Messages in this thread | | | Date | Fri, 16 May 2008 13:44:47 +0200 | From | "Francis Moreau" <> | Subject | Re: How to avoid data copies in a driver ? |
| |
Hello,
On Fri, May 16, 2008 at 10:10 AM, Jeremy Fitzhardinge <jeremy@goop.org> 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.
Thanks -- Francis
| |