RE: Parallel heap vacuum

Dear Sawada-san,

> TidStoreBeginIterateShared() is designed for multiple parallel workers
> to iterate a shared TidStore. During an iteration, parallel workers
> share the iteration state and iterate the underlying radix tree while
> taking appropriate locks. Therefore, it's available only for a shared
> TidStore. This is required to implement the parallel heap vacuum,
> where multiple parallel workers do the iteration on the shared
> TidStore.
>
> On the other hand, TidStoreBeginIterate() is designed for a single
> process to iterate a TidStore. It accepts even a shared TidStore as
> you mentioned, but during an iteration there is no inter-process
> coordination such as locking. When it comes to parallel vacuum,
> supporting TidStoreBeginIterate() on a shared TidStore is necessary to
> cover the case where we use only parallel index vacuum but not
> parallel heap scan/vacuum. In this case, we need to store dead tuple
> TIDs on the shared TidStore during heap scan so parallel workers can
> use it during index vacuum. But it's not necessary to use
> TidStoreBeginIterateShared() because only one (leader) process does
> heap vacuum.

Okay, thanks for the description. I felt it is OK to keep.

I read 0001 again and here are comments.

01. vacuumlazy.c
```
+#define LV_PARALLEL_SCAN_SHARED 0xFFFF0001
+#define LV_PARALLEL_SCAN_DESC 0xFFFF0002
+#define LV_PARALLEL_SCAN_DESC_WORKER 0xFFFF0003
```

I checked other DMS keys used for parallel work, and they seems to have name
like PARALEL_KEY_XXX. Can we follow it?

02. LVRelState
```
+ BlockNumber next_fsm_block_to_vacuum;
```

Only the attribute does not have comments Can we add like:
"Next freespace map page to be checked"?

03. parallel_heap_vacuum_gather_scan_stats
```
+ vacrel->scan_stats->vacuumed_pages += ss->vacuumed_pages;
```

Note that `scan_stats->vacuumed_pages` does not exist in 0001, it is defined
in 0004. Can you move it?

04. heap_parallel_vacuum_estimate
```
+
+ heap_parallel_estimate_shared_memory_size(rel, nworkers, &pscan_len,
+ &shared_len, &pscanwork_len);
+
+ /* space for PHVShared */
+ shm_toc_estimate_chunk(&pcxt->estimator, shared_len);
+ shm_toc_estimate_keys(&pcxt->estimator, 1);
+
+ /* space for ParallelBlockTableScanDesc */
+ pscan_len = table_block_parallelscan_estimate(rel);
+ shm_toc_estimate_chunk(&pcxt->estimator, pscan_len);
+ shm_toc_estimate_keys(&pcxt->estimator, 1);
+
+ /* space for per-worker scan state, PHVScanWorkerState */
+ pscanwork_len = mul_size(sizeof(PHVScanWorkerState), nworkers);
+ shm_toc_estimate_chunk(&pcxt->estimator, pscanwork_len);
+ shm_toc_estimate_keys(&pcxt->estimator, 1);
```

I feel pscan_len and pscanwork_len are calclated in heap_parallel_estimate_shared_memory_size().
Can we remove table_block_parallelscan_estimate() and mul_size() from here?

05. Idea

Can you update documentations?

06. Idea

AFAICS pg_stat_progress_vacuum does not contain information related with the
parallel execution. How do you think adding an attribute which shows a list of pids?
Not sure it is helpful for users but it can show the parallelism.

Best regards,
Hayato Kuroda
FUJITSU LIMITED