impl(bigtable): change from time weighted average to instant count

[scotthart]

After further conversations with the Bigtable team, we're going to simplify the RPC use tracking to a simple average of the current RPCs per channel across all the channels. Additionally, pending added channels will be included in the calculations to regulate how often channels are added as opposed to a timer.

[gemini-code-assist]

Summary of Changes

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This pull request refactors the Bigtable channel management to simplify how RPC usage is measured and how the channel pool scales. By moving from a time-weighted average to an instant count for RPCs, the system gains a more immediate and straightforward understanding of channel load. Additionally, the channel pool's sizing logic now proactively considers channels that are currently being provisioned, leading to more responsive and efficient resource allocation.

Highlights

• Simplified RPC Usage Tracking: The mechanism for tracking outstanding RPCs on Bigtable channels has been simplified from a time-weighted average to an instant count. This removes the complexity of time-based measurements and calculations.
• Pending Channel Inclusion in Pool Sizing: The DynamicChannelPool now accounts for channels that are in the process of being added (num_pending_channels_) when making pool sizing decisions, providing a more accurate view of the channel pool's state.
• Removed Pool Size Increase Cooldown: The pool_size_increase_cooldown_timer_ and its associated logic have been removed, as the new channel addition strategy no longer requires this specific cooldown mechanism.

Changelog

• google/cloud/bigtable/internal/channel_usage.h
  • Removed std::chrono and std::deque includes.
  • Removed Clock type alias and clock_ member variable.
  • Removed the average_outstanding_rpcs() method and its associated Measurement struct and measurements_ deque.
  • Updated ChannelUsage constructors to reflect the removal of time-related dependencies.
• google/cloud/bigtable/internal/channel_usage_test.cc
  • Removed the AverageOutstandingRpcs test case.
  • Removed FakeSteadyClock include and its usage in ChannelUsage constructor calls.
  • Updated ChannelUsage constructor calls to align with the simplified ChannelUsage class.
• google/cloud/bigtable/internal/dynamic_channel_pool.h
  • Removed the pool_size_increase_cooldown_timer_ member and its cancellation logic.
  • Added num_pending_channels_ to track channels currently being added.
  • Modified AddChannels to increment num_pending_channels_ when new channels are initiated and decrement it upon successful creation.
  • Removed the SetSizeIncreaseCooldownTimer method.
• google/cloud/bigtable/internal/dynamic_channel_pool_test.cc
  • Removed FakeSteadyClock include and its member variable from DynamicChannelPoolTest.
  • Removed the SetSizeIncreaseCooldownTimer method from DynamicChannelPoolTestWrapper.
  • Updated ChannelUsage constructor calls in tests to remove the fake_clock_impl_ argument.

Activity

• The author, scotthart, created this pull request to implement changes discussed with the Bigtable team regarding RPC use tracking.
• The changes simplify the RPC tracking from a time-weighted average to an instant count.
• Pending channels are now included in calculations to regulate channel additions.

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[gemini-code-assist]

Code Review

The code changes simplify the ChannelUsage class by removing the average_outstanding_rpcs functionality, including its time-based tracking (clock_ and measurements_). This also removes related includes and test cases. In DynamicChannelPool, a num_pending_channels_ counter was introduced to track channels being added. However, a review comment highlights a bug where num_pending_channels_ is incorrectly decremented by the number of successfully created stubs (new_stubs.size()) instead of the number of requested channels (new_channel_ids.size()). This can lead to an inaccurate count if some channel creations fail, potentially preventing future channel additions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.65%. Comparing base (48196b1) to head (321d68a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16047      +/-   ##
==========================================
- Coverage   92.66%   92.65%   -0.01%     
==========================================
  Files        2341     2341              
  Lines      215695   215646      -49     
==========================================
- Hits       199876   199815      -61     
- Misses      15819    15831      +12     

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