Re: Looked at TODO:Considering improving performance of computing CHAR() value lengths - Mailing list pgsql-hackers

On 08/02/2014 09:43 PM, John Cochran wrote:
> I took at look at the TODO list and got interested in the possible
> optimization of the bcTruelen() function. Read the archived messages about
> that subject and decided to see what could be done.
>
> I tested the performance of 5 different versions of bcTruelen().
> 1. The code as it exists in postgresql currently.
> 2. The code with the patch described in the above mentioned messages.
> 3. A modification of the code mentioned in 2 above using the concept of a
> sentinel.
> 4. A modification of the code mentioned in 1 above using a sentinel.
> 5. A modification of the code mentioned in 4 using knowledge of 1B and 4B
> headers.
>
> First, let me describe a sentinel. The key thing about using a sentinel is
> that with on, you can guarantee loop termination without having to check
> for an index. What you do is place a value that you're searching for 1 past
> the limit of the range you're searching for. By doing this, you can
> optimize your loop to have just 1 condition to check for instead of 2
> (search value and index limit). This mean that your loop now needs to just
> execute 3 opcodes (pointer decrement, search compare, conditional jump)
> instead of 5 (pointer decrement, search compare, conditional jump, limit
> compare, conditional jump). In the case of bcTruelen(), the sentinel value
> is anything that's non-space.
>
> I ran all 5 different versions of bcTruelen() in a benchmark program that
> simply allocated a gig of memory, populated the memory with varlena
> structures of varying length, alignment, and trailing spaces. Then called
> the different versions of bcTruelen() using a pseudo random sequence to try
> and prevent processor caching from affecting the values.
>
> Overall, the results were as follows:

Can we see the benchmark program and the actual data, please?

> 1. For small numbers of trailing space, the "4 at a time" approach was a
> consistent loser in terms of performance. If there's a large number of
> trailing spaces, the performance of the "4 at a time" approach could be
> quite impressive however.
> 2. The best typical performer was the single byte at a time approach using
> a sentinel and knowledge of the types of headers. For that routine, the
> performance compared against the current version of bcTruelen() was as
> follows
> CHAR(1) with only spaces. Current version wins. New version loses by about
> 20%
> CHAR(1) with non-space. New version wins by about 10%.
> CHAR(2) with only spaces. New version loses by about 4%.
> CHAR(2) with one space. New version wins by about 15%.
> CHAR(2) with no spaces. New version wins by about 10%
>
> Once CHAR(4) was reached, the new version was faster for all numbers of
> trailing spaces. I suspect the reason for the new routine being slower for
> short CHAR types with all spaces is because using the sentinel results in
> an extra loop iteration over the code that checks for limits each
> iteration. But once the number of iterations gets large enough, the shorter
> loop wins even though it iterated one extra time. The tests were performed
> on an Intel Core2 duo.
>
> The code effectively has 3 identical loops with different surroundings.
> 1. If the arg is pointing to a 1B style header, then there is no need to
> worry about a sentinel. No 1B header matches a space character whether the
> target system is big or little endian.
> 2. For those cases where the arg is pointing to 4B style header, you do
> have to worry about a good sentinel existing. For a little endian machine,
> such an issue happens if you're dealing with a CHAR(134217728) through
> CHAR(138412031). Unlikely to be sure. But for a big endian machine,
> CHAR(32), CHAR(288) and other values can cause problems. So for 4B style
> headers, a check is made to see if the sentinel location has a value of '
> '. If it does, it is replaced with a zero, the scan is then performed, and
> then the sentinel is then restored with the original space. It is of course
> possible to unconditionally save the value at the sentinel location, stuff
> in a zero, then restore the original value, but in doing so, more overhead
> in incurred every call, slowing down the function.

You can't modify *arg, even momentarily, because it might point directly 
to an on-disk buffer. The same Datum can be read by another process at 
the same time, or even written out to disk.

You could simply check if the byte in the header happens to be != 0x20. 
It's very likely that it is, so that you can use the optimized version.


The biggest issue with the previously discussed patches was the lack of 
performance testing. People posted results of various micro-benchmarks 
that showed different aspects, but no-one comprehensively covered all 
the interesting cases. I'd like to see a simple suite of performance 
tests that show:

1. The worst case performance.
2. Performance with a couple of typical cases. CHAR(n), where n is 
between 1-64 would be interesting, and where the number of spaces at the 
end varies.

You said you did benchmarking with a custom program, which is good, but 
it would be even better to write the benchmark as a self-contained SQL 
or bash script or similar. Then post it, so that others can easily 
repeat the same benchmarks on different platforms, and try out different 
versions of the patch.
- Heikki