Re: POC: Cleaning up orphaned files using undo logs

Hi,

On 2019-07-18 11:15:05 +0530, Amit Kapila wrote:
> On Wed, Jul 17, 2019 at 3:37 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > I'm not yet sure whether we'd want the rbtree nodes being pointed to
> > directly by the hashtable, or whether we'd want one indirection.
> >
> > e.g. either something like:
> >
> >
> > typedef struct UndoWorkerQueue
> > {
> > /* priority ordered tree */
> > RBTree *tree;
> > ....
> > }
> >
>
> I think we also need the size of rbtree (aka how many nodes/undo
> requests it has) to know whether we can add more. This information is
> available in binary heap, but here I think we need to track it in
> UndoWorkerQueue. Basically, at each enqueue/dequeue, we need to
> increment/decrement the same.
>
> > typedef struct UndoWorkerQueueEntry
> > {
> > RBTNode tree_node;
> >
> > /*
> > * Reference hashtable via key, not pointers, entries might be
> > * moved.
> > */
> > RollbackHashKey rollback_key
> > ...
> > } UndoWorkerQueueEntry;
> >
>
> In UndoWorkerQueueEntry, we might also want to include some other info
> like dbid, request_size, next_retry_at, err_occurred_at so that while
> accessing queue entry in comparator functions or other times, we don't
> always need to perform hash table search. OTOH, we can do hash_search
> as well, but may be code-wise it will be better to keep additional
> information.

The dots signal that additional fields are needed in those places.

> Another thing is we need some freelist/array for
> UndoWorkerQueueEntries equivalent to size of three queues?

I think using the slist as I proposed for the second alternative is
better?

> BTW, do you have any preference for using dynahash or simplehash for
> RollbackHashTable?

I find simplehash nicer to use in code, personally, and it's faster in
most cases...

Greetings,

Andres Freund