On Fri, Mar 15, 2024 at 07:23:46AM +0100, Ronan Dunklau wrote:
> Le mercredi 13 mars 2024, 17:55:23 CET Noah Misch a écrit :
> > On Wed, Mar 13, 2024 at 10:37:21AM +0100, Ronan Dunklau wrote:
> > > I'm a bit worried about triggering additional lseeks
> > > when we in fact have other free pages in the map, that would pass the
> > > test. I'm not sure how relevant it is given the way we search the FSM
> > > with fp_next_slot though...
> >
> > That's a reasonable thing to worry about. We could do wrong by trying too
> > hard to use an FSM slot, and we could do wrong by not trying hard enough.
> >
> > > To address that, I've given a bit of thought about enabling / disabling
> > > the
> > > auto-repair behaviour with a flag in GetPageWithFreeSpace to distinguish
> > > the cases where we know we have a somewhat up-to-date value compared to
> > > the case where we don't (as in, for heap, try without repair, then get an
> > > uptodate value to try the last block, and if we need to look at the FSM
> > > again then ask for it to be repaired) but it brings way too much
> > > complexity and would need careful thought for each AM.
> > >
> > > So please find attached a patch with the change you propose.
> > >
> > > Do you have a link to the benchmark you mention though to evaluate the
> > > patch against it ?
> >
> > Here is one from the thread that created commit 719c84c:
> > https://www.postgresql.org/message-id/flat/CA%2BTgmob7xED4AhoqLspSOF0wCMYEom
> > gHfuVdzNJnwWVoE_c60g%40mail.gmail.com
> >
> > There may be other benchmarks earlier in that thread.
>
> Thank you. I tried to run that benchmark, with 4 clients all copying 10M
> pgbench_accounts row concurrently. With and without the patch, I ran the
> benchmark 5 times, and took the average.
Does "that benchmark" refer to "unlogged tables, 4 parallel copies", "logged
tables, 4 parallel copies", or something else?
> master: 15.05s
> patched: 15.24s (+1.25%)
>
> If I remove the best and worst run for each of those, the difference falls at
> +0.7%.
To get some additional perspective on the benchmark, how hard would it be to
run one or both of the following? Feel free to decline if difficult.
- Make GetPageWithFreeSpace() just "return InvalidBlockNumber", then rerun the
benchmark. This should be a lot slower. If not, the bottleneck is
somewhere unexpected, and we'd need a different benchmark.
- Get profiles with both master and patched. (lseek or freespace.c functions
rising by 0.1%-1% would fit what we know.)
Distinguishing a 1% change from a 0% change would need different techniques
still. If we're genuinely slowing bulk loads by ~1% to account for the
possibly of a flawed post-recovery FSM, that's sad, but I'm inclined to accept
the loss. A persistent error in an innocent INSERT is unacceptable, and the
alternatives we discussed upthread have their own substantial trouble. Other
opinions?
Thanks,
nm
