Обсуждение: Thoughts on NBASE=100000000
Hello hackers,
I'm not hopeful this idea will be fruitful, but maybe we can find solutions
to the problems together.
The idea is to increase the numeric NBASE from 1e4 to 1e8, which could possibly
give a significant performance boost of all operations across the board,
on 64-bit architectures, for many inputs.
Last time numeric's base was changed was back in 2003, when d72f6c75038 changed
it from 10 to 10000. Back then, 32-bit architectures were still dominant,
so base-10000 was clearly the best choice at this time.
Today, since 64-bit architectures are dominant, NBASE=1e8 seems like it would
have been the best choice, since the square of that still fits in
a 64-bit signed int.
Changing NBASE might seem impossible at first, due to the existing numeric data
on disk, and incompatibility issues when numeric data is transferred on the
wire.
Here are some ideas on how to work around some of these:
- Incrementally changing the data on disk, e.g. upon UPDATE/INSERT
and supporting both NBASE=1e4 (16-bit) and NBASE=1e8 (32-bit)
when reading data.
- Due to the lack of a version field in the NumericVar struct,
we need a way to detect if a Numeric value on disk uses
the existing NBASE=1e4, or NBASE=1e8.
One hack I've thought about is to exploit the fact that NUMERIC_NBYTES,
defined as:
#define NUMERIC_NBYTES(num) (VARSIZE(num) - NUMERIC_HEADER_SIZE(num))
will always be divisible by two, since a NumericDigit is an int16 (2 bytes).
The idea is then to let "NUMERIC_NBYTES divisible by three"
indicate NBASE=1e8, at the cost of one to three extra padding bytes.
Another important aspect is disk space utilization, which is of course better
for NBASE=1e4, since it packs the data more tightly.
I think this is the main disadvantage of NBASE=1e8, but perhaps users would be
willing to sacrifice some disk, if they would get better run-time performance.
As said initially, this might be completely unrealistic,
but interested to hear if anyone else have had similar dreams.
Regards,
Joel
On Sun, 7 Jul 2024, 22:40 Joel Jacobson, <joel@compiler.org> wrote: > > Hello hackers, > > I'm not hopeful this idea will be fruitful, but maybe we can find solutions > to the problems together. > > The idea is to increase the numeric NBASE from 1e4 to 1e8, which could possibly > give a significant performance boost of all operations across the board, > on 64-bit architectures, for many inputs. > > Last time numeric's base was changed was back in 2003, when d72f6c75038 changed > it from 10 to 10000. Back then, 32-bit architectures were still dominant, > so base-10000 was clearly the best choice at this time. > > Today, since 64-bit architectures are dominant, NBASE=1e8 seems like it would > have been the best choice, since the square of that still fits in > a 64-bit signed int. Back then 64-bit was by far not as dominant (server and consumer chips with AMD64 ISA only got released that year after the commit), so I don't think 1e8 would have been the best choice at that point in time. Would be better now, yes, but not back then. > Changing NBASE might seem impossible at first, due to the existing numeric data > on disk, and incompatibility issues when numeric data is transferred on the > wire. > > Here are some ideas on how to work around some of these: > > - Incrementally changing the data on disk, e.g. upon UPDATE/INSERT > and supporting both NBASE=1e4 (16-bit) and NBASE=1e8 (32-bit) > when reading data. I think that a dynamic decision would make more sense here. At low precision, the overhead of 4+1 bytes vs 2 bytes is quite significant. This sounds important for overall storage concerns, especially if the padding bytes (mentioned below) are added to indicate types. > - Due to the lack of a version field in the NumericVar struct, > we need a way to detect if a Numeric value on disk uses > the existing NBASE=1e4, or NBASE=1e8. > One hack I've thought about is to exploit the fact that NUMERIC_NBYTES, > defined as: > #define NUMERIC_NBYTES(num) (VARSIZE(num) - NUMERIC_HEADER_SIZE(num)) > will always be divisible by two, since a NumericDigit is an int16 (2 bytes). > The idea is then to let "NUMERIC_NBYTES divisible by three" > indicate NBASE=1e8, at the cost of one to three extra padding bytes. Do you perhaps mean NUMERIC_NBYTES *not divisible by 2*, i.e. an uneven NUMERIC_NBYTES as indicator for NBASE=1e8, rather than only multiples of 3? I'm asking because there are many integers divisible by both 2 and 3 (all integer multiples of 6; that's 50% of the multiples of 3), so with the multiple-of-3 scheme we might need up to 5 pad bytes to get to the next multiple of 3 that isn't also a multiple of 2. Additionally, if the last digit woud've fit in NBASE_1e4, then the 1e8-based numeric value could even be 7 bytes larger than the equivalent 1e4-based numeric. While I don't think this is worth implementing for general usage, it could be worth exploring for the larger numeric values, where the relative overhead of the larger representation is lower. Kind regards, Matthias van de Meent Neon (https://neon.tech)
On Mon, Jul 8, 2024, at 12:45, Matthias van de Meent wrote: > On Sun, 7 Jul 2024, 22:40 Joel Jacobson, <joel@compiler.org> wrote: >> Today, since 64-bit architectures are dominant, NBASE=1e8 seems like it would >> have been the best choice, since the square of that still fits in >> a 64-bit signed int. > > Back then 64-bit was by far not as dominant (server and consumer chips > with AMD64 ISA only got released that year after the commit), so I > don't think 1e8 would have been the best choice at that point in time. > Would be better now, yes, but not back then. Oh, grammar mistake by me! I meant to say it "would be the best choice", in line with what I wrote above: >> Last time numeric's base was changed was back in 2003, when d72f6c75038 changed >> it from 10 to 10000. Back then, 32-bit architectures were still dominant, >> so base-10000 was clearly the best choice at this time. >> Changing NBASE might seem impossible at first, due to the existing numeric data >> on disk, and incompatibility issues when numeric data is transferred on the >> wire. >> >> Here are some ideas on how to work around some of these: >> >> - Incrementally changing the data on disk, e.g. upon UPDATE/INSERT >> and supporting both NBASE=1e4 (16-bit) and NBASE=1e8 (32-bit) >> when reading data. > > I think that a dynamic decision would make more sense here. At low > precision, the overhead of 4+1 bytes vs 2 bytes is quite significant. > This sounds important for overall storage concerns, especially if the > padding bytes (mentioned below) are added to indicate types. Right, I agree. Another idea: It seems possible to reduce the disk space for numerics that fit into one byte, i.e. 0 <= val <= 255, which could be communicated via NUMERIC_NBYTES=1. At least the value 0 should be quite common. >> - Due to the lack of a version field in the NumericVar struct, >> we need a way to detect if a Numeric value on disk uses >> the existing NBASE=1e4, or NBASE=1e8. >> One hack I've thought about is to exploit the fact that NUMERIC_NBYTES, >> defined as: >> #define NUMERIC_NBYTES(num) (VARSIZE(num) - NUMERIC_HEADER_SIZE(num)) >> will always be divisible by two, since a NumericDigit is an int16 (2 bytes). >> The idea is then to let "NUMERIC_NBYTES divisible by three" >> indicate NBASE=1e8, at the cost of one to three extra padding bytes. > > Do you perhaps mean NUMERIC_NBYTES *not divisible by 2*, i.e. an > uneven NUMERIC_NBYTES as indicator for NBASE=1e8, rather than only > multiples of 3? Oh, yes of course! Thinko. > While I don't think this is worth implementing for general usage, it > could be worth exploring for the larger numeric values, where the > relative overhead of the larger representation is lower. Yes, I agree it's definitively seems like a win for larger numeric values. Not sure about smaller numeric values, maybe it's possible to improve upon. Regards, Joel