On 09/24/2016 06:06 AM, Amit Kapila wrote:
On Fri, Sep 23, 2016 at 8:22 PM, Tomas Vondra
<tomas.von...@2ndquadrant.com> wrote:
So I'm using 16GB shared buffers (so with scale 300 everything fits into
shared buffers), min_wal_size=16GB, max_wal_size=128GB, checkpoint timeout
1h etc. So no, there are no checkpoints during the 5-minute runs, only those
triggered explicitly before each run.

Thanks for clarification.  Do you think we should try some different
settings *_flush_after parameters as those can help in reducing spikes
in writes?

I don't see why that settings would matter. The tests are on unlogged tables, so there's almost no WAL traffic and checkpoints (triggered explicitly before each run) look like this:

checkpoint complete: wrote 17 buffers (0.0%); 0 transaction log file(s) added, 0 removed, 13 recycled; write=0.062 s, sync=0.006 s, total=0.092 s; sync files=10, longest=0.004 s, average=0.000 s; distance=309223 kB, estimate=363742 kB

So I don't see how tuning the flushing would change anything, as we're not doing any writes.

Moreover, the machine has a bunch of SSD drives (16 or 24, I don't remember at the moment), behind a RAID controller with 2GB of write cache on it.

Also, I think instead of 5 mins, read-write runs should be run for 15
mins to get consistent data.

Where does the inconsistency come from?

Thats what I am also curious to know.

Lack of warmup?

Can't say, but at least we should try to rule out the possibilities.
I think one way to rule out is to do slightly longer runs for
Dilip's test cases and for pgbench we might need to drop and
re-create database after each reading.

My point is that it's unlikely to be due to insufficient warmup, because the inconsistencies appear randomly - generally you get a bunch of slow runs, one significantly faster one, then slow ones again.

I believe the runs to be sufficiently long. I don't see why recreating the database would be useful - the whole point is to get the database and shared buffers into a stable state, and then do measurements on it.

I don't think bloat is a major factor here - I'm collecting some additional statistics during this run, including pg_database_size, and I can see the size oscillates between 4.8GB and 5.4GB. That's pretty negligible, I believe.

I'll let the current set of benchmarks complete - it's running on 4.5.5 now, I'll do tests on 3.2.80 too.

Then we can re-evaluate if longer runs are needed.

Considering how uniform the results from the 10 runs are (at least
on 4.5.5), I claim  this is not an issue.

It is quite possible that it is some kernel regression which might
be fixed in later version. Like we are doing most tests in cthulhu
which has 3.10 version of kernel and we generally get consistent
results. I am not sure if later version of kernel say 4.5.5 is a net
win, because there is a considerable difference (dip) of performance
in that version, though it produces quite stable results.

Well, the thing is - the 4.5.5 behavior is much nicer in general. I'll always prefer lower but more consistent performance (in most cases). In any case, we're stuck with whatever kernel version the people are using, and they're likely to use the newer ones.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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