Re: [PoC] pgstattuple2: block sampling to reduce physical read

On 11/10/13 17:08, Satoshi Nagayasu wrote:
> (2013/10/11 7:32), Mark Kirkwood wrote:
>> On 11/10/13 11:09, Mark Kirkwood wrote:
>>> On 16/09/13 16:20, Satoshi Nagayasu wrote:
>>>> (2013/09/15 11:07), Peter Eisentraut wrote:
>>>>> On Sat, 2013-09-14 at 16:18 +0900, Satoshi Nagayasu wrote:
>>>>>> I'm looking forward to seeing more feedback on this approach,
>>>>>> in terms of design and performance improvement.
>>>>>> So, I have submitted this for the next CF.
>>>>> Your patch fails to build:
>>>>>
>>>>> pgstattuple.c: In function ‘pgstat_heap_sample’:
>>>>> pgstattuple.c:737:13: error: ‘SnapshotNow’ undeclared (first use in
>>>>> this function)
>>>>> pgstattuple.c:737:13: note: each undeclared identifier is reported
>>>>> only once for each function it appears in
>>>> Thanks for checking. Fixed to eliminate SnapshotNow.
>>>>
>>> This seems like a cool idea! I took a quick look, and initally
>>> replicated the sort of improvement you saw:
>>>
>>>
>>> bench=# explain analyze select * from pgstattuple('pgbench_accounts');
>>> QUERY PLAN
>>>
>>> --------------------------------------------------------------------------------
>>> Function Scan on pgstattuple (cost=0.00..0.01 rows=1 width=72) (actual
>>> time=786.368..786.369 rows=1 loops=1)
>>> Total runtime: 786.384 ms
>>> (2 rows)
>>>
>>> bench=# explain analyze select * from pgstattuple2('pgbench_accounts');
>>> NOTICE: pgstattuple2: SE tuple_count 0.00, tuple_len 0.00,
>>> dead_tuple_count 0.00, dead_tuple_len 0.00, free_space 0.00
>>> QUERY PLAN
>>>
>>> --------------------------------------------------------------------------------
>>> Function Scan on pgstattuple2 (cost=0.00..0.01 rows=1 width=72) (actual
>>> time=12.004..12.005 rows=1 loops=1)
>>> Total runtime: 12.019 ms
>>> (2 rows)
>>>
>>>
>>>
>>> I wondered what sort of difference eliminating caching would make:
>>>
>>> $ sudo sysctl -w vm.drop_caches=3
>>>
>>> Repeating the above queries:
>>>
>>>
>>> bench=# explain analyze select * from pgstattuple('pgbench_accounts');
>>> QUERY PLAN
>>>
>>> --------------------------------------------------------------------------------
>>> Function Scan on pgstattuple (cost=0.00..0.01 rows=1 width=72) (actual
>>> time=9503.774..9503.776 rows=1 loops=1)
>>> Total runtime: 9504.523 ms
>>> (2 rows)
>>>
>>> bench=# explain analyze select * from pgstattuple2('pgbench_accounts');
>>> NOTICE: pgstattuple2: SE tuple_count 0.00, tuple_len 0.00,
>>> dead_tuple_count 0.00, dead_tuple_len 0.00, free_space 0.00
>>> QUERY PLAN
>>>
>>> --------------------------------------------------------------------------------
>>> Function Scan on pgstattuple2 (cost=0.00..0.01 rows=1 width=72) (actual
>>> time=12330.630..12330.631 rows=1 loops=1)
>>> Total runtime: 12331.353 ms
>>> (2 rows)
>>>
>>>
>>> So the sampling code seems *slower* when the cache is completely cold -
>>> is that expected? (I have not looked at how the code works yet - I'll
>>> dive in later if I get a chance)!
> Thanks for testing that. It would be very helpful to improve the
> performance.
>
>> Quietly replying to myself - looking at the code the sampler does 3000
>> random page reads... I guess this is slower than 163935 (number of pages
>> in pgbench_accounts) sequential page reads thanks to os readahead on my
>> type of disk (WD Velociraptor). Tweaking the number of random reads (i.e
>> the sample size) down helps - but obviously that can impact estimation
>> accuracy.
>>
>> Thinking about this a bit more, I guess the elapsed runtime is not the
>> *only* theng to consider - the sampling code will cause way less
>> disruption to the os page cache (3000 pages vs possibly lots more than
>> 3000 for reading an entire ralation).
>>
>> Thoughts?
> I think it could be improved by sorting sample block numbers
> *before* physical block reads in order to eliminate random access
> on the disk.
>
> pseudo code:
> --------------------------------------
> for (i=0 ; i<SAMPLE_SIZE ; i++)
> {
> sample_block[i] = random();
> }
>
> qsort(sample_block);
>
> for (i=0 ; i<SAMPLE_SIZE ; i++)
> {
> buf = ReadBuffer(rel, sample_block[i]);
>
> do_some_stats_stuff(buf);
> }
> --------------------------------------
>
> I guess it would be helpful for reducing random access thing.
>
> Any comments?

Ah yes - that's a good idea (rough patch to your patch attached)!

bench=# explain analyze select * from pgstattuple2('pgbench_accounts');
NOTICE: pgstattuple2: SE tuple_count 0.00, tuple_len 0.00,
dead_tuple_count 0.00, dead_tuple_len 0.00, free_space 0.00
QUERY PLAN

--------------------------------------------------------------------------------
Function Scan on pgstattuple2 (cost=0.00..0.01 rows=1 width=72) (actual
time=9968.318..9968.319 rows=1 loops=1)
Total runtime: 9968.443 ms
(2 rows)

It would appear that we are now not any worse than a complete sampling...

Cheers

Mark