On Tuesday, March 15, 2016 7:39 PM, "p...@cmicdo.com" <p...@cmicdo.com>
wrote:
> Your results are close enough to mine, I think, to prove the point.
> And, I agree that the EDB benchmark is not necessary reflective of a
> real-world scenario.
>
> However, the cache I'm referring to is PG's shared_buffer cache.
> You can see the first run of the select causing a lot of disk reads.
> The second identical run, reads purely from shared_buffers.
>
> What I don't understand is, why does a slightly different select from
> the *same* table during the same session cause shared_buffers to be
> blown out and re-read??
>
> I will see if I can try YCSB next week (I'm in workshops all week...)
>
> Thanks!
I was able to try YCSB today on both PG 9.5.1 and Mongo 3.2. At first, PG
was running 4 times slower than Mongo. Then I remembered about unlogged
tables (which I think is the way Mongo is all the time.), and remade
the PG table as UNLOGGED. In a 50/50 read/update test over 1M records,
PG ran in 0.62 of the time of Mongo.
PG Load:
--------
[OVERALL], RunTime(ms), 104507.0
[OVERALL], Throughput(ops/sec), 9568.737022400413
[CLEANUP], Operations, 1.0
[CLEANUP], AverageLatency(us), 293.0
[CLEANUP], MinLatency(us), 293.0
[CLEANUP], MaxLatency(us), 293.0
[CLEANUP], 95thPercentileLatency(us), 293.0
[CLEANUP], 99thPercentileLatency(us), 293.0
[INSERT], Operations, 1000000.0
[INSERT], AverageLatency(us), 101.329235
[INSERT], MinLatency(us), 88.0
[INSERT], MaxLatency(us), 252543.0
[INSERT], 95thPercentileLatency(us), 121.0
[INSERT], 99thPercentileLatency(us), 141.0
[INSERT], Return=OK, 1000000
PG Run:
-------
[OVERALL], RunTime(ms), 92763.0
[OVERALL], Throughput(ops/sec), 10780.16019318047
[READ], Operations, 499922.0
[READ], AverageLatency(us), 79.1722428698877
[READ], MinLatency(us), 69.0
[READ], MaxLatency(us), 19935.0
[READ], 95thPercentileLatency(us), 94.0
[READ], 99thPercentileLatency(us), 112.0
[READ], Return=OK, 499922
[CLEANUP], Operations, 1.0
[CLEANUP], AverageLatency(us), 222.0
[CLEANUP], MinLatency(us), 222.0
[CLEANUP], MaxLatency(us), 222.0
[CLEANUP], 95thPercentileLatency(us), 222.0
[CLEANUP], 99thPercentileLatency(us), 222.0
[UPDATE], Operations, 500078.0
[UPDATE], AverageLatency(us), 98.96430156895525
[UPDATE], MinLatency(us), 83.0
[UPDATE], MaxLatency(us), 26655.0
[UPDATE], 95thPercentileLatency(us), 127.0
[UPDATE], 99thPercentileLatency(us), 158.0
[UPDATE], Return=OK, 500078
Mongo Load:
-----------
[OVERALL], RunTime(ms), 133308.0
[OVERALL], Throughput(ops/sec), 7501.425270801452
[CLEANUP], Operations, 1.0
[CLEANUP], AverageLatency(us), 1822.0
[CLEANUP], MinLatency(us), 1822.0
[CLEANUP], MaxLatency(us), 1822.0
[CLEANUP], 95thPercentileLatency(us), 1822.0
[CLEANUP], 99thPercentileLatency(us), 1822.0
[INSERT], Operations, 1000000.0
[INSERT], AverageLatency(us), 130.830678
[INSERT], MinLatency(us), 90.0
[INSERT], MaxLatency(us), 7147519.0
[INSERT], 95thPercentileLatency(us), 159.0
[INSERT], 99thPercentileLatency(us), 226.0
[INSERT], Return=OK, 1000000
Mongo Run:
---------
[OVERALL], RunTime(ms), 149150.0
[OVERALL], Throughput(ops/sec), 6704.65973851827
[READ], Operations, 500837.0
[READ], AverageLatency(us), 98.13153980237084
[READ], MinLatency(us), 69.0
[READ], MaxLatency(us), 28271.0
[READ], 95thPercentileLatency(us), 166.0
[READ], 99thPercentileLatency(us), 186.0
[READ], Return=OK, 500837
[CLEANUP], Operations, 1.0
[CLEANUP], AverageLatency(us), 2387.0
[CLEANUP], MinLatency(us), 2386.0
[CLEANUP], MaxLatency(us), 2387.0
[CLEANUP], 95thPercentileLatency(us), 2387.0
[CLEANUP], 99thPercentileLatency(us), 2387.0
[UPDATE], Operations, 499163.0
[UPDATE], AverageLatency(us), 195.21505600375028
[UPDATE], MinLatency(us), 118.0
[UPDATE], MaxLatency(us), 4513791.0
[UPDATE], 95thPercentileLatency(us), 211.0
[UPDATE], 99thPercentileLatency(us), 252.0
[UPDATE], Return=OK, 499163
>
>
> On Monday, March 14, 2016 3:34 AM, Dmitry Dolgov <9erthali...@gmail.com>
wrote:
>
>
> Hi, Paul
>
> I agree with Oleg, EDB benchmarks are strange sometimes. I did the same
benchmarks several months ago. I never noticed the cache influence back then,
so I tried to reproduce your situation now (on a 5*10^6 records although). I
started to play with db cache (using `echo 3 > /proc/sys/vm/drop_cache`), and I
see difference in time execution for two subsequent queries, but `explain` info
are almost identical, e.g. `shared hit & read`:
>
> ....
