It turned out that Nicolas was correct! I found that the CPU was broken and not spinning at all.
With consecutive parallel query execution, the CPU temperature hits 100C almost immediately after 1 or 2 iterations. So the processor starts throttling way below baseline clk frequency to something like 1.2G or even 1G.
I waited until the new Fan came to report back, and now this weird behavior went away.
Thanks,
Shijia
On Wed, Dec 18, 2019 at 7:44 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Laurenz Albe <laurenz.albe@cybertec.at> writes: > On Tue, 2019-12-17 at 11:11 -0500, Jeff Janes wrote: >> If it is doing a seq scan (I don't know if it is) they intentionally use a >> small ring buffer to, so they evict their own recently used blocks, rather >> than evicting other people's blocks. So these blocks won't build up in >> shared_buffers very rapidly just on the basis of repeated seq scans.
> Sure, but according to the execution plans it is doing a Parallel Index Only Scan.
Nonetheless, the presented test case consists of repeatedly doing the same query, in a fresh session each time. If there's not other activity then this should reach some sort of steady state. The table is apparently fairly large, so I don't find it surprising that the steady state fails to be 100% cached.
