What is "u-tagged"?

By: Jeff S. (fakity.delete@this.fake.com), December 14, 2018 10:23 pm
Room: Moderated Discussions
anon (anon.delete@this.yumous.com) on December 14, 2018 7:51 pm wrote:
> What happens if you get several utag matches from the same set? Do you serialize the actual tag check?

Lines with the same utags cannot reside in L1 at the same time. On a load of a colliding line, the previous line is evicted but left available in L2 (assuming strict inclusivity of L1 by L2). You can therefore use hash collisions to construct things like degenerate tight pointer chase cycles with a ~3x worse throughput than what you might expect.

> Also, increasing the number of ways by 50% will increase the number of collisions unless the
> utag size grows. Now I suck at math as well (no offense :P) so I am not able to quantify it.

It is so-called Birthday Problem math, at least if you assume uniform flat distributions from a good hash function and not intentionally positioning data to avoid or create collisions.

> I also do not see any reason why Intel would not have been playing the utag trick with
> 8-ways, so I doubt this is what enabled moving to 12-ways (Assuming they are not going
> 96-sets 8-way VIVT). Maybe it is a 64-set/8-ways decoupled sectored cache :D

Intel definitely has not used utags historically. You don't even need to hunt for hash collisions to verify, you just need to mmap() the same page twice and read two virtual views of the same data therein in alternation and see that you aren't getting >10 cycle read latencies.
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Sunny Cove wideSeni2018/12/12 02:58 PM
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    Sunny Cove wideJeff S.2018/12/12 10:26 PM
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          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
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