Re: Unexpected splice "always copy" behavior observed

Mathieu Desnoyers
2010-05-19T16:01:41Z

* Nick Piggin (npiggin@suse.de) wrote:
> On Wed, May 19, 2010 at 11:45:42AM -0400, Steven Rostedt wrote:
> > On Wed, 2010-05-19 at 17:33 +0200, Miklos Szeredi wrote:
> > > On Wed, 19 May 2010, Linus Torvalds wrote:
> > > > Btw, since you apparently have a real case - is the "splice to file"
> > > > always just an append? IOW, if I'm not right in assuming that the only
> > > > sane thing people would reasonable care about is "append to a file", then
> > > > holler now.
> > >
> > > Virtual machines might reasonably need this for splicing to a disk
> > > image.
> >
> > This comes down to balancing speed and complexity. Perhaps a copy is
> > fine in this case.
> >
> > I'm concerned about high speed tracing, where we are always just taking
> > pages from the trace ring buffer and appending them to a file or sending
> > them off to the network. The slower this is, the more likely you will
> > lose events.
> >
> > If the "move only on append to file" is easy to implement, I would
> > really like to see that happen. The speed of splicing a disk image for a
> > virtual machine only impacts the patience of the user. The speed of
> > splicing tracing output, impacts how much you can trace without losing
> > events.
>
> It's not "easy" to implement :) What's your ring buffer look like?
> Is it a normal user address which the kernel does copy_to_user()ish
> things into? Or a mmapped special driver?
>
> If the latter, it get's even harder again. But either way if the
> source pages just have to be regenerated anyway (eg. via page fault
> on next access), then it might not even be worthwhile to do the
> splice move.

Steven and I use pages to which we write directly by using the page address from
the linear memory mapping returned by page_address(). These pages have no other
mapping. They are moved to the pipe, and then from the pipe to a file (or to the
network). It's possibly the simplest scenario you could think of for splice().

Thanks,

Mathieu

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.