Re: Slow query due to slow I/O

Claudio Freire

December 12, 2013 1:35 PM

On Thu, Dec 12, 2013 at 6:16 PM, Bryce Covert <bryce@brycecovertoperations.com> wrote:

Thanks a lot for the help. I'm not familiar with explain buffers, but here's the results:

 Limit  (cost=0.00..648.71 rows=50 width=8) (actual time=653.681..52328.707 rows=50 loops=1)
   Buffers: shared hit=7875 read=9870
   ->  GroupAggregate  (cost=0.00..55672.36 rows=4291 width=8) (actual time=653.671..52328.480 rows=50 loops=1)

         Filter: ((sum(electricusage.usage) >= 3284::numeric) AND (sum(electricusage.usage) <= 3769::numeric))

         Buffers: shared hit=7875 read=9870
         ->  Nested Loop  (cost=0.00..55262.93 rows=34506 width=8) (actual time=432.129..52200.465 rows=30711 loops=1)
               Buffers: shared hit=7875 read=9870
               ->  Index Scan using premiseaccount_bucket_58c70392619aa36f on premiseaccount premiseaccount  (cost=0.00..15433.71 rows=4291 width=4) (actual time=338.160..10014.780 rows=3437 loops=1)
                     Index Cond: ((bucket)::text = '85349_single-family'::text)
                     Buffers: shared hit=744 read=2692
               ->  Index Scan using electricusage_premise_account_id on electricusage electricusage  (cost=0.00..9.17 rows=9 width=8) (actual time=11.430..12.235 rows=9 loops=3437)
                     Index Cond: (premise_account_id = premiseaccount.id)
                     Filter: (from_date >= '2012-11-20 00:00:00+00'::timestamp with time zone)
                     Buffers: shared hit=7131 read=7178
 Total runtime: 52329.028 ms
(15 rows)

and the iostat results...
Linux 3.11.6-x86_64-linode35 (preview-aps-new)     12/12/2013     _x86_64_    (8 CPU)

...

Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
xvda              0.10    47.10  152.40    7.80     2.20     0.21    30.92     1.36    8.50    8.03   17.53   6.14  98.33
xvdb              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

This means it's doing random I/O, and that your disk is a single 7200RPM drive (152 r/s being typical for that hardware).

You can improve this by either:

1 - Turning that random I/O pattern into sequential, or

2 - Getting better I/O.

I'll assume 2 isn't available to you on linode, so for 1, you could try lowering effective_cache_size substantially. It seems you're not getting nearly as much caching as you think (ie 2GB). However, I doubt there's a plan that can get you significantly better performance given your hardware.

You may shave a few seconds, though, if you increase work_mem. It seems it should have used a bitmap index scan for at least one of the index scans there, and a low work_mem could be what's limiting the planner's possibilities. What are your settings in that area?

Bryce Covert

December 12, 2013 1:16 PM

Hey Claudio,

Thanks a lot for the help. I'm not familiar with explain buffers, but here's the results:

 Limit  (cost=0.00..648.71 rows=50 width=8) (actual time=653.681..52328.707 rows=50 loops=1)
   Buffers: shared hit=7875 read=9870
   ->  GroupAggregate  (cost=0.00..55672.36 rows=4291 width=8) (actual time=653.671..52328.480 rows=50 loops=1)
         Filter: ((sum(electricusage.usage) >= 3284::numeric) AND (sum(electricusage.usage) <= 3769::numeric))
         Buffers: shared hit=7875 read=9870
         ->  Nested Loop  (cost=0.00..55262.93 rows=34506 width=8) (actual time=432.129..52200.465 rows=30711 loops=1)
               Buffers: shared hit=7875 read=9870
               ->  Index Scan using premiseaccount_bucket_58c70392619aa36f on premiseaccount premiseaccount  (cost=0.00..15433.71 rows=4291 width=4) (actual time=338.160..10014.780 rows=3437 loops=1)
                     Index Cond: ((bucket)::text = '85349_single-family'::text)
                     Buffers: shared hit=744 read=2692
               ->  Index Scan using electricusage_premise_account_id on electricusage electricusage  (cost=0.00..9.17 rows=9 width=8) (actual time=11.430..12.235 rows=9 loops=3437)
                     Index Cond: (premise_account_id = premiseaccount.id)
                     Filter: (from_date >= '2012-11-20 00:00:00+00'::timestamp with time zone)
                     Buffers: shared hit=7131 read=7178
 Total runtime: 52329.028 ms
(15 rows)

and the iostat results...
Linux 3.11.6-x86_64-linode35 (preview-aps-new)     12/12/2013     _x86_64_    (8 CPU)

Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
xvda              6.94    65.68   12.16    7.64     0.36     0.29    67.06     0.44   22.11    3.56   51.66   1.15   2.28
xvdb              0.00     0.00    0.00    0.00     0.00     0.00    10.61     0.00    1.82    1.82    0.00   1.82   0.00

Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
xvda              0.10    47.10  152.40    7.80     2.20     0.21    30.92     1.36    8.50    8.03   17.53   6.14  98.33
xvdb              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
xvda              0.20     2.30  212.10    0.70     3.22     0.01    31.09     1.47    6.88    6.90    2.86   4.63  98.62
xvdb              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
xvda              0.20     8.30  183.20    5.10     2.46     0.05    27.31     1.68    8.85    6.68   86.78   5.24  98.70
xvdb              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
xvda              0.10     0.00  165.70    0.00     2.36     0.00    29.20     1.46    8.86    8.86    0.00   5.95  98.63
xvdb              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

I'm not sure how to process this except that 2.5MB/s seems really slow, and it looks like it is using postgres' cache quite a bit.

Thanks,
Bryce

Claudio Freire

December 12, 2013 12:48 PM

Well, did you run benchmarks? How many IOPS do you get from the volumes?

Try running "iostat -x -m -d 10" while the slow query is running and pasting the results (or a relevant sample of them).

Also, do run "explain (analyze, buffers)" instead of plain "explain analyze".

Bryce Covert

December 12, 2013 12:20 PM

I don't have much info on disks, since this is a virtual server on linode. It is running ubuntu 12.04, 8cpus, 4GB ram, 95GB ext3 volume (noatime). Hopefully that's helpful.

Bryce

Claudio Freire

December 12, 2013 12:15 PM

On Thu, Dec 12, 2013 at 5:10 PM, Bryce Covert

For this kind of diagnostic, you need to include hardware details.

OS? Disks? RAID?