[Engine-devel] ThreadPoolUtil with 500 threads and no queue?

[Juan Hernandez]

On 12/12/2012 05:21 PM, Michael Kublin wrote:
> 
> 
> ----- Original Message -----
>> From: "Juan Hernandez" <jhernand at redhat.com>
>> To: engine-devel at ovirt.org
>> Sent: Wednesday, December 12, 2012 6:06:30 PM
>> Subject: [Engine-devel] ThreadPoolUtil with 500 threads and no queue?
>>
>> Hello all,
>>
>> What is the reasoning behind the decision to have a pool with a
>> maximum
>> of 500 threads and no job queue (see ThreadPoolUtil.java)? Wouldn't
>> it
>> make more sense to have a much smaller thread pool and a potentially
>> large queue of jobs?
>>
>> Regards,
>> Juan Hernandez
> 
> There are three general strategies for queuing:
> 
>  1) Direct handoffs. A good default choice for a work queue is a SynchronousQueue that hands off tasks to threads without otherwise holding them. Here, an attempt to queue a task will fail if no threads are immediately available to run it, so a new thread will be constructed. This policy avoids lockups when handling sets of requests that might have internal dependencies. Direct handoffs generally require unbounded maximumPoolSizes to avoid rejection of new submitted tasks. This in turn admits the possibility of unbounded thread growth when commands continue to arrive on average faster than they can be processed.
>   2) Unbounded queues. Using an unbounded queue (for example a LinkedBlockingQueue without a predefined capacity) will cause new tasks to wait in the queue when all corePoolSize threads are busy. Thus, no more than corePoolSize threads will ever be created. (And the value of the maximumPoolSize therefore doesn't have any effect.) This may be appropriate when each task is completely independent of others, so tasks cannot affect each others execution; for example, in a web page server. While this style of queuing can be useful in smoothing out transient bursts of requests, it admits the possibility of unbounded work queue growth when commands continue to arrive on average faster than they can be processed.
>   3) Bounded queues. A bounded queue (for example, an ArrayBlockingQueue) helps prevent resource exhaustion when used with finite maximumPoolSizes, but can be more difficult to tune and control. Queue sizes and maximum pool sizes may be traded off for each other: Using large queues and small pools minimizes CPU usage, OS resources, and context-switching overhead, but can lead to artificially low throughput. If tasks frequently block (for example if they are I/O bound), a system may be able to schedule time for more threads than you otherwise allow. Use of small queues generally requires larger pool sizes, which keeps CPUs busier but may encounter unacceptable scheduling overhead, which also decreases throughput.
> 
> Why not? we are using 1).
> Actually 500 threads should be enough for very big applications

I think that 500 are maybe too much, even for a very big application,
the reasons you explain very well in 3). A resource that we can very
easily overload is the database. If those 500 threads happen to need
database connections we have a problem.

I think we should use a bounded queue and have both the number of
threads and the size of the queue configurable. Does that make sense?

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