Feature #15631: Let round_capa for ID table not allocate excess capacity for power of 2 ints >= 4

• Status: Closed
• Assignee: ko1 (Koichi Sasada)
• Target version: -

[ahorek (Pavel Rosický)]

right now round_capa value is rounded up to the next power of 2

round_capa(4) -> returns 8
round_capa(8) -> returns 16
round_capa(16) -> returns 32

round_capa(5) -> returns 8
round_capa(9) -> returns 16
round_capa(17) -> returns 32
etc.

it seems wasteful to allocate the extra items capacity, so this PR changes that to

round_capa(4) -> returns 4
round_capa(8) -> returns 8
round_capa(16) -> returns 16

round_capa(5) -> returns 8
round_capa(9) -> returns 16
round_capa(17) -> returns 32
etc.

the main purpose is to reduce memory usage especially during boot

my patch also uses BUILTIN_CLZ macro instead of shifts that makes it slightly faster

here's a benchmark

require 'benchmark/ips'

Benchmark.ips do |x|
  x.config(time: 20, warmup: 3)

  x.report('struct', "Struct.new(*('a'..'z').map { |x| x.to_sym })")
end

trunk
Warming up --------------------------------------
              struct   527.000  i/100ms
Calculating -------------------------------------
              struct      5.461k (± 5.5%) i/s -    109.089k in  20.040253s

methodmising - POW2_P (github)
Warming up --------------------------------------
              struct   544.000  i/100ms
Calculating -------------------------------------
              struct      5.570k (± 4.1%) i/s -    111.520k in  20.057245s

ahorek - BUILTIN_CLZ (id_table.c.patch)
Warming up --------------------------------------
              struct   571.000  i/100ms
Calculating -------------------------------------
              struct      5.812k (± 3.6%) i/s -    116.484k in  20.070607s

discussion https://github.com/ruby/ruby/pull/2083

[ahorek (Pavel Rosický)]

• Description updated (diff)

[methodmissing (Lourens Naudé)]

Thanks for raising this Pavel.

st_init_table_with_size(0) effectively also allocates additional capacity, but if and how quickly the hash tables mutate I'll investigate later.

References https://github.com/ruby/ruby/blob/trunk/st.c#L573-L578 , https://github.com/ruby/ruby/blob/trunk/st.c#L595 and https://github.com/ruby/ruby/blob/trunk/st.c#L332-L359

A simple peek suggests a total table size of 152 bytes on init, but will investigate time to mutation of these 0 sized tables this evening:

diff --git a/st.c b/st.c
index ed235c674e..f2b99d7771 100644
--- a/st.c
+++ b/st.c
@@ -615,6 +615,8 @@ st_init_table_with_size(const struct st_hash_type *type, st_index_t size)
 #ifdef ST_DEBUG
     st_check(tab);
 #endif
+    printf("# st_init_table_with_size(%d) -> %d (%d)\n", size, n, st_memsize(tab));
+
     return tab;
 }

linking miniruby
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(102) -> 7 (3384)
# st_init_table_with_size(255) -> 8 (7224)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(1000) -> 10 (28728)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(1000) -> 10 (28728)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(7) -> 3 (248)
# st_init_table_with_size(15) -> 4 (440)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(16) -> 5 (888)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)
# st_init_table_with_size(0) -> 2 (152)

[ko1 (Koichi Sasada)]

• Assignee set to ko1 (Koichi Sasada)

funny_falcon, do you have any opinion?
if no opinion, we'll discuss it one month later and will commit it.

in fact, I can't check algorithm, so we can try it.

ahorek:
could you give me more performance measurements?

• you should not call map in iteration (you should prepare IDs before)
• now you only measures 26 fields. could you measure other numbers, 1 to 50, for example.

Thanks,
Koichi

[ahorek (Pavel Rosický)]

Thanks for the review Koichi. I tested the patch on a rails app (redmine), but unfortunatelly there's no improvement.
0.1MB less memory after boot (^150MB total)
No mesurable difference in performance

I'll investigate the second case. Hash creation might be a better place to optimize.

[ahorek (Pavel Rosický)]

• File st.c.patch st.c.patch added

I've attached the second patch for st_init_table_with_size. In theory it should be faster, but I can't measure any difference in ruby.

[nobu (Nobuyoshi Nakada)]

It should use SIZEOF_ST_INDEX_T and nlz_intptr.

[methodmissing (Lourens Naudé)]

Pavel added a new patch for get_power2 in https://github.com/ruby/ruby/pull/2292

[ahorek (Pavel Rosický)]

• File st.c.patch st.c.patch added

[ko1 (Koichi Sasada)]

pls give us your measurements. We can't understand how it is useful.

Thanks,
Koichi

[ahorek (Pavel Rosický)]

I run several benchmark suites for both patches
https://github.com/ruby-bench/ruby-bench-suite
https://github.com/schneems/derailed_benchmarks

but all differences were within margin of error. Here's an optimized assembly comparsion that explains why:
https://user-images.githubusercontent.com/9540855/62237913-2e221380-b3d2-11e9-932a-14b09038bf91.png

saving 1-2 instructions makes no real difference.

feel free to close the issue. Thanks

• Status changed from Open to Closed

Applied in changeset git|8e13da1ee83028000e5d7f9f9526379e32765a81.

---

optimize get_power2 [Feature #15631]

Merged: https://github.com/ruby/ruby/pull/2292