Re: BUG #14159: PostgreSQL 9.6 parallel scan consume very high mutex lock

On Fri, May 27, 2016 at 5:51 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> On 2016-05-27 05:43:00 +0530, Amit Kapila wrote:
> > On Thu, May 26, 2016 at 11:03 AM, 德哥 <digoal(at)126(dot)com> wrote:
> >
> > > This is worker process's stack, when i test the hign parallel degree.
> > >
> > > [<ffffffffa00b8ff0>] ext4_llseek+0x60/0x110 [ext4]
> > > [<ffffffff81186eda>] vfs_llseek+0x3a/0x40
> > > [<ffffffff81188b96>] sys_lseek+0x66/0x80
> > > [<ffffffff8100c072>] system_call_fastpath+0x16/0x1b
> > > [<ffffffffffffffff>] 0xffffffffffffffff
> > >
> > >
> >
> > Above call stack indicates that the file seek cost has increased on
> > employing high number of workers. I think the reason is that employing
> > more number of workers to perform parallel scan of same file doesn't
work
> > very well read-ahead mechanism of OS.
>
> I don't think that's the correct conclusion. That report and profiles
> rather seems to suggest we're hitting lock contention, rather than IO
> related cost.
>
> The kernel used here is quite old (heavily patched 2.6.32 IIRC). The
> kernel guys have since made lseek not take locks in the common case. I
> suspect that upgrading to a newer kernel will change the profile
> significantly.
>

Worth trying, however I don't think we can discount the fact that using
such large number of workers can saturate I/O channel.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com