What is "u-tagged"?

By: Jeff S. (fakity.delete@this.fake.com), December 14, 2018 8:48 am
Room: Moderated Discussions
G Adair (adairg.delete@this.example.edu) on December 13, 2018 8:54 pm wrote:
> Jeff S. (fakity.delete@this.fake.com) on December 13, 2018 8:33 am wrote:
> > And some further architectural speculation:
> > Both Zen and some of IBM's newer chips are moving to u-tagged
> > L1D, which trades a minor hit rate penalty in exchange
> > for drastically reducing CAM power and complexity.
>
> Er... what is "u-tagged"? Searching for "{u,µ}-tag{,ged} cache" isn't
> turning up anything useful, and it sounds like something interesting.

Each line in a VIPT set keeps its physical tag but gets an additional hash of some subset of the bits beyond the bottom 12 of the virtual address (aka microtag/µtag/utag). When a lookup happens, the N-way set utags are checked in parallel with the TLB lookup, and only the line matching (if any) is next verified against the PT pulled from the TLB result. In comparison, traditional VIPT caches opportunistically stage the bitlines of the word/whatever column for every line in the set into a CAM while the TLB does its work then performs the PT check there, which is clearly a pretty costly way to cut latency.

The relatively minor downsides to utag designs are that virtual aliases can't co-reside in the same L1 (effectively an L1 miss/L2 hit if you use this somewhat rare pattern) and that VA hashes/utags will have some low collision rate preventing arbitrary pairs of virtual addresses from both being in the same L1 set at the same time.
< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
Sunny Cove wideSeni2018/12/12 02:58 PM
  Sunny Cove wideTravis Downs2018/12/12 09:25 PM
    Sunny Cove wideJeff S.2018/12/12 10:26 PM
      Sunny Cove wideTravis Downs2018/12/13 08:42 AM
        Sunny Cove wideanon2018/12/13 09:09 AM
          Sunny Cove wideTravis Downs2018/12/13 09:30 AM
            Sunny Cove wideJeff S.2018/12/13 09:40 AM
              Sunny Cove wideTravis Downs2018/12/13 09:55 AM
                Sunny Cove wideJeff S.2018/12/13 12:41 PM
                  Sunny Cove wideTravis Downs2018/12/13 02:03 PM
                  Non-power-of-two set sizesPaul A. Clayton2018/12/15 07:30 PM
        Sunny Cove wideJeff S.2018/12/13 09:33 AM
          Sunny Cove wideTravis Downs2018/12/13 09:50 AM
          What is "u-tagged"?G Adair2018/12/13 09:54 PM
            What is "u-tagged"?Travis Downs2018/12/13 11:22 PM
            What is "u-tagged"?Jeff S.2018/12/14 08:48 AM
              What is "u-tagged"?anon2018/12/14 08:51 PM
                What is "u-tagged"?Jeff S.2018/12/14 10:23 PM
                  What is "u-tagged"?anon2018/12/15 05:37 AM
                    What is "u-tagged"?anon2018/12/15 08:06 AM
                      What is "u-tagged"?Travis Downs2018/12/15 09:52 AM
                        What is "u-tagged"?anon2018/12/16 08:26 AM
                          What is "u-tagged"?Anon2018/12/18 04:25 AM
    Sunny Cove wideSeni2018/12/13 03:33 AM
      Sunny Cove wideKevin G2018/12/13 08:37 AM
        Sunny Cove wideTravis Downs2018/12/13 09:17 AM
          Sunny Cove wideKevin G2018/12/17 10:09 AM
            Sunny Cove wideTravis Downs2018/12/18 03:14 PM
              Sunny Cove wideKevin G2018/12/19 12:02 PM
      Sunny Cove wideTravis Downs2018/12/13 08:51 AM
        Sunny Cove wideMaynard Handley2018/12/13 11:25 AM
          Sunny Cove wideTravis Downs2018/12/13 12:23 PM
            Sunny Cove wideanon2018/12/13 02:01 PM
              Sunny Cove wideTravis Downs2018/12/13 02:22 PM
                Sunny Cove wideanon2018/12/13 04:51 PM
                  Sunny Cove wideTravis Downs2018/12/13 05:36 PM
                    Sunny Cove wideanon2018/12/14 03:57 AM
                      Sunny Cove wideLinus Torvalds2018/12/14 01:54 PM
                        Sunny Cove wideanon2018/12/14 04:25 PM
                          Sunny Cove wideLinus Torvalds2018/12/14 06:46 PM
                            Sunny Cove wideanon2018/12/15 02:57 AM
                              Sunny Cove wideanon2018/12/15 05:59 AM
                                Sunny Cove wideanon2018/12/15 06:59 AM
                                  Sunny Cove wideanon2018/12/15 07:03 AM
                                Sunny Cove widea_different_anon2018/12/15 07:45 AM
                              Sunny Cove wideSeni2018/12/15 06:25 AM
                                Sunny Cove wideanon2018/12/15 07:02 AM
                                  Sunny Cove wideLinus Torvalds2018/12/15 10:52 AM
                                    Sunny Cove wideanon2018/12/15 11:13 AM
                                      Sunny Cove wideTravis Downs2018/12/16 11:15 AM
                                      Sunny Cove wideanon2018/12/17 12:42 AM
                                        how many anons here? (NT)Michael S2018/12/17 02:46 AM
                      Sunny Cove wideTravis Downs2018/12/15 10:08 AM
                        Sunny Cove wideanon2018/12/15 10:55 AM
                          Sunny Cove wideTravis Downs2018/12/16 09:19 AM
                            Sunny Cove wideanon2018/12/16 10:37 AM
                              Sunny Cove wideTravis Downs2018/12/16 10:57 AM
                                Sunny Cove wideanon2018/12/16 12:04 PM
                                  Sunny Cove wideTravis Downs2018/12/16 07:51 PM
                          Sunny Cove wideTravis Downs2018/12/16 11:32 AM
        Sunny Cove wideSeni2018/12/13 04:20 PM
          Fair enough! (NT)Travis Downs2018/12/13 04:43 PM
  Sunny Cove wide-.-2018/12/13 04:37 AM
    Sunny Cove wideanon2018/12/13 09:06 AM
      Sunny Cove wideTravis Downs2018/12/13 09:39 AM
        Sunny Cove wideanon2018/12/13 12:09 PM
          Sunny Cove wideTravis Downs2018/12/13 12:27 PM
            Sunny Cove wideanon2018/12/13 01:11 PM
    Sunny Cove wideTravis Downs2018/12/13 09:23 AM
      Sunny Cove wideanonymous22018/12/13 03:20 PM
        Sunny Cove wideTravis Downs2018/12/13 05:00 PM
          Sunny Cove wideanon³2018/12/13 10:34 PM
            Sunny Cove wideTravis Downs2018/12/16 07:53 PM
Reply to this Topic
Name:
Email:
Topic:
Body: No Text
How do you spell green?