Hello. At Wed, 23 Sep 2020 05:37:24 +0000, "tsunakawa.ta...@fujitsu.com" <tsunakawa.ta...@fujitsu.com> wrote in > From: Jamison, Kirk/ジャミソン カーク <k.jami...@fujitsu.com>
# Wow. I'm surprised to read it.. > > I revised the patch based from my understanding of Horiguchi-san's comment, > > but I could be wrong. > > Quoting: > > > > " > > + /* Get the number of blocks for the supplied relation's > > fork */ > > + nblocks = smgrnblocks(smgr_reln, > > forkNum[fork_num]); > > + Assert(BlockNumberIsValid(nblocks)); > > + > > + if (nblocks < BUF_DROP_FULLSCAN_THRESHOLD) > > > > As mentioned upthread, the criteria whether we do full-scan or > > lookup-drop is how large portion of NBUFFERS this relation-drop can be > > going to invalidate. So the nblocks above should be the sum of number > > of blocks to be truncated (not just the total number of blocks) of all > > designated forks. Then once we decided to do lookup-drop method, we > > do that for all forks." > > One takeaway from Horiguchi-san's comment is to use the number of blocks to > invalidate for comparison, instead of all blocks in the fork. That is, use > > nblocks = smgrnblocks(fork) - firstDelBlock[fork]; > > Does this make sense? > > What do you think is the reason for summing up all forks? I didn't > understand why. Typically, FSM and VM forks are very small. If the main > fork is larger than NBuffers / 500, then v14 scans the entire shared buffers > for the FSM and VM forks as well as the main fork, resulting in three scans > in total. I thought of summing up smgrnblocks(fork) - firstDelBlock[fork] of all folks. I don't mind omitting non-main forks but a comment to explain the reason or reasoning would be needed. reards. -- Kyotaro Horiguchi NTT Open Source Software Center
