BankForks - SharableBank for root

[apfitzge]

Problem

• Many services lock BankForks just to get the root bank
  • some use the RootBankCache, which reduces contention but doesn't eliminate when root changes

Summary of Changes

• Similar to PohRecorder: SharedWorkingBank #7280 but for root bank of BankForks
• Will be following up with a sharable_working_bank as well

Fixes #

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.94595% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.8%. Comparing base (1c9366c) to head (1d5e7c5).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #7300     +/-   ##
=========================================
- Coverage    82.8%    82.8%   -0.1%     
=========================================
  Files         801      800      -1     
  Lines      363392   363365     -27     
=========================================
- Hits       300960   300934     -26     
+ Misses      62432    62431      -1     

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[akhi3030]

FWIW, I was also looking into addressing a similar issue in anza-xyz/alpenglow#322.

[AshwinSekar]

LGTM, root bank is only written by one thread p infrequently (400 ms), but has many readers. Seems arcswap will be plenty performant here.

Would wait to see if Brennan/Akhi have any objections

[akhi3030]

Seems like a fine strategy.

I am curious if in practice we can notice any performance difference between this strategy and the simpler one of:

#[derive(Clone)]
pub struct SharableBank(Arc<parking_lot::RwLock<Bank>>);

impl SharableBank {
    pub fn load(&self) -> Arc<Bank> {
        self.0.read().clone()
    }

    fn store(&self, bank: Arc<Bank>) {
        *self.0.write() = bank;
    }
}

Especially because the data being swapped is inside an Arc so cheap to copy.

We could consider doing some benchmarking but I don't think we need to waste time on such micro optimisations at this point in time.

[apfitzge]

Seems like a fine strategy.

I am curious if in practice we can notice any performance difference between this strategy and the simpler one of:

#[derive(Clone)]
pub struct SharableBank(Arc<parking_lot::RwLock<Bank>>);

impl SharableBank {
    pub fn load(&self) -> Arc<Bank> {
        self.0.read().clone()
    }

    fn store(&self, bank: Arc<Bank>) {
        *self.0.write() = bank;
    }
}

Especially because the data being swapped is inside an Arc so cheap to copy.

We could consider doing some benchmarking but I don't think we need to waste time on such micro optimisations at this point in time.

Practically I'm not sure we'll see much difference since the lock-time would be so small. with rwlock there's still waiting for lock to be freed, possibly a syscall? i didn't look at the code of parking_lot.