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Re: speed of unaligned accesses that cross page-boundaries

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scott@slp53.sl.home (Scott Lurndal) writes:
>Juha Nieminen <nospam@thanks.invalid> writes:
>>Bonita Montero <Bonita.Montero@gmail.com> wrote:
>>> I just wanted to measure the performance of unaligned accesses.
>>> For accesses within a page the difference was below the measurement
>>> error. Then I read that some processors put a penalty on unaligned
>>> accesses that cross page-boundaries. So I wrote a little benchmark
>>> that tries to measure this. But for my PCs, a Ryzen Threadripper
>>> 3990X, a Ryzen 7 1800X and a Phenom X4 945 all measurements were
>>> also within the measurement error.
>>
>>IIRC both Intel and AMD got rid of the unaligned access penalty
>>at some point (relatively recently). You would need an older CPU
>>in order to get the penalty.
>
>The barrel shifter penalty is gone.  However, it is unavoidable
>that such an access will need to access _two_ L1D cache lines, and
>the hardware thread/core cannot do both accesses in parallel.  Note that
>Load-to-use penalties on x86 processors are about 4 clocks when
>they hit in L1D.  Much of that additional latency  will be hidden
>by out-of-order execution, unless there is an immediate dependency
>upon the value loaded.

Clock      Operation
-----      -----------------
  1         CAM the VA against the L1D TLB to get the PA  [Assumes TLB hit]
  2         Using bits from the PA to select the set, CAM the ways [Assumes L1D hit]
  3         Load the data from SRAM onto the core bus.
  4         load the data from the core bus into the register


This needs to be done for both cache lines, sequentially.

Now, given that BM is using the C++ compiler, it is likely
the compiler is hoisting the load and/or reordering instructions
to cover that expected latency.  Adding in the cores out-of-order
execution, I doubt that BM can derive any valid latency information
relative to misaligned cache/page crossing accesses using C++.  Note that
an access that crosses a page boundary must by definition also
cross a cache line boundary.

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speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-20 18:04 +0100
  Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-20 18:08 +0100
  Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-03-20 18:12 +0000
    Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-20 19:36 +0100
  Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-03-20 16:13 -0700
    Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-21 06:58 +0100
    Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-03-21 14:49 +0000
  Re: speed of unaligned accesses that cross page-boundaries Juha Nieminen <nospam@thanks.invalid> - 2022-03-21 05:59 +0000
    Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-21 08:56 +0100
    Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-03-21 14:53 +0000
      Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-03-21 16:59 +0000
    Re: speed of unaligned accesses that cross page-boundaries Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-04-18 20:59 -0700
    Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-19 06:45 +0200
      Re: speed of unaligned accesses that cross page-boundaries Juha Nieminen <nospam@thanks.invalid> - 2022-04-19 12:51 +0000
        Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-19 15:17 +0200
      Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-04-19 14:11 +0000
        Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-19 16:15 +0200
          Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-04-27 01:52 -0700
            Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-27 20:02 +0200
            Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-28 08:19 +0200
              Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-28 12:19 +0200
                Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-04-28 16:00 -0700
                Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-29 05:52 +0200
                Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-05-04 18:06 -0700
                Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-05 06:13 +0200
                Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-05-04 21:27 -0700
                Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-05 07:37 +0200
                Re: speed of unaligned accesses that cross page-boundaries Öö Tiib <ootiib@hot.ee> - 2022-05-05 04:34 -0700
                Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-05-05 12:16 -0700

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