Andres Freund <andres@2ndquadrant.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.2050004@sigaev.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; else 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