Re: [PERFORM] Hash join gets slower as work_mem increases?
Tomas Vondra wrote: > Yes, that's clearly the culprit here. In both cases we estimate here are > only ~4000 tuples in the hash, and 9.3 sizes the hash table to have at > most ~10 tuples per bucket (in a linked list). > > However we actually get ~3M rows, so there will be ~3000 tuples per > bucket, and that's extremely expensive to walk. The reason why 100MB is > faster is that it's using 2 batches, thus making the lists "just" ~1500 > tuples long. > > This is pretty much exactly the reason why I reworked hash joins in 9.5. > I'd bet it's going to be ~20x faster on that version. Thank you for the explanation! Yours, Laurenz Albe -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Hash join gets slower as work_mem increases?
Tomas Vondra wrote:
> On 01/29/2016 04:17 PM, Albe Laurenz wrote:
>> I have a query that runs *slower* if I increase work_mem.
>>
>> The execution plans are identical in both cases, except that a temp file
>> is used when work_mem is smaller.
>> What could be an explanation for this?
>> Is this known behaviour?
>
> There is a bunch of possible causes for such behavior, but it's quite
> impossible to say if this is an example of one of them as you have not
> posted the interesting parts of the explain plan. Also, knowing
> PostgreSQL version would be useful.
>
> I don't think the example you posted is due to exceeding on-CPU cache as
> that's just a few MBs per socket, so the smaller work_mem is
> significantly larger.
>
> What I'd expect to be the issue here is under-estimate of the hash table
> size, resulting in too few buckets and thus long chains of tuples that
> need to be searched sequentially. Smaller work_mem values usually limit
> the length of those chains in favor of batching.
>
> Please, post the whole explain plan - especially the info about number
> of buckets/batches and the Hash node details.
Thanks for looking at this.
Sorry, I forgot to mention that this is PostgreSQL 9.3.10.
I didn't post the whole plan since it is awfully long, I'll include hyperlinks
for the whole plan.
work_mem = '100MB' (http://explain.depesz.com/s/7b6a):
-> Hash Join (cost=46738.74..285400.61 rows=292 width=8) (actual
time=4296.986..106087.683 rows=187222 loops=1)
Hash Cond: ("*SELECT* 1_2".postadresse_id = p.postadresse_id)
Buffers: shared hit=1181177 dirtied=1, temp read=7232 written=7230
[...]
-> Hash (cost=18044.92..18044.92 rows=4014 width=8) (actual
time=4206.892..4206.892 rows=3096362 loops=1)
Buckets: 1024 Batches: 2 (originally 1) Memory Usage: 102401kB
Buffers: shared hit=1134522 dirtied=1, temp written=5296
work_mem = '500MB' (http://explain.depesz.com/s/Cgkl):
-> Hash Join (cost=46738.74..285400.61 rows=292 width=8) (actual
time=3802.849..245970.049 rows=187222 loops=1)
Hash Cond: ("*SELECT* 1_2".postadresse_id = p.postadresse_id)
Buffers: shared hit=1181175 dirtied=111
[...]
-> Hash (cost=18044.92..18044.92 rows=4014 width=8) (actual
time=3709.584..3709.584 rows=3096360 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 120952kB
Buffers: shared hit=1134520 dirtied=111
Does that support your theory?
There is clearly an underestimate here, caused by correlated attributes, but
is that the cause for the bad performance with increased work_mem?
Yours,
Laurenz Albe
--
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Hash join gets slower as work_mem increases?
On 02/01/2016 10:38 AM, Albe Laurenz wrote:
Tomas Vondra wrote:
...
I didn't post the whole plan since it is awfully long, I'll include hyperlinks
for the whole plan.
work_mem = '100MB' (http://explain.depesz.com/s/7b6a):
-> Hash Join (cost=46738.74..285400.61 rows=292 width=8) (actual
time=4296.986..106087.683 rows=187222 loops=1)
Hash Cond: ("*SELECT* 1_2".postadresse_id = p.postadresse_id)
Buffers: shared hit=1181177 dirtied=1, temp read=7232 written=7230
[...]
-> Hash (cost=18044.92..18044.92 rows=4014 width=8) (actual
time=4206.892..4206.892 rows=3096362 loops=1)
Buckets: 1024 Batches: 2 (originally 1) Memory Usage: 102401kB
Buffers: shared hit=1134522 dirtied=1, temp written=5296
work_mem = '500MB' (http://explain.depesz.com/s/Cgkl):
-> Hash Join (cost=46738.74..285400.61 rows=292 width=8) (actual
time=3802.849..245970.049 rows=187222 loops=1)
Hash Cond: ("*SELECT* 1_2".postadresse_id = p.postadresse_id)
Buffers: shared hit=1181175 dirtied=111
[...]
-> Hash (cost=18044.92..18044.92 rows=4014 width=8) (actual
time=3709.584..3709.584 rows=3096360 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 120952kB
Buffers: shared hit=1134520 dirtied=111
Does that support your theory?
There is clearly an underestimate here, caused by correlated attributes, but
is that the cause for the bad performance with increased work_mem?
Yes, that's clearly the culprit here. In both cases we estimate here are
only ~4000 tuples in the hash, and 9.3 sizes the hash table to have at
most ~10 tuples per bucket (in a linked list).
However we actually get ~3M rows, so there will be ~3000 tuples per
bucket, and that's extremely expensive to walk. The reason why 100MB is
faster is that it's using 2 batches, thus making the lists "just" ~1500
tuples long.
This is pretty much exactly the reason why I reworked hash joins in 9.5.
I'd bet it's going to be ~20x faster on that version.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
--
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance
[PERFORM] Hash join gets slower as work_mem increases?
I have a query that runs *slower* if I increase work_mem.
The execution plans are identical in both cases, except that a temp file
is used when work_mem is smaller.
The relevant lines of EXPLAIN ANALYZE output are:
With work_mem='100MB':
-> Hash Join (cost=46738.74..285400.61 rows=292 width=8) (actual
time=4296.986..106087.683 rows=187222 loops=1)
Hash Cond: ("*SELECT* 1_2".postadresse_id = p.postadresse_id)
Buffers: shared hit=1181177 dirtied=1, temp read=7232 written=7230
With work_mem='500MB':
-> Hash Join (cost=46738.74..285400.61 rows=292 width=8) (actual
time=3802.849..245970.049 rows=187222 loops=1)
Hash Cond: ("*SELECT* 1_2".postadresse_id = p.postadresse_id)
Buffers: shared hit=1181175 dirtied=111
I ran operf on both backends, and they look quite similar, except that the
number of samples is different (this is "opreport -c" output):
CPU: Intel Sandy Bridge microarchitecture, speed 2899.8 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a unit mask
of 0x00 (No unit mask) count 9
samples %image name symbol name
---
112 0.0019 postgres ExecProcNode
3020116 49.9904 postgres ExecScanHashBucket
3021162 50.0077 postgres ExecHashJoin
3020116 92.8440 postgres ExecScanHashBucket
3020116 49.9207 postgres ExecScanHashBucket [self]
3020116 49.9207 postgres ExecScanHashBucket
8190 0.1354 vmlinux apic_timer_interrupt
What could be an explanation for this?
Is this known behaviour?
Yours,
Laurenz Albe
--
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Hash join gets slower as work_mem increases?
Hi > I ran operf on both backends, and they look quite similar, except that the > number of samples is different (this is "opreport -c" output): > > CPU: Intel Sandy Bridge microarchitecture, speed 2899.8 MHz (estimated) > Counted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a unit > mask of 0x00 (No unit mask) count 9 > samples %image name symbol name > > --- > 112 0.0019 postgres ExecProcNode > 3020116 49.9904 postgres ExecScanHashBucket > 3021162 50.0077 postgres ExecHashJoin > 3020116 92.8440 postgres ExecScanHashBucket > 3020116 49.9207 postgres ExecScanHashBucket [self] > 3020116 49.9207 postgres ExecScanHashBucket > 8190 0.1354 vmlinux apic_timer_interrupt > > What could be an explanation for this? > Is this known behaviour? > one issue was fixed in 9.5 large hash table can introduce a lot of outs from L1, L2 caches. Pavel > > Yours, > Laurenz Albe > > -- > Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-performance >
