Re: In PG12, query with float calculations is slower than PG11

Hi,

I have been testing with newer compiler (clang-7)
and result is a bit different at least with clang-7.
Compiling PG 12.1 (even without patch) with clang-7
results in __isinf() no longer being a bottleneck,
that is, you don't see it in profiler at all.

So, there is no issue for people who use the modern clang toolchain,
but maybe that's not everyone.
So there would still be some interest in doing something about this.

* clang

bash-4.2$ which clang
/opt/rh/llvm-toolset-7.0/root/usr/bin/clang

bash-4.2$ clang -v
clang version 7.0.1 (tags/RELEASE_701/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/rh/llvm-toolset-7.0/root/usr/bin
Found candidate GCC installation:
/opt/rh/devtoolset-7/root/usr/lib/gcc/x86_64-redhat-linux/7
Found candidate GCC installation:
/opt/rh/devtoolset-8/root/usr/lib/gcc/x86_64-redhat-linux/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.8.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.8.5
Selected GCC installation:
/opt/rh/devtoolset-8/root/usr/lib/gcc/x86_64-redhat-linux/8
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64

** pg_config

---
CONFIGURE = '--prefix=/var/lib/pgsql/pgsql/12.1'
'CC=/opt/rh/llvm-toolset-7.0/root/usr/bin/clang'
'PKG_CONFIG_PATH=/opt/rh/llvm-toolset-7.0/root/usr/lib64/pkgconfig'
CC = /opt/rh/llvm-toolset-7.0/root/usr/bin/clang
---

* result(PostgreSQL 12.1 (even without patch))

postgres=# EXPLAIN (ANALYZE on, VERBOSE on, BUFFERS on)
select (2 * a) , (2 * b) , (2 * c), (2 * d), (2 * e)
from realtest;

QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
Seq Scan on public.realtest (cost=0.00..288697.59 rows=10000115 width=40)
(actual time=0.012..3878.284 rows=10000001 loops=1)
Output: ('2'::double precision * a), ('2'::double precision * b),
('2'::double precision * c), ('2'::double precision * d), ('2'::double
precision * e)
Buffers: shared hit=63695
Planning Time: 0.038 ms
Execution Time: 4533.767 ms
(5 rows)

Samples: 5K of event 'cpu-clock', Event count (approx.): 1275000000
Overhead Command Shared Object Symbol
33.92% postgres postgres [.] ExecInterpExpr
13.27% postgres postgres [.] float84mul
10.86% postgres [vdso] [.] __vdso_clock_gettime
5.49% postgres postgres [.] tts_buffer_heap_getsomeattrs
3.96% postgres postgres [.] ExecScan
3.25% postgres libc-2.17.so [.] __clock_gettime
3.16% postgres postgres [.] heap_getnextslot
2.41% postgres postgres [.] tts_virtual_clear
2.39% postgres postgres [.] SeqNext
2.22% postgres postgres [.] InstrStopNode

Best Regards,
Keisuke Kuroda

2020年2月7日(金) 3:48 Andres Freund <andres(at)anarazel(dot)de>:

> Hi,
>
> On 2020-02-06 11:03:51 -0500, Tom Lane wrote:
> > Andres seems to be of the opinion that the compiler should be willing
> > to ignore the semantic requirements of the C standard in order
> > to rearrange the code back into the cheaper order. That sounds like
> > wishful thinking to me ... even if it actually works on his compiler,
> > it certainly isn't going to work for everyone.
>
> Sorry, but, uh, what are you talking about? Please tell me which single
> standards violation I'm advocating for?
>
> I was asking about the inlining bit because the first email of the topic
> explained that as the problem, which I don't believe can be the full
> explanation - and it turns out it isn't. As Amit Langote's followup
> email explained, there's the whole issue of the order of checks being
> inverted - which is clearly bad. And wholly unrelated to inlining.
>
> And I asked about __isinf() being used because there are issues with
> accidentally ending up with the non-intrinsic version of isinf() when
> not using gcc, due to badly written standard library headers.
>
>
> > The patch looks unduly invasive to me, but I think that it might be
> > right that we should go back to a macro-based implementation, because
> > otherwise we don't have a good way to be certain that the function
> > parameter won't get evaluated first.
>
> I'd first like to see some actual evidence of this being a problem,
> rather than just the order of the checks.
>
>
> > (Another reason to do so is so that the file/line numbers generated
> > for the error reports go back to being at least a little bit useful.)
> > We could use local variables within the macro to avoid double evals,
> > if anyone thinks that's actually important --- I don't.
>
> I don't think that's necessarily a good idea. In fact, I think we should
> probably do the exact opposite, and move the error messages further out
> of line. All these otherwise very small functions having their own
> ereports makes them much bigger. Our low code density, and the resulting
> rate of itlb misses, is pretty significant cost (cf [1]).
>
> master:
> text data bss dec hex filename
> 36124 44 65 36233 8d89 float.o
> error messages moved out of line:
> text data bss dec hex filename
> 32883 44 65 32992 80e0 float.o
>
> Taking int4pl as an example - solely because it is simpler assembly to
> look at - we get:
>
> master:
> 0x00000000004ac190 <+0>: mov 0x30(%rdi),%rax
> 0x00000000004ac194 <+4>: add 0x20(%rdi),%eax
> 0x00000000004ac197 <+7>: jo 0x4ac19c <int4pl+12>
> 0x00000000004ac199 <+9>: cltq
> 0x00000000004ac19b <+11>: retq
> 0x00000000004ac19c <+12>: push %rbp
> 0x00000000004ac19d <+13>: lea 0x1a02c4(%rip),%rsi #
> 0x64c468
> 0x00000000004ac1a4 <+20>: xor %r8d,%r8d
> 0x00000000004ac1a7 <+23>: lea 0x265da1(%rip),%rcx #
> 0x711f4f <__func__.26823>
> 0x00000000004ac1ae <+30>: mov $0x30b,%edx
> 0x00000000004ac1b3 <+35>: mov $0x14,%edi
> 0x00000000004ac1b8 <+40>: callq 0x586060 <errstart>
> 0x00000000004ac1bd <+45>: lea 0x147e0e(%rip),%rdi #
> 0x5f3fd2
> 0x00000000004ac1c4 <+52>: xor %eax,%eax
> 0x00000000004ac1c6 <+54>: callq 0x5896a0 <errmsg>
> 0x00000000004ac1cb <+59>: mov $0x3000082,%edi
> 0x00000000004ac1d0 <+64>: mov %eax,%ebp
> 0x00000000004ac1d2 <+66>: callq 0x589540 <errcode>
> 0x00000000004ac1d7 <+71>: mov %eax,%edi
> 0x00000000004ac1d9 <+73>: mov %ebp,%esi
> 0x00000000004ac1db <+75>: xor %eax,%eax
> 0x00000000004ac1dd <+77>: callq 0x588fb0 <errfinish>
>
> out-of-line error:
> 0x00000000004b04e0 <+0>: mov 0x30(%rdi),%rax
> 0x00000000004b04e4 <+4>: add 0x20(%rdi),%eax
> 0x00000000004b04e7 <+7>: jo 0x4b04ec <int4pl+12>
> 0x00000000004b04e9 <+9>: cltq
> 0x00000000004b04eb <+11>: retq
> 0x00000000004b04ec <+12>: push %rax
> 0x00000000004b04ed <+13>: callq 0x115e17 <out_of_range_err>
>
> With the out-of-line error, we can fit multiple of these functions into one
> cache line. With the inline error, not even one.
>
> Greetings,
>
> Andres Freund
>
> [1] https://twitter.com/AndresFreundTec/status/1214305610172289024
>