3 Replies Latest reply on Jul 18, 2018 5:14 PM by Intel Corporation

    Load_Latency performance counter ambiguity


      The May 2018 Combined SDM, Chapter 19, Section 2 and Section 6 list the performance counters for skylake and haswell, respectively.


      Under section 2 you will find the following 8 events:

      Event          Umask

      Number       Value

      CDH             01H       MEM_TRANS_RETIRED.LOAD_LATENCY_GT_2

      CDH             01H       MEM_TRANS_RETIRED.LOAD_LATENCY_GT_4

      CDH             01H       MEM_TRANS_RETIRED.LOAD_LATENCY_GT_8




      CDH            01H      MEM_TRANS_RETIRED.LOAD_LATENCY_GT_256

      CDH            01H      MEM_TRANS_RETIRED.LOAD_LATENCY_GT_512


      Their description reads: "Counts loads when the latency from first dispatch to completion is greater than <X> cycles." for the correspoding

      value of X; 2, 4, 8, etc. In particular, there is no indication in the description that these counters measure randomly sampled memory loads. In fact,

      as stated I would expect a precise count of these events up to skidding in perf record.


      Under section 6, among others, you will find:

      Event         Umask

      Number      Value           Event Mask Mnemonic                                   Description

      CDH           01H             MEM_TRANS_RETIRED.LOAD_LATENCY  Randomly sampled loads whose latency is above a user defined threshold. [Specify threshold in MSR 3FAH]


      My question is: Can the "MEM_TRANS_RETIRED.LOAD_LATENCY" be used to emulate the former 8 performance counters showing up for Skylake, or are

      the semantics as stated in the description correct thus prohibiting this emulation by proxy?


      I am aware that the Events and Umask are the same, but I am unsure if the implementation of these in hardware are consistent across haswell and skylake. I would

      like to get an official answer from Intel.



      Thank you.


      Message was edited by: Khronus Taizen Added omitted word: "find"

        • 1. Re: Load_Latency performance counter ambiguity
          Intel Corporation
          This message was posted on behalf of Intel Corporation

          Hello Khronuskrazen
          Thank you for joining the community.
          Looking into your inquiry about the performance counter I have noticed that in order to provide you with better assistance I recommend to visit the following link https://software.intel.com/en-us/articles/how-to-create-a-support-request-at-online-service-center to submit the request for support on the topic.
          Hope this helps.
          Leonardo C.

          • 2. Re: Load_Latency performance counter ambiguity

            Hi Leonardo,


            I appreciate the recommendation, however please observe the following:

                 1. I do not own any Intel Products in regards to this question. It is purely a logical consistency question.

                 2. My question is stated in very precise terms. I see no reason why Intel, and any of its engineers, would be unable to answer my question here.

                 3. As much as I like signing up for things, filling out seemingly pointless forms, and not finding options applicable to my situation in a predefined list I'd much prefer to hope technical folks at Intel could understand my plight and get permission to respond.

                      3a. That was exactly my situation when I followed your link.

                      3b. The permission thing was a joke in case it wasn't clear.


            But also, could you or someone else from Intel please address my question? I think it is a fair and reasonable question to be addressed in this forum without having to fill out electronic paper work.



            Thank you.


            P.s. Being logical here: could you provide me with "regular assistance" and if that is not enough then we will think about the strange "provide you with better assistance" business: presumably better assistance than normal? Can I get the normal assistance first?

            • 3. Re: Load_Latency performance counter ambiguity
              Intel Corporation
              This message was posted on behalf of Intel Corporation

              Hello Khronuskrazen

              Thank you for your response.
              Allow me to share with you that for the specific inquiry that you have related to the processor families performance counters the appropriate group of support would be in Intel® developer zone.
              Please refer to the following links:
              Intel® Developer Zone
              Join the Intel® Developer Zone
              Leonardo C.