On Sat, Oct 25, 2014 at 12:38 PM, Andreas Karlsson <andreas@proxel.se> wrote:
Hi,
There was recently talk about if we should start using 128-bit integers (where available) to speed up the aggregate functions over integers which uses numeric for their internal state. So I hacked together a patch for this to see what the performance gain would be.
What the patch does is switching from using numerics in the aggregate state to int128 and then convert the type from the 128-bit integer in the final function.
The functions where we can make use of int128 states are:
The initial benchmark results look very promising. When summing 10 million int8 I get a speedup of ~2.5x and similarly for var_samp() on 10 million int4 I see a speed up of ~3.7x. To me this indicates that it is worth the extra code. What do you say? Is this worth implementing?
The current patch still requires work. I have not written the detection of int128 support yet, and the patch needs code cleanup (for example: I used an int16_ prefix on the added functions, suggestions for better names are welcome). I also need to decide on what estimate to use for the size of that state.
The patch should work and pass make check on platforms where __int128_t is supported.
The simple benchmarks:
CREATE TABLE test_int8 AS SELECT x::int8 FROM generate_series(1, 10000000) x;
Before:
# SELECT sum(x) FROM test_int8; sum ---------------- 50000005000000 (1 row)
Time: 2521.217 ms
After:
# SELECT sum(x) FROM test_int8; sum ---------------- 50000005000000 (1 row)
Time: 1022.811 ms
CREATE TABLE test_int4 AS SELECT x::int4 FROM generate_series(1, 10000000) x;
Before:
# SELECT var_samp(x) FROM test_int4; var_samp -------------------- 8333334166666.6667 (1 row)
Time: 3808.546 ms
After:
# SELECT var_samp(x) FROM test_int4; var_samp -------------------- 8333334166666.6667 (1 row)
