Re: SLRU statistics

On Fri, Feb 28, 2020 at 08:19:18PM -0300, Alvaro Herrera wrote:
>On 2020-Jan-21, Tomas Vondra wrote:
>
>> On Tue, Jan 21, 2020 at 05:09:33PM +0900, Masahiko Sawada wrote:
>
>> > I've not tested the performance impact but perhaps we might want to
>> > disable these counter by default and controlled by a GUC. And similar
>> > to buffer statistics it might be better to inline
>> > pgstat_count_slru_page_xxx function for better performance.
>>
>> Hmmm, yeah. Inlining seems like a good idea, and maybe we should have
>> something like track_slru GUC.
>
>I disagree with adding a GUC.  If a performance impact can be measured
>let's turn the functions to static inline, as already proposed.  My
>guess is that pgstat_count_slru_page_hit() is the only candidate for
>that; all the other paths involve I/O or lock acquisition or even WAL
>generation, so the impact won't be measurable anyhow.  We removed
>track-enabling GUCs years ago.
>

Did we actually remove track-enabling GUCs? I think we still have

  - track_activities
  - track_counts
  - track_io_timing
  - track_functions

But maybe I'm missing something?

That being said, I'm not sure we need to add a GUC. I'll do some
measurements and we'll see. Maybe the statis inline will me enough.

>BTW, this comment:
>            /* update the stats counter of pages found in shared buffers */
>
>is not strictly true, because we don't use what we normally call "shared
>buffers"  for SLRUs.
>

Oh, right. Will fix.

>Patch applies cleanly.  I suggest to move the page_miss() call until
>after SlruRecentlyUsed(), for consistency with the other case.
>

OK.

>I find SlruType pretty odd, and the accompanying "if" list in
>pg_stat_get_slru() correspondingly so.  Would it be possible to have
>each SLRU enumerate itself somehow?  Maybe add the name in SlruCtlData
>and query that, somehow.  (I don't think we have an array of SlruCtlData
>anywhere though, so this might be a useless idea.)
>

Well, maybe. We could have a system to register SLRUs dynamically, but
the trick here is that by having a fixed predefined number of SLRUs
simplifies serialization in pgstat.c and so on. I don't think the "if"
branches in pg_stat_get_slru() are particularly ugly, but maybe we could
replace the enum with a registry of structs, something like rmgrlist.h.
It seems like an overkill to me, though.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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