[GitHub] [bookkeeper] wuzhanpeng opened a new issue #3085: BP-49: Support reading ahead in async mode

[GitBox]

wuzhanpeng opened a new issue #3085:
URL: https://github.com/apache/bookkeeper/issues/3085


   **BP**
   
   > Follow the instructions at 
http://bookkeeper.apache.org/community/bookkeeper_proposals/ to create a 
proposal.
   
   This is the master ticket for tracking BP-49 :
   
   ### Motivation
   
   #### Current Design of Read-ahead
   
   Under the current design of read-ahead, every `read-entry` request that the 
entry data is required to be read from main storage eventually, will force a 
read-ahead operation through the method 
`org.apache.bookkeeper.bookie.storage.ldb.SingleDirectoryDbLedgerStorage.fillReadAheadCache`.
 This method will read several entries after the current position and load them 
into the read-cache, among which the amount of entries is controlled by the 
`dbStorage_readAheadCacheBatchSize`.
   
   In this mode, once a miss of read-cache occurs, the elapsed time of reading 
an entry is equivalent to the sum of the time of reading that entry plus 
reading several entries after the entry, because the process of read-ahead is 
synchronous.
   
   Synchronous read-ahead is a simple and effective solution in scenarios where 
read latency is less of a concern. However, we found that when the cluster has 
a large number of catch-up reads, and the p99 latency cannot be ignored, 
synchronous read-ahead may introduce a lot of latency glitches. Therefore, we 
decided to introduce an asynchronous read-ahead mode to reduce the latency for 
the catch-up reads.
   
   #### Proposed Approach
   
   Instead of modifying the original synchronous read-ahead logic, we 
introduced an independent asynchronous read-ahead module named 
`ReadAheadManager`. The user can select a specific read-ahead mode through 
configuration parameters. The async read-ahead module will provide an interface 
for reading entry for upper-layer logic.
   
   #### Evaluation Results
   
   Before diving into the details, let's take a look at the performance 
optimizations brought by asynchronous read-ahead.
   
   > The upgrade time is 2022/3/2 12:00
   
   ##### Hit / Miss Count of Read Cache
   <img width="1074" alt="image" 
src="https://user-images.githubusercontent.com/14341827/157023558-a57c520b-8cb8-4aac-9a53-ffb4344a43c3.png";>
   
   ##### AVG Time of Read-Entry Op
   <img width="525" alt="image" 
src="https://user-images.githubusercontent.com/14341827/157023689-6b3e6b08-3e14-41e3-8602-6af8f8b4dad8.png";>
   
   <!-- add a proposal PR link below -->
   Proposal PR - #abc


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