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Re: Possible Bug: high CPU usage for stats collector in 8.2

From: Darcy Buskermolen <darcyb(at)commandprompt(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Possible Bug: high CPU usage for stats collector in 8.2
Date: 2007-03-01 18:07:27
Message-ID: 200703011007.27717.darcyb@commandprompt.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wednesday 28 February 2007 14:31, Joshua D. Drake wrote:
> Joshua D. Drake wrote:
> > Darcy Buskermolen wrote:
> >> I'm observing high CPU usage (95%) of a 2.6GHz opteron by the stats
> >> collector on an 8.2.3 box  investigation has lead me to belive that the
> >> stats file is written a lot more often that once every 500ms  the
> >> following shows this behavior.

And just so that noone thinks it's linux specific, I tried the same test on a 
FreeBSD box with 8.2 on it and got the following.  Ignore the time portion of 
the results, but what is important is the number of calls occuring in the 
timeframe (given this test took about 38:30, it should have resulted in about 
4620 calls to rename)

% time     seconds                      usecs/call      calls      err syscall
------ ----------- ----------- --------- --------- ----------------
149.23    1170797349.233781   464000015      2519188 2            write
 57.71     452768432.575458      2036010019   71515       20238  poll
 17.59     138017435.049569      -1944030463  20766                    getppid
-136.71  -1072628999.112811   -1254412922  20317                    rename
-136.71  -1072629000.340448   -1254412960  20317                    close
-136.71  -1072629001.605166   -1254412996  20317                    setitimer
-146.81  -1151850347.801977   -861500230    20316                     open
-146.81  -1151850349.967150   -861500320    20316                     fstat
-226.75  -1779034289.1666093 -639263959    50831                     recvfrom
------ ----------- ----------- --------- --------- ----------------

real    38m29.897s
user    0m40.641s
sys     0m47.487s

....


>
> I have just done a test separate from darcy on my workstation:
>
>
> jd(at)scratch:~/82$ strace -c -p 16130
> Process 16130 attached - interrupt to quit
> Process 16130 detached
> % time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
>  50.00    0.167103           3     60264     10225 poll
>  28.31    0.094600           9     10398           rename
>  16.89    0.056457           5     10398           open
>   2.48    0.008278           0     50023           recvfrom
>   1.14    0.003804           0     10398           setitimer
>   0.53    0.001781           0     20796           write
>   0.43    0.001432           0     10398           close
>   0.21    0.000690           0     10398           munmap
>   0.02    0.000057           0     10398           mmap
>   0.00    0.000000           0     10398           fstat
>   0.00    0.000000           0     10398     10225 rt_sigreturn
>   0.00    0.000000           0     10414           getppid
> ------ ----------- ----------- --------- --------- ----------------
> 100.00    0.334202                224681     20450 total
>
>
> Query:
>
> time for i in `bin/psql -p8000 -d postgres -c "select
> generate_series(1,10000)"`; do bin/psql -p8000 -d postgres -qc "select 1
> from pg_database where datname = 'postgres'"; done;
>
> Time:
>
> real    2m5.077s
> user    0m28.414s
> sys     0m39.762s
>
>
> PostgreSQL 8.2.0, Ubuntu Edgy 64bit.
>
> Seems like something is extremely wonky here.
>
> Joshua D. Drake
>
> >> PostgreSQL 8.2.3 on x86_64-redhat-linux-gnu, compiled by GCC gcc (GCC)
> >> 3.4.6 20060404 (Red Hat 3.4.6-3)
> >>
> >> I ran a
> >> time for i in `psql -qtc "select generate_series(1,10000)"`; do psql 
> >> -qtc "select 1 from test where msg_id=$i" ; done
> >>
> >> which took
> >> real    1m23.288s
> >> user    0m24.142s
> >> sys     0m21.536s
> >>
> >>
> >> to execute, during which time I ran a strace on the stats collector
> >> which produces the following output.  From this it looks like the stats
> >> file is getting rewritten for each connection teardown, not just every
> >> 500ms.
> >>
> >> Process 10061 attached - interrupt to quit
> >> Process 10061 detached
> >> % time     seconds  usecs/call     calls    errors syscall
> >> ------ ----------- ----------- --------- --------- ----------------
> >>  68.14   28.811963          17   1663827           write
> >>  18.22    7.701885         123     62808     12793 poll
> >>  11.31    4.783082         365     13101           rename
> >>   0.58    0.246169           5     50006           recvfrom
> >>   0.57    0.241073          18     13101           open
> >>   0.43    0.182816          14     13101           munmap
> >>   0.18    0.076176           6     13101           mmap
> >>   0.17    0.072746           6     13101           close
> >>   0.14    0.060483           5     13101           setitimer
> >>   0.10    0.041344           3     13101     12793 rt_sigreturn
> >>   0.09    0.039240           3     13101           fstat
> >>   0.06    0.024041           2     13110           getppid
> >> ------ ----------- ----------- --------- --------- ----------------
> >> 100.00   42.281018               1894559     25586 total
> >>
> >> As you can see rename was called more than the theroitcal 167  times for
> >> 500ms slices that elapsed during the test
> >>
> >> Compared to  PostgreSQL 8.1.8 on x86_64-pc-linux-gnu, compiled by GCC
> >> gcc-4.0.gcc-opt (GCC) 4.0.3 (Ubuntu 4.0.3-1ubuntu5)
> >> [All be it this is slower hardware..]
> >>
> >> time for i in `psql -qtc "select generate_series(1,10000)"`; do psql 
> >> -qtc "select 1 from test where msg_id=$i" ; done
> >>
> >> which took
> >> real    9m25.380s
> >> user    6m51.254s
> >> sys     1m47.687s
> >> (and therefor should be about 1130 stat write cycles)
> >>
> >> and yielded the following strace
> >>
> >> % time     seconds  usecs/call     calls    errors syscall
> >> ------ ----------- ----------- --------- --------- ----------------
> >>  93.64   20.422006         334     61212           select
> >>   3.49    0.760963           7    110192           read
> >>   1.82    0.396654          19     21128           write
> >>   0.64    0.139679         126      1112           rename
> >>   0.27    0.057970          52      1112           open
> >>   0.06    0.012177          11      1112           munmap
> >>   0.04    0.008901           8      1112           mmap
> >>   0.03    0.006402           6      1112           close
> >>   0.02    0.004282           4      1112           fstat
> >> ------ ----------- ----------- --------- --------- ----------------
> >> 100.00   21.809034                199204           total
> >>
> >>
> >>
> >> During this run the stats collector does not even show and CPU usage
> >> according to top.
> >>
> >>
> >> both 8.1 and 8.2 have the following postgresql.conf parameters
> >>
> >> stats_command_string =  off
> >> stats_start_collector = on
> >> stats_block_level = on
> >> stats_row_level = on

-- 


Darcy Buskermolen
The PostgreSQL company, Command Prompt Inc.
http://www.commandprompt.com/

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