From: Roland Dreier <rdreier@cisco.com>
To: Jes Sorensen <jes@sgi.com>
Cc: "Bryan O'Sullivan" <bos@pathscale.com>, Andi Kleen <ak@suse.de>,
linux-kernel@vger.kernel.org, discuss@x86-64.org
Subject: Re: Why is wmb() a no-op on x86_64?
Date: Wed, 18 Jan 2006 12:07:23 -0800 [thread overview]
Message-ID: <ada4q41wb9g.fsf@cisco.com> (raw)
In-Reply-To: <yq04q41qxcw.fsf@jaguar.mkp.net> (Jes Sorensen's message of "18 Jan 2006 12:06:39 -0500")
Bryan> On x86_64, we fiddle with the MTRRs to enable write
Bryan> combining, which makes a huge difference to performance.
Bryan> It's not clear to me what we should even do on other
Bryan> architectures, since the only generic entry point that even
Bryan> exposes write combining is pci_mmap_page_range, which is
Bryan> for PCI mmap through userspace, and half the arches I've
Bryan> looked at ignore its write_combine parameter.
Jes> A job for mmiowb() perhaps?
I don't think the semantics of mmiowb() do what is desired here.
mmiowb() is all about ordering writes between separate CPUs, and the
issue at hand here is that write-combining buffers might reorder
writes from a single CPU.
- R.
prev parent reply other threads:[~2006-01-18 20:07 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-01-18 16:23 Why is wmb() a no-op on x86_64? Bryan O'Sullivan
2006-01-18 16:29 ` Andi Kleen
2006-01-18 16:52 ` Bryan O'Sullivan
2006-01-18 17:06 ` Jes Sorensen
2006-01-18 17:23 ` Bryan O'Sullivan
2006-01-18 17:31 ` [discuss] " Andi Kleen
2006-01-19 10:03 ` Jes Sorensen
2006-01-18 20:07 ` Roland Dreier [this message]
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