On Thu, May 31, 2018 at 10:04 AM, Ron <ronljohnso...@gmail.com> wrote:
> On 05/31/2018 08:52 AM, Melvin Davidson wrote: > > > > On Thu, May 31, 2018 at 1:13 AM, Charles Clavadetscher < > clavadetsc...@swisspug.org> wrote: > >> Hi Melvin >> >> As an answer to a previous post you wrote: >> >> "Also, your main problem is that when you have two exact same queries >> executing at the same time, they will cause contention in >> the disk, and neither one will make much progress." >> >> Could you elaborate a little more on the meaning of "contention in the >> disk"? >> What is it that happens? >> >> Thank you and have a good day. >> Regards >> Charles >> >> >> > >Could you elaborate a little more on the meaning of "contention in the > disk"? > >What is it that happens? > > To simplify, you have two users/jobs, both wanting the exact same > information. So the system instructs the disk to get > that information from the disk, which causes the disk head to "seek" to > the position of the first eligible row and > continues positioning to other eligible rows. Now the job is not > exclusive, so the system temporarily switches to the > other job, which causes the disk to go back to the first row and work from > there. The switching back and forth continues, > so that instead of one job finishing quickly, they both have to take turns > waiting for needed information. That takes > a lot longer, > > Try this, Select a table that has a lot of rows, ideally 1M+. Then start a > query with a WHERE clause and see how long > it takes. Then submit the same query from 5 separate connections > simultaneously and see how long that takes. > > > Why isn't the OS caching the disk blocks, and why isn't Postgres using the > cached data? > > > -- > Angular momentum makes the world go 'round. > > Why isn't the OS caching the disk blocks, and why isn't Postgres using the cached data? It does, but the cache is for each connection/job. They are not shared. -- *Melvin Davidson* *Maj. Database & Exploration Specialist* *Universe Exploration Command – UXC* Employment by invitation only!
