remove par_iter on update index below threshold

[jeffwashington]

Problem

Improve perf for common call with low # accounts to update.
See #24692

The largest call I saw on a mnb snapshot and brief ledger replay was 302 accounts being updated at a time.

Summary of Changes

Fixes #

[HaoranYi]

make it const? hoping that the compiler can const propagating and inline lambda for the special case of 1.

[HaoranYi]

lgtm

[jeffwashington]

data on my mnb run for all # accounts > 1% of the total:

# accounts | COUNT | % | cumulative %
-- | -- | -- | --
1 | 504160 | 61% | 61%
4 | 97797 | 12% | 73%
2 | 56329 | 7% | 80%
6 | 53812 | 7% | 86%
8 | 23438 | 3% | 89%
10 | 16963 | 2% | 91%
9 | 12817 | 2% | 93%
12 | 11043 | 1% | 94%
14 | 6911 | 1% | 95%
16 | 5323 | 1% | 96%

[jeffwashington]

Note that any individual pubkey update index call could be a page fault currently. With a threshold of 1, we are clearly better not adding parallelism. It gets more complicated to know with len > 1.

[jeffwashington]

To get to a cumulative ~1% of calls, we have to include all calls with len > 64.
Here are the largest 10 len values:

len | COUNT of len | % | cumulative %
-- | -- | -- | --
302 | 15 | 0% | 0%
264 | 15 | 0% | 0%
262 | 1 | 0% | 0%
232 | 9 | 0% | 0%
230 | 2 | 0% | 0%
196 | 15 | 0% | 0%
192 | 1 | 0% | 0%
190 | 1 | 0% | 0%
188 | 11 | 0% | 0%
186 | 3 | 0% | 0%

[brooksprumo]

Looks good to me as well.

Pull request has been modified.

[jeffwashington]

doh. my metrics were from testnet...

[codecov]

Codecov Report

Merging #25699 (25c6cdf) into master (3ed2e0c) will increase coverage by 6.6%.
The diff coverage is 53.5%.

@@             Coverage Diff             @@
##           master   #25699       +/-   ##
===========================================
+ Coverage    75.4%    82.1%     +6.6%     
===========================================
  Files          40      621      +581     
  Lines        2345   170485   +168140     
  Branches      338        0      -338     
===========================================
+ Hits         1769   139978   +138209     
- Misses        459    30507    +30048     
+ Partials      117        0      -117