Re: Compressed TOAST Slicing

Hi!

> 12 марта 2019 г., в 10:22, Andrey Borodin <x4mmm@yandex-team.ru> написал(а):
> 
> 3. And I'd use memmove despite the comment why we do not do that. It is SSE-optimized and cache-optimized nowadays.


So, I've pushed idea a little bit and showed that decompress byte-copy cycle to Vladimir Leskov.
while (len--)
{
    *dp = dp[-off];
    dp++;
}

He advised me to use algorithm that splits copied regions into smaller non-overlapping subregions with exponentially increasing size.

while (off <= len)
{
    memcpy(dp, dp - off, off);
    len -= off;
    dp += off;
    off *= 2;
}
memcpy(dp, dp - off, len);

On original Paul's test without patch of this thread this optimization gave about x2.5 speedup.
I've composed more detailed tests[0] and tested against current master. Now it only gives 20%-25% of decompression speedup, but I think it is still useful.

Best regards, Andrey Borodin.

[0] Here's the test

create table if not exists slicingtest1 as select repeat('0', 10000) as a from generate_series(1,10000);
create table if not exists slicingtest2 as select repeat('01', 10000) as a from generate_series(1,10000);
create table if not exists slicingtest3 as select repeat('012', 10000) as a from generate_series(1,10000);
create table if not exists slicingtest4 as select repeat('0123', 10000) as a from generate_series(1,10000);
create table if not exists slicingtest5 as select repeat('01234', 10000) as a from generate_series(1,10000);
create table if not exists slicingtest6 as select repeat('012345', 10000) as a from generate_series(1,10000);
create table if not exists slicingtest7 as select repeat('0123456', 10000) as a from generate_series(1,10000);
create table if not exists slicingtest8 as select repeat('01234567', 10000) as a from generate_series(1,10000);
create table if not exists slicingtest16 as select repeat('0123456789ABCDEF', 10000) as a from generate_series(1,10000);
create table if not exists slicingtest32 as select repeat('0x1x2x3x4x5x6x7x8x9xAxBxCxDxExFx', 10000) as a from generate_series(1,10000);
create table if not exists slicingtest64 as select repeat('0xyz1xyz2xyz3xyz4xyz5xyz6xyz7xyz8xyz9xyzAxyzBxyzCxyzDxyzExyzFxyz', 10000) as a from generate_series(1,10000);

\timing off
select sum(length(a)) from slicingtest1; -- do for every stride lenght
\timing on
select sum(length(a)) from slicingtest1;