Re: [PATCH 0/2] RISC-V: Probe for misaligned access speed

[Evan Green]

On Sat, Jun 24, 2023 at 3:08 AM Conor Dooley <conor@xxxxxxxxxx> wrote:
>
> On Fri, Jun 23, 2023 at 03:20:14PM -0700, Evan Green wrote:
> >
> > The current setting for the hwprobe bit indicating misaligned access
> > speed is controlled by a vendor-specific feature probe function. This is
> > essentially a per-SoC table we have to maintain on behalf of each vendor
> > going forward. Let's convert that instead to something we detect at
> > runtime.
> >
> > We have two assembly routines at the heart of our probe: one that
> > does a bunch of word-sized accesses (without aligning its input buffer),
> > and the other that does byte accesses. If we can move a larger number of
> > bytes using misaligned word accesses than we can with the same amount of
> > time doing byte accesses, then we can declare misaligned accesses as
> > "fast".
> >
> > The tradeoff of reducing this maintenance burden is boot time. We spend
> > 4-6 jiffies per core doing this measurement (0-2 on jiffie edge
> > alignment, and 4 on measurement). The timing loop was based on
> > raid6_choose_gen(), which uses (16+1)*N jiffies (where N is the number
> > of algorithms). On my THead C906, I found measurements to be stable
> > across several reboots, and looked like this:
> >
> > [    0.047582] cpu0: Unaligned word copy 1728 MB/s, byte copy 402 MB/s, misaligned accesses are fast
> >
> > I don't have a machine where misaligned accesses are slow, but I'd be
> > interested to see the results of booting this series if someone did.
>
> Can you elaborate on "results" please? Otherwise,
>
> [    0.333110] smp: Bringing up secondary CPUs ...
> [    0.370794] cpu1: Unaligned word copy 2 MB/s, byte copy 231 MB/s, misaligned accesses are slow
> [    0.411368] cpu2: Unaligned word copy 2 MB/s, byte copy 231 MB/s, misaligned accesses are slow
> [    0.451947] cpu3: Unaligned word copy 2 MB/s, byte copy 231 MB/s, misaligned accesses are slow
> [    0.462628] smp: Brought up 1 node, 4 CPUs
>
> [    0.631464] cpu0: Unaligned word copy 2 MB/s, byte copy 229 MB/s, misaligned accesses are slow
>
> btw, why the mixed usage of "unaligned" and misaligned"?

Yes, this is exactly what I was hoping for in terms of results, thank
you. I'll clean up the diction and choose one word. I think my brain
attributed subtle differences between unaligned (as in, without regard
for alignment, the behavior of the copies) and misaligned (ie.
deliberately out of alignment, the type of access we're testing), but
I'm not sure I'm even fully consistent to those, so I'll fix it.

-Evan

>
> Cheers,
> Conor.