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

 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.

@@ -76,6 +80,13 @@ SharedFileSetInit(SharedFileSet *fileset, dsm_segment *seg)
  /* Register our cleanup callback. */
  if (seg)
  on_dsm_detach(seg, SharedFileSetOnDetach, PointerGetDatum(fileset));
+ else
+ {
+ if (filesetlist == NULL)
+ on_proc_exit(SharedFileSetOnProcExit, 0);
+
+ filesetlist = lcons((void *)fileset, filesetlist);
+ }
 }

>
> 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.

> 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.  Am I missing
something?

> 3.
> +static List * filesetlist = NULL;
> +
>  static void SharedFileSetOnDetach(dsm_segment *segment, Datum datum);
> +static void SharedFileSetOnProcExit(int status, Datum arg);
>  static void SharedFileSetPath(char *path, SharedFileSet *fileset, Oid
> tablespace);
>  static void SharedFilePath(char *path, SharedFileSet *fileset, const
> char *name);
>  static Oid ChooseTablespace(const SharedFileSet *fileset, const char *name);
> @@ -76,6 +80,13 @@ SharedFileSetInit(SharedFileSet *fileset, dsm_segment *seg)
>   /* Register our cleanup callback. */
>   if (seg)
>   on_dsm_detach(seg, SharedFileSetOnDetach, PointerGetDatum(fileset));
> + else
> + {
> + if (filesetlist == NULL)
> + on_proc_exit(SharedFileSetOnProcExit, 0);
>
> We use NIL for list initialization and comparison.  See lock_files usage.

Right.

> 4.
> +SharedFileSetOnProcExit(int status, Datum arg)
> +{
> + ListCell *l;
> +
> + /* Loop over all the pending  shared fileset entry */
> + foreach (l, filesetlist)
> + {
> + SharedFileSet *fileset = (SharedFileSet *) lfirst(l);
> + SharedFileSetDeleteAll(fileset);
> + }
>
> We can initialize filesetlist as NIL after the for loop as it will
> make the code look clean.

ok

Thanks for your feedback on this.  I will reply to other comments separately.

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