Integrated: 8276662: Scalability bottleneck in SymbolTable::lookup_common()

[Derek White]

On Mon, 15 Nov 2021 22:31:13 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.

This pull request has now been integrated.

Changeset: 1d7cef33
Author:    Derek White <drwhite at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/1d7cef33c5ff24695463a03c58c7ca350ec190fc
Stats:     7 lines in 1 file changed: 7 ins; 0 del; 0 mod

8276662: Scalability bottleneck in SymbolTable::lookup_common()

Reviewed-by: redestad, dholmes, iklam, shade

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