Is getting object from Thread local storage expensive?

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  1. #1
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    Is getting object from Thread local storage expensive?

    Is there a big performance hit in frequently retrieving an object from thread local storage? (Assume that it's stored once per thread and never changed but is retrieved many times)

  2. #2
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    Define "big", "frequently", and "many". If you have an existing program about which you are wondering, profile it and see if there is indeed a penalty.

  3. #3
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    What architecture is this on?

    Most x86 OS's use FS or GS segment register as a pointer to the TLS base, so the access to get the TLS pointer will require an extra attribute, but it shouldn't make much difference compared to using a pointer to get the data in the first place. I doubt you could measure the difference here.

    If the TLS base is not in a segment register, then it may require a bit more "effort" from the processor to dig out the correct data.

    For example, in some systems, it involves a "GetThreadId()" system call, and then an index into a table - that could easily take a few hundred or more clock-cycles.

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