On Wed, Mar 12, 2025 at 3:12 AM Masahiko Sawada <sawada.m...@gmail.com> wrote: > > On Tue, Mar 11, 2025 at 6:00 AM Amit Kapila <amit.kapil...@gmail.com> wrote: > > > > On Mon, Mar 10, 2025 at 11:57 PM Masahiko Sawada <sawada.m...@gmail.com> > > wrote: > > > > > > On Sun, Mar 9, 2025 at 11:12 PM Amit Kapila <amit.kapil...@gmail.com> > > > wrote: > > > > > > > > > > > > > However, in the heap vacuum phase, the leader process needed > > > > > to process all blocks, resulting in soft page faults while creating > > > > > Page Table Entries (PTEs). Without the patch, the backend process had > > > > > already created PTEs during the heap scan, thus preventing these > > > > > faults from occurring during the heap vacuum phase. > > > > > > > > > > > > > This part is again not clear to me because I am assuming all the data > > > > exists in shared buffers before the vacuum, so why the page faults > > > > will occur in the first place. > > > > > > IIUC PTEs are process-local data. So even if physical pages are loaded > > > to PostgreSQL's shared buffer (and paga caches), soft page faults (or > > > minor page faults)[1] can occur if these pages are not yet mapped in > > > its page table. > > > > > > > Okay, I got your point. BTW, I noticed that even for the case where > > all the data is in shared_buffers, the performance improvement for > > workers greater than two does decrease marginally. Am I reading the > > data correctly? If so, what is the theory, and do we have > > recommendations for a parallel degree? > > The decrease you referred to is that the total vacuum execution time? >
Right. > When it comes to the execution time of phase 1, it seems we have good > scalability. For example, with 2 workers (i.e.3 workers working > including the leader in total) it got about 3x speed up, and with 4 > workers it got about 5x speed up. Regarding other phases, the phase 3 > got slower probably because of PTEs stuff, but I don't investigate why > the phase 2 also slightly got slower with more than 2 workers. > Could it be possible that now phase-2 needs to access the shared area for TIDs, and some locking/unlocking causes such slowdown? -- With Regards, Amit Kapila.
