Re: How to avoid data copies in a driver ?

From: Francis Moreau
Date: Fri May 16 2008 - 07:45:04 EST


On Fri, May 16, 2008 at 10:10 AM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
> Francis Moreau wrote:
>> I'm suprised because what I need doens't seem so uncommon, usually
>> devices send or
>> receive data to/from files. So a helper (system call ?) to achieve
>> that other than the basic
>> read/write seems needed, no ?
> It's fairly rare to have an application which requires moving data to file
> with absolutely no processing; normally there's at least a bit of
> massaging/parsing/etc.

Well, every cases where a client <-> server exchange files. I wouldn't call
that a rare case...

> If that's really what you want to do, maybe you can
> do it with splice? I haven't looked at it at all, but the intention is that
> you can splice file descriptors together, so you can splice your device fd
> to a file fd and have it all just work...

yes but the kernel I'm working on (2.6.16) doens't have splice support.

But relay may sound a good idea, and the version I use has sendfile support.

The drawback of relay: the user application can receive a file from the kernel
but the app can't send a file to the kernel.

But I can be wrong since I haven't look at relay closely yet.

> Alternatively you could read() from your device into a mmaped file. That's
> a single copy from device to file, which is about the best you can do
> without going to heroic lengths.

Don't know for now.

I think using sendfile with relay may be a good answer without me becoming
a hero ;)

> Also, it really depends on your application. Is it a high-bandwidth thing
> in which the copy is a huge cost? Or do you want to eliminate the copies
> because it seems like a nice thing to do?

No it has a real cost. The application receives files which are usually larger
than 1Go.

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