Re: [HACKERS] Vacuum: allow usage of more than 1GB of work mem

On Fri, Jan 20, 2017 at 6:24 AM, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> On Thu, Jan 19, 2017 at 8:31 PM, Claudio Freire <klaussfreire@gmail.com> wrote:
>> On Thu, Jan 19, 2017 at 6:33 AM, Anastasia Lubennikova
>> <a.lubennikova@postgrespro.ru> wrote:
>>> 28.12.2016 23:43, Claudio Freire:
>>>
>>> Attached v4 patches with the requested fixes.
>>>
>>>
>>> Sorry for being late, but the tests took a lot of time.
>>
>> I know. Takes me several days to run my test scripts once.
>>
>>> create table t1 as select i, md5(random()::text) from
>>> generate_series(0,400000000) as i;
>>> create index md5_idx ON  t1(md5);
>>> update t1 set md5 = md5((random() * (100 + 500))::text);
>>> vacuum;
>>>
>>> Patched vacuum used 2.9Gb of memory and vacuumed the index in one pass,
>>> while for old version it took three passes (1GB+1GB+0.9GB).
>>> Vacuum duration results:
>>>
>>> vanilla:
>>> LOG: duration: 4359006.327 ms  statement: vacuum verbose t1;
>>> patched:
>>> LOG: duration: 3076827.378 ms  statement: vacuum verbose t1;
>>>
>>> We can see 30% vacuum speedup. I should note that this case can be
>>> considered
>>> as favorable to vanilla vacuum: the table is not that big, it has just one
>>> index
>>> and disk used is a fast fusionIO. We can expect even more gain on slower
>>> disks.
>>>
>>> Thank you again for the patch. Hope to see it in 10.0.
>>
>> Cool. Thanks for the review and the tests.
>>
>
> I encountered a bug with following scenario.
> 1. Create table and disable autovacuum on that table.
> 2. Make about 200000 dead tuples on the table.
> 3. SET maintenance_work_mem TO 1024
> 4. VACUUM
>
> @@ -729,7 +759,7 @@ lazy_scan_heap(Relation onerel, int options,
> LVRelStats *vacrelstats,
>                          * not to reset latestRemovedXid since we want
> that value to be
>                          * valid.
>                          */
> -                       vacrelstats->num_dead_tuples = 0;
> +                       lazy_clear_dead_tuples(vacrelstats);
>                         vacrelstats->num_index_scans++;
>
>                         /* Report that we are once again scanning the heap */
>
> I think that we should do vacrelstats->dead_tuples.num_entries = 0 as
> well in lazy_clear_dead_tuples(). Once the amount of dead tuples
> reached to maintenance_work_mem, lazy_scan_heap can never finish.

That's right.

I added a test for it in the attached patch set, which uncovered
another bug in lazy_clear_dead_tuples, and took the opportunity to
rebase.

On Mon, Jan 23, 2017 at 1:06 PM, Alvaro Herrera
<alvherre@2ndquadrant.com> wrote:
> I pushed this patch after rewriting it rather completely.  I added
> tracing notices to inspect the blocks it was prefetching and observed
> that the original coding was failing to prefetch the final streak of
> blocks in the table, which is an important oversight considering that it
> may very well be that those are the only blocks to read at all.
>
> I timed vacuuming a 4000-block table in my laptop (single SSD disk;
> dropped FS caches after deleting all rows in table, so that vacuum has
> to read all blocks from disk); it changes from 387ms without patch to
> 155ms with patch.  I didn't measure how much it takes to run the other
> steps in the vacuum, but it's clear that the speedup for the truncation
> phase is considerable.
>
> ĄThanks, Claudio!

Cool.

Though it wasn't the first time this idea has been floating around, I
can't take all the credit.


On Fri, Jan 20, 2017 at 6:25 PM, Alvaro Herrera
<alvherre@2ndquadrant.com> wrote:
> FWIW, I think this patch is completely separate from the maint_work_mem
> patch and should have had its own thread and its own commitfest entry.
> I intend to get a look at the other patch next week, after pushing this
> one.

That's because it did have it, and was left in limbo due to lack of
testing on SSDs. I just had to adopt it here because otherwise tests
took way too long.

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