Re: Wait free LW_SHARED acquisition - v0.9

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
- 48.61% postgres postgres [.] s_lock
- s_lock
+ 96.67% StrategyGetBuffer
+ 1.19% UnpinBuffer
+ 0.90% PinBuffer
+ 0.70% hash_search_with_hash_value
+ 3.11% postgres postgres [.] GetSnapshotData
+ 2.47% postgres postgres [.] StrategyGetBuffer
+ 1.93% postgres [kernel.kallsyms] [k] copy_user_generic_string
+ 1.28% postgres postgres [.] hash_search_with_hash_value
- 1.27% postgres postgres [.] LWLockAttemptLock
- LWLockAttemptLock
- 97.78% LWLockAcquire
+ 38.76% ReadBuffer_common
+ 28.62% _bt_getbuf
+ 8.59% _bt_relandgetbuf
+ 6.25% GetSnapshotData
+ 5.93% VirtualXactLockTableInsert
+ 3.95% VirtualXactLockTableCleanup
+ 2.35% index_fetch_heap
+ 1.66% StartBufferIO
+ 1.56% LockReleaseAll
+ 1.55% _bt_next
+ 0.78% LockAcquireExtended
+ 1.47% _bt_next
+ 0.75% _bt_relandgetbuf

to me. Now that's with the client count 496, but it's similar with lower
counts.

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

> Patch_ver/Client_count 175 HEAD 248374 PATCH 235669
> > > Now probably these shouldn't matter much in case backend needs to
> > > wait for other Exclusive locker, but I am not sure what else could be
> > > the reason for dip in case we need to have Exclusive LWLocks.
> >
> > Any chance to get a profile?
>
> 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?

Greetings,

Andres Freund

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