Re: Clarification on some settings

(Sorry if this ends up being a duplicate post, I sent a reply yesterday,
but it doesn't appear to have gone through... I think I typo'd the address
but never got a bounce.)

Hi,
   Thanks for your initial help. I have some more questions below.

At 05:02 AM 5/12/2004, Shridhar Daithankar wrote:
>Doug Y wrote:
>
>>Hello,
>>   I've been having some performance issues with a DB I use. I'm trying
>> to come up with some performance recommendations to send to the "adminstrator".
>>
>>Ok for what I'm uncertain of...
>>shared_buffers:
>>According to http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html
>>Its more of a staging area and more isn't necessarily better. That psql
>>relies on the OS to cache data for later use.
>>But according to
>>http://www.ca.postgresql.org/docs/momjian/hw_performance/node3.html its
>>where psql caches previous data for queries because the OS cache is
>>slower, and should be as big as possible without causing swap.
>>Those seem to be conflicting statements. In our case, the "administrator"
>>kept increasing this until performance seemed to increase, which means
>>its now 250000 (x 8k is 2G).
>>Is this just a staging area for data waiting to move to the OS cache, or
>>is this really the area that psql caches its data?
>
>It is the area where postgresql works. It updates data in this area and
>pushes it to OS cache for disk writes later.
>
>By experience, larger does not mean better for this parameter. For
>multi-Gig RAM machines, the best(on an average for wide variety of load)
>value found to be around 10000-15000. May be even lower.
>
>It is a well known fact that raising this parameter unnecessarily
>decreases the performance. You indicate that best performance occurred at
>250000. This is very very large compared to other people's experience.

Ok. I think I understand a bit better now.

>>effective_cache_size:
>>Again, according to the Varlena guide this tells psql how much system
>>memory is available for it to do its work in.
>>until recently, this was set at the default value of 1000. It was just
>>recently increased to 180000 (1.5G)
>>according to
>>http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html
>>it should be about 25% of memory?
>
>No rule of thumb. It is amount of memory OS will dedicate to psotgresql
>data buffers. Depending uponn what else you run on machine, it could be
>straight-forward or noodly value to calculate. For a 4GB machine, 1.5GB is
>quite good but coupled with 2G of shared buffers it could push the
>machines to swap storm. And swapping shared buffers is a big performance hit.

We don't seem to be swapping much:

# top

   2:21pm  up 236 days, 19:12,  1 user,  load average: 1.45, 1.09, 1.00
53 processes: 51 sleeping, 2 running, 0 zombie, 0 stopped
CPU0 states: 30.3% user,  9.1% system,  0.0% nice, 60.0% idle
CPU1 states: 32.0% user,  9.3% system,  0.0% nice, 58.1% idle
Mem:  3863468K av, 3845844K used,   17624K free, 2035472K shrd,  198340K buff
Swap: 1052248K av,    1092K used, 1051156K free                 1465112K cached

looks like at some point it did swap a little, but from running vmstat, I
can't seem to catch it actively swapping.

>>Finally sort_mem:
>>Was until recently left at the default of 1000. Is now 16000.
>
>Sort memory is per sort not per query or per connection. So depending upon
>how many concurrent connections you entertain, it could take quite a chuck
>of RAM.

Right I understand that. How does one calculate the size of a sort? Rows *
width from an explain?

>>Increasing the effective cache and sort mem didn't seem to make much of a
>>difference. I'm guessing the eff cache was probably raised a bit too
>>much, and shared_buffers is way to high.
>
>I agree. For shared buffers start with 5000 and increase in batches on
>1000. Or set it to a high value and check with ipcs for maximum shared
>memory usage. If share memory usage peaks at 100MB, you don't need more
>than say 120MB of buffers.

My results from ipcs seems confusing... says its using the full 2G of
shared cache:

# ipcs

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x0052e2c1 6389760    postgres  600        2088370176 4

------ Semaphore Arrays --------
key        semid      owner      perms      nsems      status
0x0052e2c1 424378368  postgres  600        17
0x0052e2c2 424411137  postgres  600        17
0x0052e2c3 424443906  postgres  600        17
0x0052e2c4 424476675  postgres  600        17
0x0052e2c5 424509444  postgres  600        17
0x0052e2c6 424542213  postgres  600        17
0x0052e2c7 424574982  postgres  600        17
0x0052e2c8 424607751  postgres  600        17
0x0052e2c9 424640520  postgres  600        17
0x0052e2ca 424673289  postgres  600        17
0x0052e2cb 424706058  postgres  600        17
0x0052e2cc 424738827  postgres  600        17
0x0052e2cd 424771596  postgres  600        17
0x0052e2ce 424804365  postgres  600        17
0x0052e2cf 424837134  postgres  600        17
0x0052e2d0 424869903  postgres  600        17
0x0052e2d1 424902672  postgres  600        17
0x00018d45 505544721  root      777        1

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages


>>What can I do to help determine what the proper settings should be and/or
>>look at other possible choke points. What should I look for in iostat,
>>mpstat, or vmstat as red flags that cpu, memory, or i/o bound?
>
>Yes. vmstat is usually a lot of help to locate the bottelneck.

What would I be looking for here?

# vmstat 2 10
    procs                      memory    swap          io     system
  cpu
  r  b  w   swpd   free   buff  cache  si  so    bi    bo   in    cs  us
sy  id
  0  0  0   1092  14780 198120 1467164   0   0     0    0   0     0   0  0   0
  0  0  0   1092  19488 198120
1467204   0   0     0    0  240   564  11   5  84
  0  0  0   1092  19520 198120
1467300   0   0     0   210  443  1094  29   8  63
  0  0  0   1092  15832 198120
1467356   0   0     4   110  368  1455  27   5  68
  3  0  0   1092  10956 198120
1467464   0   0     4   336  417  1679  33  10  57
  1  0  0   1092  17840 198124
1465980   0   0  200   334  581  1914  63  14  23
  1  0  0   1092  16556 198124
1466012   0   0     0   226  397  1069  30   4  66
  0  0  0   1092  19096 198124
1466028   0   0     0   160  230   314  12   2  86
  2  0  1   1092  16100 198128
1466748   0   0    28  1484  711  1578  23  12  65
  0  0  0   1092  20140 198128
1466780   0   0     0   414  291   746  15   8  77

I'm guessing what I should look at is the io: bi & bo ? when I run some
particularly large queries I see bo activity so I'm speculating that that
means its reading pages from disk, correct?

>>DB maintenance wise, I don't believe they were running vacuum full until
>>I told them a few months ago that regular vacuum analyze no longer cleans
>>out dead tuples. Now normal vac is run daily, vac full weekly
>>(supposedly). How can I tell from the output of vacuum if the vac fulls
>>aren't being done, or not done often enough? Or from the system tables,
>>what can I read?
>
>In 7.4 you can do vacuum full verbose and it will tell you the stats at
>the end. For 7.3.x, its not there.
>
>I suggest you vacuum full database once.(For large database, dumping
>restoring might work faster. Dump/restore and vacuum full both lock the
>database exclusively i.e. downtime. So I guess faster the better for you.
>But there is no tool/guideline to determine which way to go.)

Ok they had not done a full vacuum in a long time. I them run vacuumdb
--full --analyze --verbose and dump it into a file. What should I look for
to see if it was useful?

for example:
INFO:  Pages 118200: Changed 74, reaped 117525, Empty 0, New 0; Tup 575298:
Vac 11006, Keep/VTL 0/0, UnUsed 2454159, MinLen 68, MaxLen 1911; Re-using:
Free/Avai
l. Space 774122944/774122944; EndEmpty/Avail. Pages 0/118200.
         CPU 9.41s/1.33u sec elapsed 97.35 sec.

Is there any documentation on what those numbers represent?

Also do we need to use REINDEX on the indexes, or does vacuum full take
case of that?


>>Is there anywhere else I can look for possible clues? I have access to
>>the DB super-user, but not the system root/user.
>
>Other than hardware tuning, find out slow/frequent queries. Use explain
>analyze to determine why they are so slow. Forgetting to typecast a where
>clause and using sequential scan could cost you lot more than mistuned
>postgresql configuration.

Right. One example I can think of is one particular query takes about 120
seconds to run (explain analyze), but if I set enable_seqscan to off, it
takes about 10 seconds.

>>Thank you for your time. Please let me know any help or suggestions you
>>may have. Unfortunately upgrading postgres, OS, kernel, or re-writing
>>schema is most likely not an option.
>
>I hope you can change your queries.

For the most part we're not having too much trouble, just some newer
queries were building for some new features is what we're seeing trouble with.


>HTH
>
>  Shridhar