Re: speedup tidbitmap patch: cache page

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)2ndquadrant(dot)com>
Cc: David Rowley <dgrowleyml(at)gmail(dot)com>, Teodor Sigaev <teodor(at)sigaev(dot)ru>, Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: speedup tidbitmap patch: cache page
Date: 2015-01-16 17:35:34
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Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> On 2015-01-16 12:15:35 -0500, Tom Lane wrote:
>> It strikes me that this patch leaves some lookups on the table,
>> specifically that it fails to avoid repeated hash_search lookups
>> inside tbm_page_is_lossy() in the situation where we're adding
>> new TIDs to an already-lossified page. Is it worth adding a few
>> more lines to handle that case as well?

> There was a alternative version (v2.3 in 549950FB(dot)2050004(at)sigaev(dot)ru) of
> the patch that cached the lossyness of a page, but Teodor/David didn't
> find it to be beneficial in their benchmarking.

> Teodor's argument was basically that it's completely lost in the noise
> due to the much bigger overhead of rechecks.

That's a fair point, but on reflection it seems like a patch that covered
this case too wouldn't actually be any more complicated, so why not?
v2.3 is pretty brute-force and I agree it's not very attractive, but
I was thinking about something like

BlockNumber cached_blkno = InvalidBlockNumber;
PagetableEntry *page = NULL;

inside loop:

/* look up the target page unless we already have it */
if (blk != cached_blkno)
if (tbm_page_is_lossy(tbm, blk))
page = NULL;
page = tbm_get_pageentry(tbm, blk);
cached_blkno = blk;
if (page == NULL)
continue; /* page is already marked lossy */

The "reset" after calling tbm_lossify() would just need to be
"cached_blkno = InvalidBlockNumber".

regards, tom lane

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