On Mon, Nov 4, 2019 at 11:58 PM Andres Freund <and...@anarazel.de> wrote: > > Hi, > > On 2019-11-04 12:59:02 -0500, Jeff Janes wrote: > > On Mon, Nov 4, 2019 at 1:54 AM Amit Kapila <amit.kapil...@gmail.com> wrote: > > > > > For parallel vacuum [1], we were discussing what is the best way to > > > divide the cost among parallel workers but we didn't get many inputs > > > apart from people who are very actively involved in patch development. > > > I feel that we need some more inputs before we finalize anything, so > > > starting a new thread. > > > > > > > Maybe a I just don't have experience in the type of system that parallel > > vacuum is needed for, but if there is any meaningful IO throttling which is > > active, then what is the point of doing the vacuum in parallel in the first > > place? > > I am wondering the same - but to be fair, it's pretty easy to run into > cases where VACUUM is CPU bound. E.g. because most pages are in > shared_buffers, and compared to the size of the indexes number of tids > that need to be pruned is fairly small (also [1]). That means a lot of > pages need to be scanned, without a whole lot of IO going on. The > problem with that is just that the defaults for vacuum throttling will > also apply here, I've never seen anybody tune vacuum_cost_page_hit = 0, > vacuum_cost_page_dirty=0 or such (in contrast, the latter is the highest > cost currently). Nor do we reduce the cost of vacuum_cost_page_dirty > for unlogged tables. > > So while it doesn't seem unreasonable to want to use cost limiting to > protect against vacuum unexpectedly causing too much, especially read, > IO, I'm doubtful it has current practical relevance. >
IIUC, you mean to say that it is of not much practical use to do parallel vacuum if I/O throttling is enabled for an operation, is that right? > I'm wondering how much of the benefit of parallel vacuum really is just > to work around vacuum ringbuffers often massively hurting performance > (see e.g. [2]). > Yeah, it is a good thing to check, but if anything, I think a parallel vacuum will further improve the performance with larger ring buffers as it will make it more CPU bound. -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com
