From: Segher Boessenkool <segher@kernel•crashing.org>
To: Scott Wood <scottwood@freescale•com>
Cc: Christophe Leroy <christophe.leroy@c-s•fr>,
Benjamin Herrenschmidt <benh@kernel•crashing.org>,
Paul Mackerras <paulus@samba•org>,
Michael Ellerman <mpe@ellerman•id.au>,
linuxppc-dev@lists•ozlabs.org, linux-kernel@vger•kernel.org
Subject: Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop
Date: Thu, 6 Aug 2015 18:25:06 -0500 [thread overview]
Message-ID: <20150806232506.GB22196@gate.crashing.org> (raw)
In-Reply-To: <1438901145.2097.170.camel@freescale.com>
On Thu, Aug 06, 2015 at 05:45:45PM -0500, Scott Wood wrote:
> > The original loop was already optimal, as the comment said.
>
> The comment says that bdnz has zero overhead. That doesn't mean the adde
> won't stall waiting for the load result.
adde is execution serialising on those cores; it *always* stalls,
that is, it won't run until it is next to complete.
> > The new code adds extra instructions and a mispredicted branch.
>
> Outside the main loop.
Sure, I never said it was super-bad or anything.
> > You also might get less overlap between the loads and adde (I didn't check
> > if there is any originally): those instructions are no longer
> > interleaved.
> >
> > I think it is a stupid idea to optimise code for all 32-bit PowerPC
> > CPUs based on solely what is best for a particularly simple, slow
> > implementation; and that is what this patch is doing.
>
> The simple and slow implementation is the one that needs optimizations the
> most.
And, on the other hand, optimising for atypical (mostly) in-order
single-issue chips without branch folding, hurts performance on
other chips the most. Well, dual-issue in-order might be worse :-P
> If this makes performance non-negligibly worse on other 32-bit chips, and is
> an important improvement on 8xx, then we can use an ifdef since 8xx already
> requires its own kernel build. I'd prefer to see a benchmark showing that it
> actually does make things worse on those chips, though.
And I'd like to see a benchmark that shows it *does not* hurt performance
on most chips, and does improve things on 8xx, and by how much. But it
isn't *me* who has to show that, it is not my patch.
If these csum routines actually matter for performance that much, there
really *should* be chip-specific implementations.
Segher
next prev parent reply other threads:[~2015-08-06 23:25 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-08-05 13:29 [PATCH v2 0/2] powerpc32: Optimise csum_partial() Christophe Leroy
2015-08-05 13:29 ` [PATCH v2 1/2] powerpc32: optimise a few instructions in csum_partial() Christophe Leroy
2015-08-05 13:29 ` [PATCH v2 2/2] powerpc32: optimise csum_partial() loop Christophe Leroy
2015-08-06 0:30 ` Segher Boessenkool
2015-08-06 2:31 ` Scott Wood
2015-08-06 4:39 ` Segher Boessenkool
2015-08-06 22:45 ` Scott Wood
2015-08-06 23:25 ` Segher Boessenkool [this message]
2015-08-17 10:56 ` leroy christophe
2015-08-17 11:00 ` leroy christophe
2015-08-17 13:05 ` leroy christophe
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