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pull mode vs push mode

onkarkk

 Hi,

Could any help me out in understanding the difference between pull mode and push mode . I am really not understanding this theory . please help me out.

Thanks in advance,

regards,

Krishna Kishore 

  • I will assume you are referring to eRM or OVM generation of transactions.

    Push mode (recommended by other methodologies) refers to the generator creating transactions and pushing them to the BFM or Driver which then uses the transaction to drive the DUT. This has a problem that the generator is creating transactions before the driver requests them and therefore might not reflect constraints that the test is requiring, i.e. not reactive. This results in the possibility that those transactions will be discarded and new transactions having to being generated.

    Pull mode refers to the generator creating transactions in response to the driver requesting them. This is more efficient in that the driver can add additional constraints before requesting the transaction and therefore get only the transactions needed. This is recommended and built-in to eRM/OVM.

    Tim

  •  Thankyou Tim,

     Now i understood bit clearly..

    regards,

    Krishna Kishore 

  • Hi Tim,

    I was referring to the topic pull vs push mode, I am not clear that how constraint will not reflect that the test is requireng in push mode due to the transaction generated before to driver's request. Consider the sequence started body() will start then will call do action, so item will be generated according to the constraints given then date will be send_to_bfm(). The same case for pull_mode generator will create transaction to the drivers request and this also will call the do action on data as per constraint. So both cases doing the action on data item so how it will differe on constraint. Also where will be the push mode useful.

    Thanks,

    rsl

  • hi, The main issue here is when does the sequence driver picks the next "do". In Pull mode, this decision is made right before injecting the item. In Push mode - this decision might be made many cycles before the injection. Consider, for example, the case when the driver picks the next "do" from sequence A. Cycles passes, and in the mean time - seq B proceeds (perhaps it was waiting for some event), and now it grabs the driver. But the driver already picked another "do", not the B's. This results with a scenario not matching user's intention. While developing layered sequences examples (some are included in uvm/uvm_examples), we came to realize several use models when Push mode just wouldn't work (or - more accurate - requires too much user code). Because in layered sequences, there is a delicate synchronization between high layer, low layer, their drivers and BFMs. To your question - we came to conclusion that Pull mode is the recommended mode. We yet have not seen a use model where Push is preferred.
  • In UVM/OVM sequencers (uvm_push_sequencer vs the uvm_sequencer (pull)), the difference in the kind of TLM connection. A push sequencer has a "port" and pull sequencer has an "export". Because sequencers and drivers have a 2-way handshake, the sequencer will still wait for one transaction to finish before randomizing the next. The reason for using a push would be that if the driver doesn't need a dedicated thread (the sequencer calls the driver) it could be higher performance.

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