On Fri, Aug 31, 2018 at 3:50 PM, Andres Freund <and...@anarazel.de> wrote:
> On 2018-08-31 19:31:47 -0300, Alvaro Herrera wrote: > > On 2018-Aug-31, David Pacheco wrote: > > > > > From reading the 9.6.3 source, it looks like the autovacuum process > > > itself is single-threaded, and it reads pages essentially linearly > > > from the relation (possibly skipping some). When the autovacuum > > > process needs to modify a page, it doesn't write it directly, but > > > rather marks the buffer dirty. The page will be written later, > > > > Unless there's some bug, there is a BufferAccessStrategy that only lets > > a few dozen buffers go unwritten before the autovac worker process > > itself is forced to write some. > > I've not re-checked, but I'm not sure that's true if the buffer is > already in s_b, which it'll be for many workloads. > Does that mean this analysis from above is accurate? It looks to me like the autovacuum process is effectively generating work > (in > the form of async writes) that's being distributed implicitly to the > various > backend processes, creating latency for any other query that happens to > require > a buffer (including read-only queries). Thanks, Dave
