From: Christoph Hellwig <hch@infradead.org>
To: Chuck Lever <chuck.lever@oracle.com>
Cc: Christoph Hellwig <hch@infradead.org>,
Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
simo@redhat.com
Subject: Re: [PATCH RFC 04/10] SUNRPC: Add common byte-swapped RPC header constants
Date: Sun, 3 Feb 2019 23:53:36 -0800 [thread overview]
Message-ID: <20190204075336.GA28337@infradead.org> (raw)
In-Reply-To: <52468C38-9E9C-49A7-B44B-2BE302A33145@oracle.com>
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.
next prev parent reply other threads:[~2019-02-04 7:53 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-01 19:57 [PATCH RFC 00/10] SUNRPC GSS overhaul Chuck Lever
2019-02-01 19:57 ` [PATCH RFC 01/10] SUNRPC: Remove some dprintk() call sites from auth functions Chuck Lever
2019-02-04 19:04 ` J. Bruce Fields
2019-02-04 19:07 ` Chuck Lever
2019-02-01 19:57 ` [PATCH RFC 02/10] SUNRPC: Remove rpc_xprt::tsh_size Chuck Lever
2019-02-01 19:57 ` [PATCH RFC 03/10] SUNRPC: Add build option to disable support for insecure enctypes Chuck Lever
2019-02-01 19:57 ` [PATCH RFC 04/10] SUNRPC: Add common byte-swapped RPC header constants Chuck Lever
2019-02-02 2:30 ` Tom Talpey
2019-02-02 22:46 ` Chuck Lever
2019-02-03 15:00 ` Trond Myklebust
2019-02-03 16:49 ` Chuck Lever
2019-02-03 18:58 ` Trond Myklebust
2019-02-02 17:02 ` Christoph Hellwig
2019-02-02 22:49 ` Chuck Lever
2019-02-04 7:53 ` Christoph Hellwig [this message]
2019-02-04 14:16 ` Chuck Lever
2019-02-04 14:32 ` Christoph Hellwig
2019-02-04 14:56 ` Chuck Lever
2019-02-04 19:37 ` J. Bruce Fields
2019-02-05 1:57 ` Tom Talpey
2019-02-01 19:57 ` [PATCH RFC 05/10] SUNRPC: Use struct xdr_stream when constructing RPC Call header Chuck Lever
2019-02-01 19:57 ` [PATCH RFC 06/10] SUNRPC: Clean up rpc_verify_header() Chuck Lever
2019-02-01 19:58 ` [PATCH RFC 07/10] SUNRPC: Use struct xdr_stream when decoding RPC Reply header Chuck Lever
2019-02-01 19:58 ` [PATCH RFC 08/10] SUNRPC: Introduce trace points in rpc_auth_gss.ko Chuck Lever
2019-02-01 19:58 ` [PATCH RFC 09/10] SUNRPC: Remove xdr_buf_trim() Chuck Lever
2019-02-04 19:46 ` J. Bruce Fields
2019-02-04 19:49 ` Chuck Lever
2019-02-04 20:00 ` Bruce Fields
2019-02-04 20:07 ` Chuck Lever
2019-02-04 20:11 ` Bruce Fields
2019-02-01 19:58 ` [PATCH RFC 10/10] SUNRPC: Add SPDX IDs to some net/sunrpc/auth_gss/ files Chuck Lever
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