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Subject: Re: [PATCH RFC 04/10] SUNRPC: Add common byte-swapped RPC header
 constants
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <20190204075336.GA28337@infradead.org>
Date:   Mon, 4 Feb 2019 09:16:54 -0500
Cc:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>, simo@redhat.com
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To:     Christoph Hellwig <hch@infradead.org>
Sender: linux-nfs-owner@vger.kernel.org
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> On Feb 4, 2019, at 2:53 AM, Christoph Hellwig <hch@infradead.org> wrote:
> 
> On Sat, Feb 02, 2019 at 05:49:35PM -0500, Chuck Lever wrote:
>>>> Byte-swapping causes a CPU pipeline bubble on some processors. When
>>>> a decoder is comparing an on-the-wire value for equality, byte-
>>>> swapping can be avoided by comparing it directly to a pre-byte-
>>>> swapped constant value.
>>> 
>>> Which ones?
>> 
>> I assume you mean on which processors have I observed CPU cycle
>> spikes around bswap instructions.
> 
> Yes.
> 
>> I've seen this behavior only
>> on Intel processors of various families.
> 
> Interesting. In general we should not do separate byte swap instructions
> on x86, as MOVBE can be used to do a load or store with an included
> byteswap, and I thought the whole point for that was that they could
> be handled in the same cycle.
> 
> In fact https://www.agner.org/optimize/instruction_tables.pdf
> says that movbe is generally a single cycle instruction.
> 
>> Would you prefer a different justification for this clean-up?
> 
> I don't really care about the cleanup, it is just that the explanation
> goes against conventional wisdom, which is why I was a little surpised.
> 
> And that is not just the cycles, but also as Trond pointed out
> that the Linux byte swapping macro on constants should usually be
> optimized away at compile time anyway.

They are. The problem is that we are byte-swapping the incoming wire
data and then comparing to CPU-endian constants in some hot paths.
It's better to leave the incoming data alone and compare to a pre-
byte-swapped constant. This patch adds some of these constants that
were missing, in preparation for fixing the hot paths.

That is apparently not clear from the patch description, so I will
endeavor to improve it.


--
Chuck Lever