Re: pgsql: Use pre-fetching for ANALYZE

Greetings,

* Andres Freund (andres(at)anarazel(dot)de) wrote:
> On 2022-06-02 19:30:16 -0700, Andres Freund wrote:
> > On 2021-03-16 18:48:08 +0000, Stephen Frost wrote:
> > > Use pre-fetching for ANALYZE
> > >
> > > When we have posix_fadvise() available, we can improve the performance
> > > of an ANALYZE by quite a bit by using it to inform the kernel of the
> > > blocks that we're going to be asking for. Similar to bitmap index
> > > scans, the number of buffers pre-fetched is based off of the
> > > maintenance_io_concurrency setting (for the particular tablespace or,
> > > if not set, globally, via get_tablespace_maintenance_io_concurrency()).
> >
> > I just looked at this as part of debugging a crash / hang in the AIO patch.
> >
> > The code does:
> >
> > block_accepted = table_scan_analyze_next_block(scan, targblock, vac_strategy);
> >
> > #ifdef USE_PREFETCH
> >
> > /*
> > * When pre-fetching, after we get a block, tell the kernel about the
> > * next one we will want, if there's any left.
> > *
> > * We want to do this even if the table_scan_analyze_next_block() call
> > * above decides against analyzing the block it picked.
> > */
> > if (prefetch_maximum && prefetch_targblock != InvalidBlockNumber)
> > PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, prefetch_targblock);
> > #endif
> >
> > I.e. we lock a buffer and *then* we prefetch another buffer. That seems like a
> > quite bad idea to me. Why are we doing IO while holding a content lock, if we
> > can avoid it?

At the end, we're doing a posix_fadvise() which is a kernel call but
hopefully wouldn't do actual IO when we call it. Still, agreed that
it'd be better to do that without holding locks and no objection to
making such a change.

> It also seems decidedly not great from a layering POV to do the IO in
> analyze.c. There's no guarantee that the tableam maps blocks in a way that's
> compatible with PrefetchBuffer(). Yes, the bitmap heap scan code does
> something similar, but a) that is opt in by the AM, b) there's a comment
> saying it's quite crufty and should be fixed.

Certainly open to suggestions. Are you thinking it'd make sense to add
a 'prefetch_block' method to TableAmRoutine? Or did you have another
thought?

Thanks!

Stephen