[JBoss JIRA] (JBJCA-1305) Performance degradation using fair semaphores in heavily contended workloads

Tuesday, 8 December 2015

    [
https://issues.jboss.org/browse/JBJCA-1305?page=com.atlassian.jira.plugin...
] 

Tom Jenkinson edited comment on JBJCA-1305 at 12/8/15 5:25 AM:
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...
From
https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Semaphore....:
"Generally, semaphores used to control resource access should be initialized as fair,
to ensure that no thread is starved out from accessing a resource. When using semaphores
for other kinds of synchronization control, the throughput advantages of non-fair ordering
often outweigh fairness considerations." 
As I understand it this semaphore is for accessing a resource so the guidance from the SDK
is that fair would be desirable for that type of work load. However, if there are still
use-cases that guard access to resources where none-fair is advantageous, a configurable
option may therefore be appropriate.

was (Author: tomjenkinson):
...
From
https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Semaphore....:
"Generally, semaphores used to control resource access should be initialized as fair,
to ensure that no thread is starved out from accessing a resource. When using semaphores
for other kinds of synchronization control, the throughput advantages of non-fair ordering
often outweigh fairness considerations." 
As I understand it this semaphore is for accessing a resource so the guidance from the SDK
is that fair would be desirable for that type of work load. However, if there are still
use-cases that guard access to resources where none-fair is advantageous, a configurable
option may therefore be appropriate. Please can you create a forum thread to discuss this
further?

...
 Performance degradation using fair semaphores in heavily contended
workloads
 ----------------------------------------------------------------------------

                 Key: JBJCA-1305
                 URL: https://issues.jboss.org/browse/JBJCA-1305
             Project: IronJacamar
          Issue Type: Enhancement
          Components: Core
    Affects Versions: WildFly/IronJacamar 1.3.1.Final
            Reporter: John O'Hara
            Assignee: Jesper Pedersen

 Under heavy contention, a fair semaphore causes performance degradation for threads
trying to acquire a permit from the semaphore.  Changing the semaphore to not fair
alleviates the performance degradation under heavy contention. 

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