[jira] [Updated] (ARROW-10038) [C++] SetCpuThreadPoolCapacity(1) spins up nCPUs threads

["Antoine Pitrou (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-10038?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Antoine Pitrou updated ARROW-10038:
-----------------------------------
    Summary: [C++] SetCpuThreadPoolCapacity(1) spins up nCPUs threads  (was: SetCpuThreadPoolCapacity(1) spins up nCPUs threads)

> [C++] SetCpuThreadPoolCapacity(1) spins up nCPUs threads
> --------------------------------------------------------
>
>                 Key: ARROW-10038
>                 URL: https://issues.apache.org/jira/browse/ARROW-10038
>             Project: Apache Arrow
>          Issue Type: Bug
>          Components: C++
>    Affects Versions: 1.0.1
>            Reporter: Adam Hooper
>            Priority: Minor
>
> When I call {{arrow::SetCpuThreadPoolCapacity(1);}}, Arrow does this:
> 1. Spins up a singleton {{ThreadPool}} with _the default thread count_;
> 2. Sets the number of threads on that {{ThreadPool}} to 1 -- killing the extra threads.
> On my Intel system, I'm forced to spin up four threads to set the CPU thread-pool capacity to 1. This goes against the spirit of the API method -- or at least, my understanding of it (and my experience with other thread pools).
> My workaround, for calling code: instead of calling {{arrow::SetCpuThreadPoolCapacity(1)}}, call {{setenv("OMP_NUM_THREADS", "1", 1)}}.
> Brainstorming, here are some ideas for Arrow's global thread pool that would stop launching {{>limit}} threads to set the limit:
> * {{cpu_thread_pool_capacity}} could be a global variable, not an attribute on the global {{ThreadPool}}. API users would be expected to set the thread-pool capacity _before_ creating the thread pool. (They're probably doing this anyway.)
> * {{SetCpuThreadPoolCapacity()}} could call {{setenv("OMP_NUM_THREADS", ...)}}
> * {{ThreadPool}} could create threads on-demand instead of in the ctor. An unused {{ThreadPool}} would launch zero threads -- resolving ARROW-10033 as a side-effect



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