Re: [HACKERS] Parallel Select query performance and shared buffers

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On 2013-12-04 14:27:10 -0200, Claudio Freire wrote:
> On Wed, Dec 4, 2013 at 9:19 AM, Metin Doslu <metin@xxxxxxxxxxxxx> wrote:
> >
> > Here are the results of "vmstat 1" while running 8 parallel TPC-H Simple
> > (#6) queries:  Although there is no need for I/O, "wa" fluctuates between 0
> > and 1.
> >
> > procs -----------memory---------- ---swap-- -----io---- --system--
> > -----cpu-----
> >  r  b   swpd   free   buff    cache     si   so    bi    bo    in     cs us sy  id wa st
> >  0  0      0 30093568  84892 38723896    0    0     0     0    22     14  0  0 100  0  0
> >  8  1      0 30043056  84892 38723896    0    0     0     0 27080  52708 16 14  70  0  0
> >  8  1      0 30006600  84892 38723896    0    0     0     0 44952 118286 43 44  12  1  0
> >  8  0      0 29986264  84900 38723896    0    0     0    20 28043  95934 49 42   8  1  0
> >  7  0      0 29991976  84900 38723896    0    0     0     0  8308  73641 52 42   6  0  0
> >  0  0      0 30091828  84900 38723896    0    0     0     0  3996  30978 23 24  53  0  0
> >  0  0      0 30091968  84900 38723896    0    0     0     0    17    23   0  0 100  0  0
> 
> 
> Notice the huge %sy

My bet is on transparent hugepage defragmentation. Alternatively it's
scheduler overhead, due to superflous context switches around the buffer
mapping locks.

I'd strongly suggest doing a "perf record -g -a <wait a bit, ctrl-c>;
perf report" run to check what's eating up the time.

Greetings,

Andres Freund

-- 
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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