitchTo(ApplyContext);
> > + ent->subxact_fileset = palloc(sizeof(SharedFileSet));
> >   SharedFileSetInit(ent->subxact_fileset, NULL);
> > + MemoryContextSwitchTo(oldctx);
> >   fd = BufFileCreateShared(ent->subxact_fileset, path);
> >
> > Why is this change required for this patch and why we only cover
> > SharedFileSetInit in the Apply context and not BufFileCreateShared?
> > The comment is also not very clear on this point.
>
> Because only the sharedfileset and the filesetlist which is allocated
> under SharedFileSetInit, are required in the permanent context.
> BufFileCreateShared, only creates the Buffile and VFD which will be
> required only within the current stream so transaction context is
> enough.
>

Okay, then add some more comments to explain it or if you have
explained it elsewhere, then add a reference for the same.

> > 2.
> > +void
> > +SharedFileSetUnregister(SharedFileSet *input_fileset)
> > +{
> > + bool found = false;
> > + ListCell *l;
> > +
> > + Assert(filesetlist != NULL);
> > +
> > + /* Loop over all the pending shared fileset entry */
> > + foreach (l, filesetlist)
> > + {
> > + SharedFileSet *fileset = (SharedFileSet *) lfirst(l);
> > +
> > + /* remove the entry from the list and delete the underlying files */
> > + if (input_fileset->number == fileset->number)
> > + {
> > + SharedFileSetDeleteAll(fileset);
> > + filesetlist = list_delete_cell(filesetlist, l);
> >
> > Why are we calling SharedFileSetDeleteAll here when in the caller we
> > have already deleted the fileset as per below code?
> > BufFileDeleteShared(ent->stream_fileset, path);
> > + SharedFileSetUnregister(ent->stream_fileset);
> >
> > I think it will be good if somehow we can remove the fileset from
> > filesetlist during BufFileDeleteShared.  If that is possible, then we
> > don't need a separate API for SharedFileSetUnregister.
>
> But the filesetlist is maintained at the sharedfileset level, so even
> if we delete from BufFileDeleteShared, we need to call an API from the
> sharedfileset layer to unregister the fileset.
>

Sure, but isn't it better if we can call such an API from BufFileDeleteShared?


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