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Re: write alignment matters?
On Thu, Jun 23, 2011 at 06:36:31AM -0700, Matt Thomas wrote:
> On Jun 23, 2011, at 4:41 AM, Thor Lancelot Simon wrote:
> > On Thu, Jun 23, 2011 at 03:36:25PM +0700, Robert Elz wrote:
> >> Date: Wed, 22 Jun 2011 19:30:55 -0400 (EDT)
> >> From: der Mouse <mouse%Rodents-Montreal.ORG@localhost>
> >> Message-ID:
> >> <201106222330.TAA28359%Sparkle.Rodents-Montreal.ORG@localhost>
> >> | But the interface is much older than that, and, even if it's not
> >> | codified, there's a lot of history behind the notion that userland
> >> | alignment of write() buffers affects, at most, performance, to the
> >> | point where I consider it part of the interface.
> >> Not on access to raw devices it isn't, and never was - what Erik Fair
> >> said (Message-id:
> >> <5F005E6A-5441-4BEC-BB3C-4A9B79584BFB%netbsd.org@localhost>)
> >> was 100% correct - if you're using a raw device, it is up to the
> >> application to meet whatever the requirements of that particular device
> >> are, because one of the properties of raw devices is that they don't
> >> do any kind of rebuffering of data (and the driver must not - that is
> >> a part of the interface contract).
> > That doesn't seem like it can really be right. There are plenty of
> > systems where devices cannot DMA from user addresses.
> Really? That seems unlikely.
Any system where some devices have a limited range of DMA addresses
will see this kind of "raw" I/O get bounced, no? Think of an ISA
And then there are weird cases like the pmax where the device can do
DMA but there's a buffer in the middle anyway.
Thor Lancelot Simon tls%panix.com@localhost
"All of my opinions are consistent, but I cannot present them all
at once." -Jean-Jacques Rousseau, On The Social Contract
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