Re: Wait free LW_SHARED acquisition - v0.9

On Fri, Oct 10, 2014 at 8:11 PM, Andres Freund <andres(at)2ndquadrant(dot)com>
wrote:
> On 2014-10-10 17:18:46 +0530, Amit Kapila wrote:
> > On Fri, Oct 10, 2014 at 1:27 PM, Andres Freund <andres(at)2ndquadrant(dot)com>
> > wrote:
> > > > Observations
> > > > ----------------------
> > > > a. The patch performs really well (increase upto ~40%) incase all
the
> > > > data fits in shared buffers (scale factor -100).
> > > > b. Incase data doesn't fit in shared buffers, but fits in RAM
> > > > (scale factor -3000), there is performance increase upto 16 client
> > count,
> > > > however after that it starts dipping (in above config unto ~4.4%).
> > >
> > > Hm. Interesting. I don't see that dip on x86.
> >
> > Is it possible that implementation of some atomic operation is costlier
> > for particular architecture?
>
> Yes, sure. And IIRC POWER improved atomics performance considerably for
> POWER8...
>
> > I have tried again for scale factor 3000 and could see the dip and this
> > time I have even tried with 175 client count and the dip is
approximately
> > 5% which is slightly more than 160 client count.
>
> FWIW, the profile always looks like:

For my tests on Power8, the profile looks somewhat similar to below
profile mentioned by you, please see this mail:
http://www.postgresql.org/message-id/CAA4eK1Je9ZBLHsfiavHD18GDwXUx21zFqPJgq_Dz_ZoA35nLpQ@mail.gmail.com

However on Power7, the profile looks different which I have
posted above thread.

>
> BTW, that profile *clearly* indicates we should make StrategyGetBuffer()
> smarter.

Yeah, even bgreclaimer patch is able to achieve the same, however
after that the contention moves to somewhere else as you can see
in above link.

> >
> > Here it goes..
> >
> > Lwlock_contention patches - client_count=128
> > ----------------------------------------------------------------------
> >
> > + 7.95% postgres postgres [.] GetSnapshotData
> > + 3.58% postgres postgres [.] AllocSetAlloc
> > + 2.51% postgres postgres [.] _bt_compare
> > + 2.44% postgres postgres [.]
> > hash_search_with_hash_value
> > + 2.33% postgres [kernel.kallsyms] [k] .__copy_tofrom_user
> > + 2.24% postgres postgres [.] AllocSetFreeIndex
> > + 1.75% postgres postgres [.]
> > pg_atomic_fetch_add_u32_impl
>
> Uh. Huh? Normally that'll be inline. That's compiled with gcc? What were
> the compiler settings you used?

Nothing specific, for performance tests where I have to take profiles
I use below:
./configure --prefix=<installation_path> CFLAGS="-fno-omit-frame-pointer"
make

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