Re: [HACKERS] GUC for cleanup indexes threshold.

On 2017-09-22 16:22, Sokolov Yura wrote:
> On 2017-09-22 11:21, Masahiko Sawada wrote:
>> On Fri, Sep 22, 2017 at 4:16 PM, Kyotaro HORIGUCHI
>> <horiguchi.kyotaro@lab.ntt.co.jp> wrote:
>>> At Fri, 22 Sep 2017 15:00:20 +0900, Masahiko Sawada 
>>> <sawada.mshk@gmail.com> wrote in 
>>> <CAD21AoD6zgb1W6ps1aXj0CcAB_chDYiiTNtEdpMhkefGg13-GQ@mail.gmail.com>
>>>> On Tue, Sep 19, 2017 at 3:31 PM, Kyotaro HORIGUCHI
>>>> <horiguchi.kyotaro@lab.ntt.co.jp> wrote:
>>>> > I was just looking the thread since it is found left alone for a
>>>> > long time in the CF app.
>>>> >
>>>> > At Mon, 18 Sep 2017 16:35:58 -0700, Peter Geoghegan <pg@bowt.ie> wrote in
<CAH2-WzkhJhAXD+6DdBp7D8WYLfJ3D0m=AZbGsiw=USUjTmuv-g@mail.gmail.com>
>>>> >> On Wed, Apr 5, 2017 at 3:50 PM, Andres Freund <andres@anarazel.de> wrote:
>>>> >> > Hi,
>>>> >> >
>>>> >> > On 2017-04-01 03:05:07 +0900, Masahiko Sawada wrote:
>>>> >> >> On Fri, Mar 31, 2017 at 11:44 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>>>> >> >> [ lots of valuable discussion ]
>>>> >> >
>>>> >> > I think this patch clearly still is in the design stage, and has
>>>> >> > received plenty feedback this CF.  I'll therefore move this to the next
>>>> >> > commitfest.
>>>> >>
>>>> >> Does anyone have ideas on a way forward here? I don't, but then I
>>>> >> haven't thought about it in detail in several months.
>>>> >
>>>> > Is the additional storage in metapage to store the current status
>>>> > of vaccum is still unacceptable even if it can avoid useless
>>>> > full-page scan on indexes especially for stable tables?
>>>> >
>>>> > Or, how about additional 1 bit in pg_stat_*_index to indicate
>>>> > that the index *don't* require vacuum cleanup stage. (default
>>>> > value causes cleanup)
>>>> 
>>>> You meant that "the next cycle" is the lazy_cleanup_index() function
>>>> called by lazy_scan_heap()?
>>> 
>>> Both finally call btvacuumscan under a certain condition, but
>>> what I meant by "the next cycle" is the lazy_cleanup_index call
>>> in the next round of vacuum since abstract layer (index am) isn't
>>> conscious of the detail of btree.
>>> 
>>>> > index_bulk_delete (or ambulkdelete) returns the flag in
>>>> > IndexBulkDeleteResult then lazy_scan_heap stores the flag in
>>>> > stats and in the next cycle it is looked up to decide the
>>>> > necessity of index cleanup.
>>>> >
>>>> 
>>>> Could you elaborate about this? For example in btree index, the 
>>>> index
>>>> cleanup skips to scan on the index scan if index_bulk_delete has 
>>>> been
>>>> called during vacuuming because stats != NULL. So I think we don't
>>>> need such a flag.
>>> 
>>> The flag works so that successive two index full scans don't
>>> happen in a vacuum round. If any rows are fully deleted, just
>>> following btvacuumcleanup does nothing.
>>> 
>>> I think what you wanted to solve here was the problem that
>>> index_vacuum_cleanup runs a full scan even if it ends with no
>>> actual work, when manual or anti-wraparound vacuums.  (I'm
>>> getting a bit confused on this..) It is caused by using the
>>> pointer "stats" as the flag to instruct to do that. If the
>>> stats-as-a-flag worked as expected, the GUC doesn't seem to be
>>> required.
>> 
>> Hmm, my proposal is like that if a table doesn't changed since the
>> previous vacuum much we skip the cleaning up index.
>> 
>> If the table has at least one garbage we do the lazy_vacuum_index and
>> then IndexBulkDeleteResutl is stored, which causes to skip doing the
>> btvacuumcleanup. On the other hand, if the table doesn't have any
>> garbage but some new tuples inserted since the previous vacuum, we
>> don't do the lazy_vacuum_index but do the lazy_cleanup_index. In this
>> case, we always do the lazy_cleanup_index (i.g, we do the full scan)
>> even if only one tuple is inserted. That's why I proposed a new GUC
>> parameter which allows us to skip the lazy_cleanup_index in the case.
>> 
>>> 
>>> Addition to that, as Simon and Peter pointed out
>>> index_bulk_delete can leave not-fully-removed pages (so-called
>>> half-dead pages and pages that are recyclable but not registered
>>> in FSM, AFAICS) in some cases mainly by RecentGlobalXmin
>>> interlock. In this case, just inhibiting cleanup scan by a
>>> threshold lets such dangling pages persist in the index. (I
>>> conldn't make such a many dangling pages, though..)
>>> 
>>> The first patch in the mail (*1) does that. It seems having some
>>> bugs, though..
>>> 
>>> 
>>> Since the dangling pages persist until autovacuum decided to scan
>>> the belonging table again, we should run a vacuum round (or
>>> index_vacuum_cleanup itself) even having no dead rows if we want
>>> to clean up such pages within a certain period. The second patch
>>> doesn that.
>>> 
>> 
>> IIUC half-dead pages are not relevant to this proposal. The proposal
>> has two problems;
>> * By skipping index cleanup we could leave recyclable pages that are
>> not marked as a recyclable.
>> * we stash an XID when a btree page is deleted, which is used to
>> determine when it's finally safe to recycle the page
>> 
>> Regards,
>> 
>> --
>> Masahiko Sawada
>> NIPPON TELEGRAPH AND TELEPHONE CORPORATION
>> NTT Open Source Software Center
> 
> Here is a small patch that skips scanning btree index if no pending
> deleted pages exists.
> 
> It detects this situation by comparing pages_deleted with pages_free.
> If they are equal, then there is no half-deleted pages, and it is
> safe to skip next lazy scan.
> 
> Flag stored in a btpo_flags. It is unset using general wal before scan.
> If no half-deleted pages found, it is set without wal (like hint bit).
> (it is safe to miss setting flag, but it is not safe to miss unsetting
> flag).
> 
> This patch works correctly:
> - if rows are only inserted and never deleted, index always skips
> scanning (starting with second scan).
> - if some rows updated/deleted, then some scans are not skipped. But
> when all half-deleted pages are marked as deleted, lazy scans start to
> be skipped.
> 
> Open question: what to do with index statistic? For simplicity this
> patch skips updating stats (just returns NULL from btvacuumcleanup).
> Probably, it should detect proportion of table changes, and do not
> skip scans if table grows too much.

Excuse me, I didn't mean to overwrite "last attachment" on commitfest 
page.

-- 
Sokolov Yura
Postgres Professional: https://postgrespro.ru
The Russian Postgres Company


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