Re: BitmapHeapScan streaming read user and prelim refactoring

On 3/24/24 18:38, Melanie Plageman wrote:
> On Sun, Mar 24, 2024 at 01:36:19PM +0100, Tomas Vondra wrote:
>>
>>
>> On 3/23/24 01:26, Melanie Plageman wrote:
>>> On Fri, Mar 22, 2024 at 08:22:11PM -0400, Melanie Plageman wrote:
>>>> On Tue, Mar 19, 2024 at 02:33:35PM +0200, Heikki Linnakangas wrote:
>>>>> On 18/03/2024 17:19, Melanie Plageman wrote:
>>>>>> I've attached v7 rebased over this commit.
>>>>>
>>>>> If we delayed table_beginscan_bm() call further, after starting the TBM
>>>>> iterator, we could skip it altogether when the iterator is empty.
>>>>>
>>>>> That's a further improvement, doesn't need to be part of this patch set.
>>>>> Just caught my eye while reading this.
>>>>
>>>> Hmm. You mean like until after the first call to tbm_[shared]_iterate()?
>>>> AFAICT, tbm_begin_iterate() doesn't tell us anything about whether or
>>>> not the iterator is "empty". Do you mean cases when the bitmap has no
>>>> blocks in it? It seems like we should be able to tell that from the
>>>> TIDBitmap.
>>>>
>>>>>
>>>>>> v7-0003-Push-BitmapHeapScan-skip-fetch-optimization-into-.patch
>>>>>
>>>>> I suggest to avoid the double negative with SO_CAN_SKIP_FETCH, and call the
>>>>> flag e.g. SO_NEED_TUPLE.
>>>>
>>>> Agreed. Done in attached v8. Though I wondered if it was a bit weird
>>>> that the flag is set in the common case and not set in the uncommon
>>>> case...
>>>
>>> v8 actually attached this time
>>
>> I tried to run the benchmarks with v8, but unfortunately it crashes for
>> me very quickly (I've only seen 0015 to crash, so I guess the bug is in
>> that patch).
>>
>> The backtrace attached, this doesn't seem right:
>>
>> (gdb) p hscan->rs_cindex
>> $1 = 543516018
>
> Thanks for reporting this! I hadn't seen it crash on my machine, so I
> didn't realize that I was no longer initializing rs_cindex and
> rs_ntuples on the first call to heapam_bitmap_next_tuple() (since
> heapam_bitmap_next_block() wasn't being called first). I've done this in
> attached v9.
>

OK, I've restarted the tests with v9.

> I haven't had a chance yet to reproduce the regressions you saw in the
> streaming read user patch or to look closely at the performance results.

So you tried to reproduce it and didn't hit the issue? Or didn't have
time to look into that yet? FWIW with v7 it failed almost immediately
(only a couple queries until hitting one triggering the issue), but v9
that's not the case (hundreds of queries without an error).

> I don't anticipate the streaming read user will have any performance
> differences in this v9 from v6, since I haven't yet rebased in Thomas'
> latest streaming read API changes nor addressed any other potential
> regression sources.
>

OK, understood. It'll be interesting to see the behavior with the new
version of Thomas' patch.

I however wonder what the plan with these patches is - do we still plan
to get some of this into v17? It seems to me we're getting uncomfortably
close to the end of the cycle, with a fairly incomplete idea of how it
affects performance.

Which is why I've been focusing more on the refactoring patches (up to
0015), to make sure those don't cause regressions if committed. And I
think that's generally true.

But for the main StreamingRead API the situation is very different.

> I tried rebasing in Thomas' latest version today and something is
> causing a crash that I have yet to figure out. v10 of this patchset will
> have his latest version once I get that fixed. I wanted to share this
> version with what I think is a bug fix for the crash you saw first.
>

Understood. I'll let the tests with v9 run for now.

regards

--
Tomas Vondra
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