RFR: 8276662: Scalability bottleneck in SymbolTable::lookup_common() [v2]

[Derek White]

On Thu, 18 Nov 2021 23:25:19 GMT, Derek White <drwhite at openjdk.org> wrote:

>> Symbol table lookup had an optimization added when the symbol table was split into a shared table (for CDS?) and a local table. The optimization tries to track which table successfully found a symbol, so it can try that table first the next time.
>> 
>> Symbol table lookup is used in many JVM operations, including classloading, serialization, and reflection.
>> 
>> At startup time, more symbols will be from the shared table, but over time lookup can will be from a mix of local and shared symbols (eg user classes still have java.lang.String fields or subclass from java.lang.Object), resulting in multiple threads fighting over the value of this global variable.
>> 
>> With enough threads and cores, this can result in "true sharing" cache line contention.
>> 
>> This fix solves the scalability issue by using a thread-local variable to track which table to search first instead of a static global. The "symbol table success history" also makes more sense logically as a thread local state than as a global state.
>> 
>> Other options would also solve the the scaling problem, but may change the behavior that we're trying to optimize, or add more overhead or complexity than warranted, such as:
>> - Statically preferring the shared or local table
>> - Checking the shared table first "early on". When enough local symbols have been added, then check the local table first.  
>> - Using a NUMA-aware set of N variables distributed over M threads.
>
> Derek White has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix scalability with THREAD_LOCAL

Thanks for the reviews everyone!
I believe all review comments have been responded to. If there's anything else, please let me know. Otherwise I plan on integrating by EOD.

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PR: https://git.openjdk.java.net/jdk/pull/6400